A systematic review on consumer acceptance of alternative proteins: Pulses, algae, insects, plant-based meat alternatives, and cultured meat

Consumers' dietary patterns have a significant impact on planetary and personal health. To address health and environmental challenges one of the many possible solutions is to substitute meat consumption with alternative protein sources. This systematic review identifies 91 articles with a focus on the drivers of consumer acceptance of five alternative proteins: pulses, algae, insects, plant-based alternative proteins, and cultured meat. This review demonstrates that acceptance of the alternative proteins included here is relatively low (compared to that of meat); acceptance of insects is lowest, followed by acceptance of cultured meat. Pulses and plant-based alternative proteins have the highest acceptance level. In general, the following drivers of acceptance consistently show to be relevant for the acceptance of various alternative proteins: motives of taste and health, familiarity, attitudes, food neophobia, disgust, and social norms. However, there are also differences in relevance between individuals and alternative proteins. For example, for insects and other novel alternative proteins the drivers of familiarity and affective processes of food neophobia and disgust seem more relevant. As part of gaining full insight in relevant drivers of acceptance, the review also shows an overview of the intervention studies that were included in the 91 articles of the review, providing implications on how consumer acceptance can be increased. The focal areas of the intervention studies included here do not fully correspond with the current knowledge of drivers. To date, intervention studies have mainly focussed on conscious deliberations, whereas familiarity and affective factors have also been shown to be key drivers. The comprehensive overview of the most relevant factors for consumer acceptance of various categories of alternative proteins thus shows large consistencies across bodies of research. Variations can be found in the nuances showing different priorities of drivers for different proteins and different segments, showing the relevance of being context and person specific for future research.

In comparison to meat, the market shares of alternative proteins remain low (Gravely & Fraser, 2018), even despite the fact that supermarkets and restaurants increasingly offer alternatives to traditional meat products or dishes, such as plant-based burgers or wraps with beans . Alternative proteins, such as pulses, algae, insects, plant-based meat alternatives, and cultured meat (e. g., Hartmann & Siegrist, 2017;Van der Weele et al., 2019), are generally considered to be healthier and more environmentally friendly than traditional animal-derived proteins (e.g., Aiking, 2011). That being said, the benefits of alternative protein production have not yet been fully scientifically documented, particularly with respect to the environment. For instance, uncertainty remains about whether cultured meat will be produced in a more environmentally sustainable manner than conventional meat (e.g., Alexander, Brown, Dias, Moran, & Rounsevell, 2019;Van der Weele et al., 2019).
However, consumers are often unaware of meat-related environmental and health issues; they are also often unwilling to change their meat-eating habits (e.g., Harguess, Crespo, & Hong, 2020;Hartmann & Siegrist, 2017;Onwezen & Van der Weele, 2016;Sanchez-Sabate & Sabaté, 2019). Given the importance of transitioning to more sustainable diets, we need to better understand why consumers might be willing to consume alternative proteins and how this knowledge can be used to develop interventions to increase consumer acceptance.
The present review of literature aims to contribute to this field of research by providing a comprehensive overview of the most relevant drivers of Western consumers' acceptance of, or willingness to buy or eat, a range of alternative proteins. As part of gaining full insight into the drivers of acceptance, we also include intervention studies that focus on increasing acceptance of alternative proteins via drivers of acceptance. We focus on the acceptance of five alternative proteins that reflect a broad range in terms of novelty, desirability, and plausibility (Van der Weele et al., 2019): pulses, algae, insects, plant-based meat alternatives, and cultured meat.
We contribute to the existing literature by focussing on specific drivers of acceptance. Multiple systematic reviews and meta-analyses have aimed to gain insight into effective ways to stimulate healthy and sustainable dietary patterns (e.g., Adriaanse, Vinkers, De Ridder, Hox, & De Wit, 2011;Bianchi, Garnett, Dorsel, Aveyard, & Jebb, 2018;Wilson, Buckley, Buckley, & Bogomolova, 2016). However, in these studies, there has been little focus on the underlying determinants that steer consumers to healthier and more sustainable food consumption, which would provide more insight into the effectiveness of interventions (O'Rourke & MacKinnon, 2018).
There are several reviews on related areas of consumer acceptance of alternative proteins, such as those regarding the consumer acceptance of innovative food (Siegrist, 2008); novel food technologies (Siegrist & Hartmann, 2020); food naturalness (Roman, Sánchez-Siles, & Siegrist, 2017); meat consumption (Sanchez-Sabate & Sabaté, 2019); and specific meat alternatives like cultured meat (Bryant & Barnett, 2018), insects (Dagevos, 2021, in press;Mancini, Moruzzo, Riccioli, & Paci, 2019), and plant-based meat substitutes (Weinrich, 2019). These reviews provide comprehensive and detailed overviews of relevant drivers of behaviour; however, they do not include multiple alternative proteins. We aim to contribute to this field of studies by providing an overview on consumer acceptance of various alternative proteins. Thereby allowing for the possibility to reflect on similarities and differences across various categories of alternative proteins. To the best of our knowledge, the only systematic review on consumer acceptance of several alternative proteins available to date is one by Hartmann and Siegrist (2017). This study includes 19 articles focussed on meat substitutes and alternative proteins such as insects and cultured meat. Hartmann and Siegrist's review paper examines consumers' awareness of the environmental impact of meat production and their willingness to change their current meat consumption behaviour in general. We add to their review in multiple ways. First, many studies on novel proteins have been recently published, thus requiring a new overview of the literature. Second, the current study provides a more comprehensive and in-depth overview of relevant drivers of consumer acceptance, thereby resulting in a deeper understanding of the drivers that determine consumers' acceptance of different alternative proteins. Third, the present review encompasses a larger variety of alternative proteins than the review by Hartmann and Siegrist (2017), including cultured meat, seaweed and microalgae, insects, pulses, and plant-based alternative proteins. Finally, we add to the literature by reflecting on differences between categories of alternative proteins, thereby providing a first indication of whether different alternative proteins are accepted for different reasons.
Our main contribution to the literature is thus that we provide an up to date, in-depth overview of drivers of acceptance of a wide range of alternative proteins. This overview can help bridging knowledge across research domains and results in a research agenda towards understanding and steering consumer acceptance of alternative proteins.

Article selection
In June 2020, a literature search was conducted in the electronic database Scopus, as this is the largest abstract and citation database of peer-reviewed literature. We used the following search query: ALL (consumption*) AND TITLE-ABS-KEY (food* AND consumer* AND (accept* OR preference* OR willing* OR buy* OR purchas* OR choice* OR behavio* OR adopt* OR perception*) AND ("cultured meat*" OR "in vitro meat*" OR "synthetic meat*" OR seaweed* OR alga* OR insect* OR lupin* OR pulse* OR legume* OR *bean* OR "dry pea*" OR chickpea* OR "cow pea*" OR "pigeon pea*" OR lentil* OR "meat alternative*" OR "meat substitute*" OR "plant-based meat*" OR "meat analogue*")). The search terms were tested and refined through multiple rounds until the resulting number of papers was manageable while simultaneously demonstrating face validity (i.e., whether important key papers were included in the search results). "Consumption", "food", and "consumer" were included to ensure a focus on consumers' food consumption. Various words for consumer acceptance were included. Finally, various words were included for the five selected alternative proteins. Regarding pulses, the definition of the United Nations Food and Agriculture Organization (FAO) 1 was utilised. The search terms were included in the keywords, title, or abstract of the article being searched. Non-English articles, conference papers, reviews, and irrelevant subject areas (i.e., biochemistry, genetics and molecular biology, immunology and microbiology, chemistry, chemical engineering, engineering, and neuroscience) were excluded, resulting in a total of 665 articles. Fig. 1 displays a flowchart of the article selection process, while Table 1 contains the inclusion and exclusion criteria used. The 665 articles were screened according to their title and abstract by two 1 FAO recognizes 11 types of pulses: dry beans, dry broad beans, dry peas, chickpeas, cow peas, pigeon peas, lentils, Bambara beans, vetches, lupins and pulses. independent raters. Interrater agreement resulted in a conformity of 79%; disagreement was resolved by a discussion resulting in full consensus. 2 Based on this screening, 103 articles were considered relevant. Most studies were excluded because they either did not contain consumer data or they did not focus primarily on consumers or alternative proteins, or Western countries.
In the next step, full-text articles were retrieved. All articles were read, and we further refined the set of included articles by excluding papers that did not fit the main purpose of our review. This additional screening resulted in a total of 88 papers included by June 2020. To assure that we do not miss any intervention studies an additional search was performed including the term "intervention" in the search query. This resulted in one extra study that was included based on the additional search including the term "intervention". 3 Two additional relevant articles were found independent of the title and abstract screening. In total, this resulted in 91 relevant articles that were included in the systematic review.

Data extraction
All relevant information from the articles was extracted. Again, disagreement was resolved by discussion and consensus. All articles were scanned for information on country, sample size, type of study (e. g., survey, experiment), type of alternative protein, independent variables, dependent variables, and main outcomes. The information is listed below in Table 2 (and in more detail in Appendix A). For a complete overview of the included studies, we refer to the supplementary materials.

Results
We first describe general findings regarding the studies (section 3.1; country, methods, etc.) and current acceptance levels of the alternative proteins, including variations in lifestyle and demographics (section 3.2), followed by specific findings on the drivers of consumer acceptance of the different alternative proteins (section 3.3). Finally, we discuss the interventions to increase consumer acceptance (section 3.4).

General findings
The literature research shows that the research field on alternative proteins is developing rapidly. For example, in 2014, there were only three studies published, whereas there were 16 studies in 2018 and 37 in 2019.
The results reveal an unequal distribution of articles across the different alternative proteins. There are 9 articles on pulses, 9 on algae, 58 on insects, 9 on plant-based meat alternatives, and 16 on cultured meat. 4 Thus, most studies focus on insects.
The majority of the studies were conducted in the Netherlands (20 studies), Italy (17 studies), Germany (13 studies), the United States (9 studies), Australia (8 studies), Belgium (7 studies), the United Kingdom (5 studies), and Switzerland (6 studies). Other countries, such as the Czech Republic, are only represented once or twice.
The types of study designs show an overrepresentation of quantitative designs: 46 of the studies include a survey, 40 studies include an experiment, seven include focus groups, and five include interviews. 5

Acceptance of the different types of alternative proteins
In the selected body of research, different outcome measures were used to refer to acceptance of alternative proteins-for example, willingness to pay, intention to try or purchase, and self-reported behaviour. Generally, the results indicate that alternative proteins have low acceptance levels (e.g., Graça et al., 2019;La Barbera, Verneau, & Coppola, 2019;Neff et al., 2018).
Numerous studies compare the acceptance of alternative proteins to that of traditional meat, revealing that alternative proteins are evaluated significantly less positively (e.g., Onwezen, van den Puttelaar, et al.,

Table 1
Inclusion and exclusion criteria for article selection.
• Social environment f : eating insects perceived to be not supported by family and/or friends (Sogari, 2015;Sogari et al., 2016). High social acceptance (Hartmann et al., 2015) and the more participants thought other participants ate mealworm-containing products (Jensen & Lieberoth, 2019), or perceived eating insects as a socially acceptable activity (Schäufele et al., 2019), the more likely they were to try insect foods themselves. Perceptions of a conspiracy theory (regarding a social movement toward growing insect consumption) (Sogari, Bogueva & Marinova, 2019). • Cultural appropriateness g : Insects were perceived as inappropriate in culture (Gallen et al., 2019;Myers & Pettigrew, 2018;. Insects that (continued on next page) 2019; Slade, 2018). For example, when participants were informed that all burgers tasted the same, 65% would purchase the beef burger, 21% would purchase the plant-based burger, 11% preferred the cultured meat burger, and 4% would purchase nothing (Slade, 2018). A small number of studies include multiple alternative proteins (e.g., Bryant et al., 2019;Circus & Robinson, 2019;de Boer et al., 2013;Gómez-Luciano, Vriesekoop, & Urbano, 2019;Grasso et al., 2019;Ianuzzi, 2019;Onwezen, van den Puttelaar, et al., 2019;Slade, 2018, Tucker, 2014. Direct comparisons indicate that insects are the least accepted alternative protein, cultured meat the second-least, and that plant-based meat substitutes are among the most accepted alternative proteins (Circus & Robinson, 2019;Grasso et al., 2019;Ianuzzi, 2019), demonstrating that consumers are less willing to accept animal-based novel proteins than plant-based novel proteins (de Boer et al., 2013;Slade, 2018). For example, participants were much less willing to taste insect snacks compared to other snacks made from lentils, seaweed, and hybrid meat (i.e., meat products in which meat has been partially replaced by more sustainable protein sources) (de Boer et al., 2013); similarly, plant-based burgers were preferred over cultured meat burgers (21% versus 11%; Slade, 2018).
Lifestyle and demographics are described below as these provide often-used descriptions of populations and segmentation criteria (Onwezen, 2018;Verain et al., 2012).
Lifestyle. The studies we examined consistently noted the relevance of dietary patterns (i.e., current meat and meat alternative consumption): Consumers with high levels of meat consumption are more were marketed more in one's own culture were preferred more (Fisher & Steenbekkers, 2018;Hartmann et al., 2015).

External attributes Interventions
•Plant-based meat alternatives have less shelf space compared to traditional meat, less promotions. Consumers found it slightly more satisfying as well as easier to shop for animal-based compared to plant-based protein sources (Gravely & Fraser, 2018).
Note. * a,b,c,d,e,f,g These superscript letters are used to link drivers with the manuscript. In this way we do not refer to all references all the time, but simply refer to the superscript letters that correspond with the driver and the related references.
receptive to cultured meat (e.g., Circus & Robinson, 2019) and products that look similar to meat   Circus & Robinson, 2019). Cultured meat was, for example, favoured by individuals with high meat attachment; edible insects were not favoured by the low and the high meat-attachment groups; and plant-based substitutes were favoured mostly by the low meat-attachment groups who primarily followed vegan and vegetarian diets (Circus & Robinson, 2019). Additionally, more general dietary lifestyle patterns influence acceptance of alternative proteins; for instance, following a green, vegetarian diet or a healthy lifestyle is an important aspect in acceptance of snacks made from lentils, seaweed (de Boer et al., 2013), and insects (green eating behaviour; Grasso et al., 2019).
Demographics. The results on demographic variables (age, gender, education, etc.) reveal variation in relevance in their association with acceptance of alternative proteins. In some studies, demographics contribute to the understanding of consumer acceptance of alternative proteins (e.g., gender Gómez-Luciano et al., 2019;Orkusz et al., 2020); in other studies, demographics are insignificant (e.g., gender Barton et al., 2020;Birch et al., 2019a;de Boer et al., 2013). The explained variance of demographic variables is generally low, as they only explain some aspects of the acceptance of alternative proteins (e.g., Grasso et al., 2019).

Drivers of acceptance
Based on a framework of acceptance of novel foods (Siegrist, 2008), we define three types of drivers and interventions. The three lines of drivers are (1) product-related factors, (2) psychological factors, and (3) external attributes (social environment, trust, and culture). The authors (experts in the field of consumer acceptance) categorised drivers of this literature overview within the abovementioned three lines. For each line of research, we first describe the general findings, followed by a reflection on the differences in drivers of acceptance of the categories of alternative proteins. To reflect on differences across alternative proteins, we use studies that include multiple alternative proteins and we counted the number of studies showing significant associations.
If we focus on healthiness (Adámek et al., 2018;Ali, 2006) and environmental benefits (Ali, 2006;Tucker, 2014;Verbeke et al., 2015), the findings reveal that consumers do not always know whether alternative proteins have specific benefits. For example, consumers do not seem to be aware of the potential health benefits and environmental benefits, such as the environmental and nutritional advantages of eating insects (Myers & Pettigrew, 2018); many participants also underestimated the ecological impact of animal production (Vanhonacker, Van Loo, Gellynck, & Verbeke, 2013). However, when consumers are aware of such potential benefits, these motivations are shown to play an important role in consumer acceptance. For example, the belief that eating insects has health benefits (Menozzi et al., 2017;Schlup & Brunner, 2018) and environmental benefits (Menozzi et al., 2017;Sogari, 2016;Vanhonacker et al., 2013;Verbeke, 2015) significantly affects consumers' intention to consume insects. More than 80% of participants were willing to eat food enriched with edible insects because they believed it to be healthy (Adámek et al., 2018). Note that environmental benefits also reveal mixed findings, and the environmental benefits are also not always included in research designs (e.g., review on insects, Dagevos, 2021).
Familiarity. b Individuals have a tendency to behave in similar ways as they are used to behave and to choose options that are already known (e. g., Pelchat & Pliner, 1995;Tuorila, Andersson, Martikainen, & Salovaara, 1998). In the context of alternative proteins, familiarity (Schlup & Brunner, 2018) is relevant in the acceptance of pulses (e.g., Florkowswki & Park, 2001), algae (Birch et al., 2019a), insects (Schlup & Brunner, 2018;Tan, van den Berg, & Stieger, 2016;Verbeke, 2015;Woolf et al., 2019), and cultured meat . Related concepts, such as previous experience with alternative proteins (i.e., past behaviour; Florkowswki & Park, 2001) and knowledge of and experience with plant-based meat alternatives (Hoek et al., 2013), are also demonstrated to be linked with acceptance. Moreover, past consumption of insects not only increases consumption but also positively impacts wider associations, such as perceptions of tastiness of insects (Tan et al, 2015(Tan et al, , 2016a(Tan et al, , 2016bCicatiello et al., 2016;Hartmann et al., 2015;Megido et al., 2016;Schlup & Brunner, 2018;Verbeke, 2015). The research on insect acceptance clarifies the underlying processes of the relevance of familiarity by illustrating, for example, that past experiences with insects 6 The superscript letters refer to Table 2. We use these superscript letters when we refer to all references related to a specific driver with the same superscript letter in Table 2.
result in a formulation of intentions based on memories; comparatively, individuals who do not have these past experiences often have existing negative associations (Tan et al., 2016a(Tan et al., , 2017a. Differences between alternative proteins in product-related factors. Table 2 shows a large range of different motivations a to consume various alternative proteins (e.g., healthiness, taste, price, convenience, etc). A direct comparison of the relevance of these motivations for the different alternative proteins is not possible, as each study utilises different questionnaires and methods. However, the results do provide some indications of noteworthy differences among the alternative proteins. For example, health motivations seem to play a more prominent role in the acceptance of plant-based proteins (Circus & Robinson, 2019;de Boer et al., 2013), whereas environmental motivations seem to be more relevant in the acceptance of animal-based proteins such as cultured meat (Circus & Robinson, 2019).
We discovered a greater number of studies demonstrating the relevance of some form of familiarity as a driver of the acceptance of insects (59%) compared to that of cultured meat (33%), plant-based meat alternatives (27%), algae (20%), and pulses (0%). Although this might occur due various reasons, for example because of a bias in research designs (i.e., familiarity more often included for novel products), or because barriers on familiarity are more easily accessible for consumers than motivations of familiarity, the findings do show indications that underscore the relevance of familiarity for insects. Individuals seem to have negative associations with and fear of trying insects; insects are still far from being accepted by consumers as a conventional part of their diets compared to plant-based proteins (de Boer et al., 2013). Moreover, familiarity seems to play a more prominent role when products are novel for a consumer, e.g., familiarity was less important for heavy users of various meat substitutes in explaining their acceptance compared to non-users and light or medium users. Non-users had a higher tendency to avoid new food .

Psychological factors
Individuals may exhibit distinct factors that explain variations in acceptance of alternative proteins. The personal characteristics that demonstrate consistent results are attitudes, c and food neophobia and disgust. d Attitudes. c Attitudes are consistently shown to be relevant in explaining intentions to consume alternative proteins; this seems to apply to all alternative proteins (Onwezen, van den Puttelaar, et al., 2019;pulses (Lemken et al., 2017), algae (Lucas et al., 2019), insects (Fisher & Steenbekkers, 2018Lombardi et al., 2019;Menozzi et al., 2017;Piha et al., 2018), plant-based meat alternatives , and cultured meat (Slade, 2018)). For example, Lombardi et al. (2019) found that negative beliefs and attitudes toward insects negatively affect the willingness to pay for insect-based products. Moreover, attitudes seem to differ between groups of individuals-for example, between non-users, light and medium users, and heavy users of meat substitutes. Non-users have a very positive attitude toward meat, while heavy users have more positive attitudes toward meat substitutes .
Novel meat alternatives may attract neophilic consumers who seek new food alternatives (e.g., out of curiosity; Sogari, 2016) and are not scared to try novel foods (i.e., drivers of food neophobia, fear, and disgust). Note that these are distinct mechanisms; for example, food neophobia and disgust have separate effects on intention to eat insects, with disgust having more explanatory power (La Barbera et al., 2018). Moons et al. (2018) indicate that sensitivity to food neophobia could differ between specific target groups (i.e., foodies) that are more or less engaged with food and food choices, suggesting that these processes may be even more relevant for specific target groups.
Differences across alternative proteins in psychological factors. The acceptance of all alternative proteins is affected by attitudes. The findings of Onwezen, van den Puttelaar, Verain, and Veldkamp (2019) demonstrate a first indication that attitudes are more relevant to meat-related products such as traditional burgers and cultured meat.
The acceptance of all alternative proteins is affected by food neophobia; however, insects reveal a broader group of affect-related feelings that explain acceptance of specific products (i.e., mostly insectbased products). General disgust, affective attitudes toward eating insects (Fisher & Steenbekkers, 2018), fear (Cicatiello et al., 2016), and possible negative evaluations by family or friends (Sogari, 2016) all negatively affect willingness to eat specific insects. In accordance with these findings, Onwezen and colleagues (2019) show that affective drivers are more relevant for innovative alternative proteins of insects and seaweed (compared to less innovative alternative proteins of pulses and fish), indicating that acceptance of innovative alternative proteins is based more on feelings than is acceptance of less innovative alternative proteins.

External attributes
As the term "alternative proteins" often refers to new food products that are perceived as innovative by consumers (e.g., insects; Onwezen, van den Puttelaar, et al., 2019), consumers use external factors to form an opinion on these products (Siegrist, 2008). The results of our review and of previous studies (Siegrist, 2008) reveal three types of external attributes: trust, e social environment, f and (cultural) appropriateness. s Trust. e Generally, a low number of studies include trust in their research (three studies out of 90 included studies). The findings on trust reveal a positive association between trust and acceptance of alternative proteins (Lensvelt & Steenbekkers, 2014;Wilks et al., 2019). For example, a general distrust in science correlates with less acceptance of cultured meat (Wilks et al., 2019). Independent promoters, conversely, appear to play a role in increasing trust in the product (algae; Balzan et al., 2016); similarly, information provided by public health institutions increase trust in insects as an alternative protein (Balzan et al., 2016). There is also variation between countries (for instance, the Netherlands demonstrates more trust than Australia) in trusting information about insect food from different organisations (Lensvelt & Steenbekkers, 2014).
Social environment. g Social norms provide information on appropriate behaviour via the perception of behaviour and opinions of others. Social norms are generally shown relevant drivers of behaviour (Onwezen, Antonides, & Bartels, 2013;White, Smith, Terry, Greenslade, & McKimmie, 2009). Multiple studies consistently refer to the influence of the social environment on acceptance of alternative proteins (social norms: Jensen & Lieberoth, 2019;Lensvelt & Steenbekkers, 2014;Onwezen, van den Puttelaar, et al., 2019 andsocial acceptance: Hartmann et al., 2015;Lemken et al., 2017;Sogari, 2015;Schäufele et al., 2019;Slade, 2018;Vainio et al., 2016;Warne et al., 2019;Wilks et al., 2019). The negative opinions of family and friends, for example, are referred to as a relevant barrier for trying pulses (Figueira et al., 2019) or edible insects in the future (Sogari et al., 2016). Moreover, in comparison to a range of acceptance variables such as disgust, emotions, attitudes, and perceived behavioural control, social norms appeared to be the most relevant factor in explaining consumer acceptance of a range of alternative proteins: fish, pulses, insects, seaweed, and cultured meat (Onwezen, van den Puttelaar, et al., 2019).
Some studies also reveal the potential of social norms to steer behaviour: The more participants perceived eating insects as a socially acceptable activity, the more they were willing to try insect products (Schäufele et al., 2019). For example, the more participants thought other participants ate products containing mealworm, the more likely they were to eat these foods themselves (Jensen & Lieberoth, 2019). The impact of social norms seems especially relevant for those who do not regularly consume those particular products (e.g., individuals without an established diet containing beans and soy products, Vainio et al., 2016).
Cultural appropriateness. Only studies on insects reveal the relevance of cultural traditions (i.e., alternative proteins not accepted because perceived as inappropriate in one's own culture; Gallen et al., 2019;Myers & Pettigrew, 2018) or food appropriateness (Tan, Tibboel, & Stieger, 2017). A study by Hartmann et al. (2015) reveals that Chinese participants rated all insect-based products more favourably than Germans regarding taste, nutritional value, familiarity, and social acceptance and were more willing to eat insect-based products (with no differences for processed versus unprocessed insects), indicating that insects that were already part of a person's culture were preferred (Hartmann et al., 2015).
Differences between alternative proteins in external attributes. Social norms are a highly relevant factor for all alternative proteins (in comparison to the included range of explanatory factors such as attitudes, perceived behavioural control, and emotions), as well as for various forms of novel insects that differ in how innovative they are perceived to be (Onwezen, van den Puttelaar, et al., 2019). Social acceptance was important for the acceptance of insects in China and Germany (Hartmann et al., 2015), indicating that social environment is also relevant for acceptance when a particular food is already part of tradition.

Interventions
Compared to studies that focus on understanding the drivers of behaviour, far fewer articles have tested the effectiveness of these drivers by means of behavioural interventions (27% of all studies have a link with interventions). We use a broad definition of interventions including all experimental designs that include one or multiple activities to manipulate the choice environment and therefore test how an intervention affects consumer acceptance (e.g., attitudes, intentions or behaviour). Below we discuss the findings of these intervention studies pertaining to the behavioural drivers that were distinguished above.

Product-related attributes
The majority of the included intervention studies focussed on testing the effectiveness of product-related attributes (13 out of 27 intervention studies included some form of claim information). Claims of health and environmental benefits are shown to support the acceptance of pulses (Lemken et al., 2017;environmental benefits: Warne et al., 2019), seaweed (Brayden et al., 2018;environmental benefits: Weinrich & Elshiewy, 2019), cultured meat (health benefits: Siegrist & Sütterlin, 2017), insects (Cavallo & Materia, 2018;de-Magistris et al., 2015), and lentil consumption (environmental information in combination with economic and nutritional information: Warne et al., 2019). A combination of both environmental and health claims has been proven to have superior results in increasing acceptance over separate claims (Lemken et al., 2017). Note that physical environment may play a role. For example, a study by Lucas et al. (2019) indicated that participants' choice of seaweed claims is strongly influenced by the location at which they purchase seaweed.
In addition to claims, framing information in a specific manner is also beneficial in promoting acceptance of alternative proteins. For example, highlighting societal benefits (more than highlighting individual benefits) increased acceptance of insects (Verneau et al., 2016); describing cultured meat in a nontechnical manner that focusses on the final product increases acceptance of cultured meat (Siegrist et al., 2018); and more specific information increased participants' willingness to pay for insects (Lombardi et al., 2019).
However, not all information was shown to be effective; for example, positioning the product with different types of information (i.e., product-related information, physiological information, social norm information, or no information) did not affect acceptance of insects (Lensvelt & Steenbekkers, 2014) or cultured meat (Mancini & Antonioli, 2019). Similarly, the 'clean meat is natural' message failed to make participants consider clean meat more natural (compared to a control condition) or willing to pay more (Bryant et al., 2019).
Finally, some studies even showed drawback effects as a result of information provision. For example, 'high-tech' framing led to the least positive attitudes (Bryant & Dillard, 2019), and communicating e-numbers or possible negative health effects decreased the perceived naturalness of cultured meat (Siegrist & Sütterlin, 2017).
Price interventions prove to be effective in increasing consumer acceptance of alternative proteins. High prices were associated with increases in expected quality, which positively influenced participants' preference for mealworm burgers and mealworm truffles (Berger et al., 2018).
Familiarity. In line with the findings on familiarity, intervention studies reveal that familiarity can be used to increase consumer acceptance (i.e., tasting increased familiarity with and acceptance of insects; Barton et al., 2020;Hartmann & Siegrist, 2016;Lensvelt & Steenbekkers, 2014;Sogari et al., 2018). For example, participants who ate tortilla chips containing cricket flour were more willing to eat unprocessed insects than people who had eaten traditional tortilla chips (containing corn flour; Hartmann & Siegrist, 2016). Consumers can also be made more familiar with and more willing to accept alternative proteins by providing them with prior information on consumption of insects (Barsics et al., 2017). Laureati et al. (2016) demonstrated that attending university courses or working in an environment in which the topics of insects and sustainability are investigated and debated positively affects consumers' willingness to incorporate insects into their diets. A similar effect is found when providing prior information via an information session.
Generally, individuals prefer products in which novel ingredients are disguised; disguising ingredients increases familiarity with algae (Balzan et al., 2016), and willingness to eat insects decreases when insects are visible in the products (even when barely visible; Ali, 2016;Baker et al., 2016;Cavallo & Materia, 2018;de-Magistris et al., 2015;Schlup & Brunner, 2018). Similarly, acceptance was lower for unprocessed insects compared to processed insects (Hartmann et al., 2015), for insects as food compared to insects as animal feed (Laureati et al., 2016), and when descriptions of insects in a menu setting were very explicit instead of vague (Baker et al., 2016).

External attributes
Social norms can be utilised to increase acceptance levels such that participants exposed to positive peer ratings of a nutrition bar expected it to be of higher quality than did participants exposed to negative peer ratings (Berger et al., 2019).
Providing information via different organisations affects the levels of trust on insects and can also be helpful to further increase the effectiveness of health and environmental claims. For example, public health institutions can play an active role in promoting consumer acceptance of insects (Balzan et al., 2016).

General discussion
Insights into consumer acceptance of alternative proteins and its drivers are needed to steer consumers towards lower meat consumption and higher consumption of alternative proteins (Dagevos & Reinders, 2018;Harguess et al., 2020;Hartmann & Siegrist, 2017;Sanchez-Sabate & Sabaté, 2019). The present systematic review provides an overview of relevant drivers of willingness to eat and try five different alternative proteins, including the effectiveness of interventions to increase consumer acceptance of these alternative proteins. The systematic review adds to the literature by a focus on drivers to understand and find ways to steer consumer acceptance, and by including a wide range of alternative proteins thereby allowing for a comprehensive overview and comparisons across categories of alternative proteins. Based on our search criteria, 91 articles were identified and included in this systematic review.

Acceptance of alternative proteins is relatively low, especially for insects followed by cultured meat
In accordance with previous findings (e.g., Dagevos & Voordouw, 2013;de Boer et al., 2013;Graça et al., 2019;Malek, Umberger, & Goddard, 2019), our first main finding was that acceptance levels of alternative proteins are relatively low in comparison to the consumption of meat (e.g., Onwezen, van den Puttelaar, et al., 2019;Slade, 2018). Furthermore, we observed variations in acceptance levels between the different alternative proteins. Plant-based meat alternatives and pulses are most accepted, insects are least accepted, and cultured meat second-least (Circus & Robinson, 2019;de Boer et al., 2013;Ianuzzi, 2019;Onwezen, van den Puttelaar, et al., 2019). Interestingly, pulses and plant-based meat alternatives are also estimated to have higher sustainability potential at this moment compared to other alternative proteins (Van der Weele et al., 2019), which implies that it is most promising-in terms of environmental impact and societal acceptance -to focus on increasing consumption of pulses and plant-based meat alternatives.
Acceptance levels vary across segmentation criteria of demographics and lifestyle. Demographic variables were generally found to be less relevant compared to social and psychological factors in understanding consumer acceptance of alternative proteins. This finding is in accordance with previous research (Beane & Ennis, 1987;Hartmann & Siegrist, 2017;Wansink, Sonka, & Park, 2004). Lifestyle showed to be of significance, consumers with high levels of meat consumption are more receptive to cultured meat (e.g., Circus & Robinson, 2019)

Relevant drivers to increase consumer acceptance
This review provides a comprehensive overview of which drivers are consistently shown to have an association with various alternative proteins. In short, these drivers include the food choice motives a of taste, healthiness (Lea et al., 2005;Moons et al., 2018), familiarity, b attitudes, c food neophobia and disgust, d and social norms f (more details below). The relevance of this wide range of drivers implies that for acceptance of alternative proteins it is relevant to use integrated approaches including drivers from different levels (e.g., personal, physical environment and social environment, Story et al., 2008).
While alternative proteins are often referred to as a specific category of food choices (e.g., Hartmann & Siegrist, 2017), we want to note that the relevance of the abovementioned drivers is consistent with the body of research on drivers of food choices in general (e.g., similar effects were found for healthy food choices, organic food choices, fruit and vegetable choices, and choices for novel foods). 7 Indicating that the acceptance of alternative proteins is not that different compared to food choices in related domains and that much more cross-learning across domains is possible. Moreover, the findings indicate that consumer acceptance of alternative proteins shows many congruencies across alternative proteins. Highlighting the relevance of future research in the context of alternative proteins to learn from related bodies of research within related areas of alternative proteins, for example there is a large body of research on acceptance of insects which can be used by related domains such as acceptance of seaweed.
That being said, our review clearly illustrates differences in alternative proteins and groups of consumers. Both directions provide a more nuanced picture regarding acceptance of alternative proteins, showing that acceptance of alternative proteins varies across products and categories and across consumers. Moreover, the relevant drivers of acceptance also show variations across consumer groups and across alternative proteins. We elaborate on these findings in more detail below.

Differences between groups of consumers
Regarding the drivers of acceptance, the results show that past consumption influences a person's motivations for eating insects (e.g., price, taste, availability; House, 2016). Variations in use of alternative proteins resulted in a different impact of familiarity, attitudes , food neophobia (Moons et al., 2018), and social norms (Vainio et al., 2016) on acceptance. Past consumption of alternative proteins is thus a relevant factor to include in understanding and targeting consumers (i.e., segmentation criteria, Onwezen, 2018) in the context of alternative proteins. Moreover, the findings on psychological factors also indicate the relevance of personal variations in understanding consumer acceptance. It is for example more relevant to target on consumer groups that are more receptive to various alternative proteins, e.g., scoring low on food neophobia, disgust and having positive attitudes and associations with alternative proteins. 7 Taste and health for example are often mentioned as the most relevant food choices (Onwezen et al, 2001(Onwezen et al, , 2019bSteptoe et al., 1995). Attitudes and social norms are part of the theory of planned behaviour which shows to have predictive value for a broad range of food products (organic food, Arvola et al., 2008;fruit, Bogers, Brug, van Assema, & Dagnelie, 2004; sustainable food, Vermeir & Verbeke, 2008). Food neopobia, disgust and trust seem to be especially related to novel food technologies (Genetic modification, Siegrist, 2008; novel foods, Tuorila & Hartmann, 2020).

Differences between alternative proteins
Although future research including designs with direct comparisons across various drivers and various alternative proteins is necessary, the results provide implications for variations in drivers of acceptance across various alternative proteins. Fig. 2 shows a highly simplified overview of the main drivers for each alternative protein. We highlight three relevant lines of differences between alternative proteins.
First, health motivations seem to be more relevant to acceptance of plant-based proteins, and environmental motivations seem more closely linked to the consumption of animal-based proteins (comparison across various alternative proteins: Circus & Robinson, 2019;de Boer et al., 2013). Second, insect-based studies reveal a broader range of affect-related drivers d-insects compared to other alternative proteins. Consumers may rely more on affective processes for novel alternatives compared to conventional alternatives (Onwezen, van den Puttelaar, et al., 2019). Third, the role of familiarity seems especially relevant to insect-based alternative proteins (e.g., Schlup & Brunner, 2018;Verbeke, 2015;Woolf et al., 2019. Taken together these findings indicate that consumer acceptance of various alternative proteins also shows variations in relevance of drivers. It is therefore recommended for future research to be specific about which proteins are researched, as research even shows variations for different products within categories. Moreover, interventions developed to increase consumer acceptance should be targeted at specific alternative proteins, as these show to be associated with specific product attributes and barriers, and personal motivations and environmental drivers.

Interventions
As noted in the results, the majority of studies on interventions included manipulations of claim information. There are indications that information on health and environmental benefits (Brayden et al., 2018;Lemken et al., 2017;Warne et al., 2019;Weinrich & Elshiewy, 2019) positively influences consumer acceptance. Informational claims can be framed to make them more effective in increasing consumer acceptance, such as highlighting societal benefits (more than highlighting individual benefits; Verneau et al., 2016), nontechnical (versus technical) descriptions of production (Siegrist et al., 2018), and including specific (instead of general) information (Lombardi et al., 2019). The results also reveal that not all information was shown to be effective; some information even resulted in negative effects (Bryant & Dillard, 2019).
In accordance with the central role of familiarity in the acceptance of alternative proteins, the results on interventions indicate various effective routes to increase familiarity and thus promote consumer acceptance of alternative proteins: Introducing alternative proteins in existing and recognizable dishes and products and decreasing the visibility of the alternative proteins (especially regarding insects) can increase the acceptance of alternative proteins (Barton et al., 2020;Hartmann & Siegrist, 2016;Lensvelt & Steenbekkers, 2014;Pambo et al., 2017;Sogari et al., 2018). For example developing processed products that fit with known products and recipes (e.g., wraps in which seaweed or insects are processed).
The body of research on designing interventions had a focus on familiarity and information. Only a small amount of intervention studies included other aspects like for example social norm or affective interventions. This demonstrates that the body of research on interventions is not fully in line with the body of literature on the drivers of acceptance of sustainable protein and on consumer acceptance in general. The results on the affective and social aspects of drivers were promising (see our discussion above), and also other bodies of literature emphasise the importance of including aspects beyond information alone-aspects such as nudging-in interventions in order to potentially change behaviour (e.g., Marchiori, Adriaanse & De Ridder, 2017).

Future research lines
The manuscripts provides a comprehensive overview of the literature. This overview also results in research gaps and lines for future research. This section focusses on what we believe are the most prominent research gaps that set the agenda for future research.
Some sustainable protein sources have been studied more than others (e.g., insects versus algae, see Table 2). More research on a wide range of specific alternative proteins is necessary to gain better insight into the acceptance of consumers regarding a wide range of alternative protein products. Additionally, there are only a few studies that include multiple novel protein sources (e.g., Circus & Robinson, 2019;de Boer et al., 2013;Ianuzzi, 2019;Onwezen, van den Puttelaar, et al., 2019). The comparisons between various products of a specific alternative protein, or across categories of alternative proteins in a comparable way (e.g., a burger from various alternative proteins), are especially crucial to gaining a better understanding of why some specific products are accepted and others are not. Moreover, there is a need for future research to find ways to increase comparisons across studies, for example the use of standardised measures, and an overarching theoretical framework. We state it is relevant to develop standardised measurements and include a wide range of variables including multiple levels (e.g., personal, social environment and physical environment, Story, 2008). In this way knowledge in relative importance across drivers can be increased.
The studies included emphasised cognitive deliberation far more than affective and unconscious factors (e.g., for pulses, there were nine individual studies on food choice motives and none on affective variables; for algae, there were 16 and two respectively). At the same time, the research suggests that affective factors are especially relevant for novel products, which are perceived as more innovative by consumers (Onwezen, van den Puttelaar, et al., 2019). Thus, a recommendation for future research is to attend more to affective factors.
Although the amount of studies on social norms was relatively low, the results show the relevance of this driver in consumer acceptance of all alternative proteins. Consumers use all kinds of information to form opinions on alternative proteins, including social context. Studies on food choices in general (Higgs, 2015) and specifically regarding food innovations have already noted the significance of the social environment (Ronteltap, Van Trijp, Renes, & Frewer, 2007). The current review adds to this knowledge by illustrating that social information is also a highly relevant driver of acceptance for all alternative proteins (e.g., Onwezen, van den Puttelaar, et al., 2019). Future research is needed to further explore the potential of social norms in consumer acceptance of alternative proteins.
Although we focussed on Western countries, a few studies included a comparison between Western and non-Western countries (Bryant et al., 2019;Hartmann et al., 2015;Tan et al., 2015). These comparisons are especially interesting in the context of insects and seaweed, because these products already form a part of the traditional diet in a number of non-Western countries. The comparison can provide valuable insight into how insects and seaweed could become part of traditional diets elsewhere. Future studies should include more representative samples in more countries to identify and explain differences between various countries and cultures.
The amount of intervention studies was relatively small compared to the total amount of studies on consumer acceptance. Moreover, most interventions studies were not performed in real life settings whereas, testing consumer responses in real-life settings is also very important (Taufik, Verain, Bouwman, & Reinders, 2019). Finally, we noticed that the interventions included often-used informational claims, which is a common approach to influencing consumer food choices (Guthrie, Mancino, & Lin, 2015). However, there are many other factors aside from information that influence behaviour; therefore, information does not always determine behavioural change (Guthrie et al., 2015;Michie, Van Stralen, & West, 2011;Rothschild, 1999). We recommend future studies to include other interventions to increase consumer acceptance of alternative proteins for example affective factors and social norms, and also test interventions in real-life for real behavioural choices.

Limitations
A common limitation of a systematic review is that publication bias has likely inflated the reported results. Publication bias causes effective studies to be published more often than ineffective studies (Rothstein, Sutton, & Borenstein, 2005, pp. 1-7). Moreover, although we used a broad search string, some studies may be missing in our overview. Including other search engines and a reference check might complement the overview in future literature reviews.
The data did not allow us to conduct a meta-analysis due to the inconsistent way in which outcome measures were reported as well as the diversity in populations, settings, and reported results (e.g., lack of effect sizes and correlation coefficients). As a result, we cannot assess the relative effectiveness in targeting each of the different determinants in the acceptance of sustainable protein sources. Future research may benefit from the use of consistent outcome measures and the reporting of effect sizes to allow a better quantitative comparison between studies (also see Bryant & Barnett, 2018).
We used an existing framework on acceptance of novel food (Siegrist, 2008) to discuss our results. Possibly the framework which is developed for novel foods is less suitable for conventional alternative proteins. Although we worked from the raw table (see Appendix A and the supplementary materials) and fitted the results within the framework it is possible that a different framework would result in different findings. For example the results on trust are only found because the framework explicitly mentioned the relevance of trust. It therefore becomes clear that this factor is under researched in the literature on alternative proteins.
The focus of the review is on acceptance of alternative proteins, as actual behaviour is often not applicable in the case of novel proteins and is not possible in the case of future proteins. However, it should be considered that an increase in intentions or acceptance does not necessarily equate to an increase in behaviour as well (Sheeran & Webb, 2016).

Conclusion
The findings of the current review provide a comprehensive overview of the consumer acceptance of multiple alternative proteins. Although consumer acceptance varies greatly across alternative proteins, such that plant-based proteins are far more accepted than others like insects, the results also reveal consistencies in the drivers of consumer acceptance. The relevant drivers for consumer acceptance of all alternative proteins were food choice motives (especially healthiness and taste), familiarity, attitudes, food neophobia, disgust, and social norms. Besides these consistencies the review also shows the relevance of being specific and taking variations between individuals and alternative proteins into account. Future research is necessary with a focus on comparisons. Comparisons across multiple drivers, comparisons across multiple alternative proteins, comparisons across countries, and comparisons across consumer segments. Moreover, intervention studies have a focus on information and familiarity whereas other drivers like social norms and affect also show to be relevant. Thus, aside from focussing on providing information on benefits and motives, there is an urgent need to find ways to make consumers more familiar with various alternative proteins and to include affective messaging and social norm incentives in real-life contexts.
Authors ( Participants who had eaten or tasted seaweed in the past were more likely (compared to those who hadn't) to eat seaweed in the next 12 months. Participants who preferred red meat, pork or chicken over fish/seafood were significantly less likely to have ever eaten seaweed, consumed seaweed in the past 12 months, or to eat seaweed in the next 12 months. Gender and age had no significant effect on likelihood to consume seaweed in the next 12 months. Respondents with a university degree are 4 times more likely than those with lower education to eat seaweed in the next 12 months, and those who are familiar (vs. unfamiliar) with seaweed products are 7.6 times more likely to do so. Food neophobia was the most significant predictor of likelihood to eat seaweed in the next 12 months, with higher food neophobia scores leading to a lower likelihood to consume seaweed. Attaching more importance to the symbolic value of food consumption, higher health-consciousness, and more snacking behaviour each led to a higher likelihood of eating seaweed in the next 12 months. Responsibility with food and food safety concern did not significantly affect this likelihood.
Food neophobia (continued on next page) Authors ( (2), intention to try food containing processed insects (3), or to introduce it into own daily diet (4), intention to try food derived from animals fed with insects (5), or to introduce it into own daily diet (6) In general, participants' intention to eat insects and to introduce them into their daily diet is low. Intention to try and to include in one's daily diet were both generally higher for food containing processed insects compared to raw insects as food. However, intention to try or to include in one's daily diet food derived from animals fed with insects received the highest scores (compared to the other measures of intention). Food neophobia is more correlated with intention to try insects than with intention to introduce them into one's daily diet.
Food neophobia is a better predictor (compared to disgust sensitivity) of intention to try processed insects and both to try and introduce into daily diet animals fed with insects. However, disgust sensitivity appears to be a better predictor of intention to introduce raw insects into own daily diet.
19. Barton et al. participants exposed to negative peer ratings. Participants' expectations also significantly differed between the negative and neutral peer rating conditions. No significant difference was found between the neutral and positive peer rating conditions. Although peer ratings did not directly influence the overall ratings of the nutrition bar, they did affect participants' pretasting expectations, which in turn influenced the overall ratings. Study 2: Participants exposed to highscore expert reviews had higher expectations of the burger's quality than did participants exposed to the middle-score reviews, who in turn had higher expectations than did participants exposed to the low-score expert reviews. Consumer acceptance was higher for participants exposed to high-score reviews compared to those exposed to middle-and lowscore reviews (there was no significant difference between the latter two groups Insects that were marketed more in one's own culture were preferred more, and those marketed less in one's own culture were preferred less. General disgust and affective attitudes towards eating insects negatively affected willingness to eat specific insects. However, neophobia was not found to influence willingness to eat insects.
Degree to which insect products are marketed in one's own culture, disgust and affective attitudes towards eating insects 28. Gallen et al. The more participants perceived eating insects as a socially acceptable activity, the more they were willing to try the insect product. Food neophobia, on the other hand, had a significant and negative effect on the willingness to try the product.
In addition, women were less willing to try the product than men. Lastly, higher visibility of insects predicted lower willingness to try the insect product.
Perceived Before tasting, participants gave the appearance of the insect-based jelly a mildly positive score, and this score significantly increased after tasting the product. The score participants gave for expected sensory-liking of the insect jelly also increased significantly after tasting.
For males, there was a small but significant increase in taste scores after the actual tasting, whereas for females there was a small but significant decrease from pre-to post-tasting scores.
Although the taste of jelly with whole cricket received higher scores than that of jelly made with cricket flour, the latter was preferred over Mealworm products were expected to and experienced as tasting very different from mealworm-free products, and participants would generally like mealworm products to taste similar to mealworm-free products.
Even with high interest and good products, willing participants hesitate to regularly consume insectbased products due to practical and socio-cultural factors (such as appropriateness of food preparation). about environmental impact of animal production and meat consumption, participants' environmentally-friendly behaviours, meat consumption frequency, enjoyment of meat consumption to pay for different food choices with lower ecological impact (e.g., plant-based foods) ecological impact of animal production. Segments of participants can be distinguished based on awareness of and concern about the environmental impact of animal production and meat consumption. These segments show that awareness of and concern about the environmental impact of animal production and meat consumption sometimes increase willingness to reduce the ecological impact of one's diet The 'clean meat is natural'message failed to make participants consider clean meat more natural (compared to the control condition).
The 'conventional meat is unnatural'-message made participants consider conventional made less natural (compared to the control condition). It also led to significantly higher WTP for clean fish sticks, and marginally higher WTP for clean chicken nuggets. The effect of this message on WTP for clean beef burgers was non-significant but followed the same direction.
The 'challenging the appeal to nature'-argument (which debunked the perception that naturalness is important) failed to make participant attach less weight to naturalness.
Not specified (most likely a west-European country, perhaps Italy)

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Online survey Insects Food neophobia is a better predictor (compared to disgust sensitivity) of intention to try processed insects and animals fed with insects. However, disgust sensitivity appears to be a better predictor of intention to introduce raw insects into own daily diet. 19. Barton et al. (2020)  Online surveys Insects Self-reported potential reasons for including edible insects and insect-based products into one's diet, opinions on edible insects as an alternative food source, meat consumption frequency, gender 55. Sogari, Menozzi, and Mora (2019) Italy 88 Experiment Insects Gender, food neophobia, initial exposure to edible insect products, willingness to try the edible insects 56. Tan et al. (2015) Thailand and the Netherlands Ecological reasons for considering alternative protein sources or meat alternatives, sensory objections to protein-based foods or meat reduction

Ethical statement
The current manuscript involves a systematic review. We did not collect data by ourselves and therefore our research did not involve human participants, human material, or human data in any direct way. Subsequently a statement on ethics approval is not required.