Life history factors, personality and the social clustering of sexual experience in adolescents

Adolescent sexual behaviour may show clustering in neighbourhoods, schools and friendship networks. This study aims to assess how experience with sexual intercourse clusters across the social world of adolescents and whether predictors implicated by life history theory or personality traits can account for its between-individual variation and social patterning. Using data on 2877 adolescents from the Avon Longitudinal Study of Parents and Children, we ran logistic multiple classification models to assess the clustering of sexual experience by approximately 17.5 years in schools, neighbourhoods and friendship networks. We examined how much clustering at particular levels could be accounted for by life history predictors and Big Five personality factors. Sexual experience exhibited substantial clustering in friendship networks, while clustering at the level of schools and neighbourhoods was minimal, suggesting a limited role for socio-ecological influences at those levels. While life history predictors did account for some variation in sexual experience, they did not explain clustering in friendship networks. Personality, especially extraversion, explained about a quarter of friends' similarity. After accounting for life history factors and personality, substantial unexplained similarity among friends remained, which may reflect a tendency to associate with similar individuals or the social transmission of behavioural norms.


Introduction
Adolescents in industrial societies like the United Kingdom vary substantially in their age at first sexual intercourse and other aspects of their sex lives [1]. Such behavioural variation between individual teenagers may cluster at higher levels of the social structure, i.e. members of particular social groups, such as friendship networks and neighbourhoods, may be more similar to each other than random members of In the study by Warner et al. [84], a model with just the normative climate variable implied a neighbourhood intraclass correlation of 7.7% [84]. The model lacks a school level, however, which makes it vulnerable to misattribution of variance. By contrast, another study from the USA found neighbourhoods to be relatively unimportant when both schools and neighbourhoods were included in a cross-classified model looking at whether someone had ever had sexual intercourse [83]. Based on the reported variances, we have calculated the intra-neighbourhood correlation as 2.9%.

This study
In this study, we use data from the Avon Longitudinal Study of Parents and Children to investigate how much of the variation in experience with sexual intercourse by 17.5 years is found at different levels of the social world-individual and family, neighbourhood, school, and friendship networksin a sample of British adolescents. Having revealed the pattern of clustering, we examine how much of it can be explained by similarity in a set of predictors believed to be important from a life history perspective as well as the Big Five personality factors.

Data and participants
The primary data source for this study was the Avon Longitudinal Study of Parents and Children (ALSPAC) [85,86]. ALSPAC is a large and ongoing birth cohort study centred on the city of Bristol (estimated population in 2015: 442 500) in the south-west of England. The original sample contained 14 451 pregnancies, with expected delivery dates between the 1 April 1991 and 31 December 1992, which resulted in 14 062 live births (13 988 children alive at 1 year of age). The study sample is broadly representative of the British population in terms of socioeconomic and medical markers although there is some overrepresentation of White, affluent Britons. Ethical approval for the study was obtained from the ALSPAC Ethics and Law Committee and the Local Research Ethics Committees. The ALSPAC website contains details of all the data that are available through a fully searchable data dictionary (http://www. bris.ac.uk/alspac/researchers/data-access/data-dictionary/).
Most of the data used in this study were collected through postal questionnaires sent to the study children's mothers every few months. Information on adolescent sexual behaviour was obtained in a computer session during their visit to the so-called Teen Focus 4 clinic (TF4; target age = 17.5). The Office for National Statistics produced the local deprivation data used below. ALSPAC has been matched to the UK government's National Pupil Database, which provided educational data (viz., an anonymized school identifier and educational achievement at the end of Key Stage 4).
Our initial study sample consisted of 4058 adolescents in the core ALSPAC sample with a valid outcome value (had sexual intercourse: yes/no). After excluding twins, individuals without a valid school or area identifier, and adolescents with missing values for more than 75% of predictors, we were left with an analysis sample of 2877 adolescents.
A comparison of this sample with the attrition sample-i.e. all individuals (singleton births) in the core ALSPAC sample alive at 1 year but not included in the analysis sample, revealed a skew of the analysis sample toward higher SES (see electronic supplementary material, S1). Adolescents in the analysis sample also tended to live in less deprived areas: only 2.8% of the study adolescents lived in the 10% most deprived areas in England.

Dependent variables
At Teen Focus 4 (target age = 17.5), participants were asked a number of questions about their sexual history, including whether they had ever had sexual intercourse with another young person. They were not asked directly about their age at first sex.
For this age group, sexual activity can be considered normative behaviour in the population under consideration (British teenagers). According to figures from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), the median age at first heterosexual intercourse in England, Scotland and Wales for the age group 16-24 years is 16 (IQR: [15][16][17][18] for both males and females [1]. Pubertal development, age and sex. A binary measure of pubertal development, based on information from a questionnaire administered when the study adolescents were about 13 years old, was included as earlier puberty has been found to predict an earlier sexual debut [93]. If a girl had started her menstrual periods or a boy's voice had changed, this was coded as 1 (versus 0 = no periods yet/no change in voice). Age at Teen Focus 4 was a covariate in all models. Finally, sex of the respondent was included to account for possible differences between girls and boys in sexual behavioural development.

Social structure
Friendship networks. Friendship network assignments were based on a questionnaire called You and Your Friends, which ALSPAC participants were sent in 2008, when they were 15-17 years old, and which asked them to list up to five friends. This questionnaire was returned by 3123 participants who nominated 14 503 friends (11 041 unique individuals) [57]. Nominated friends' names were used to link them to ALSPAC; restricting the sample to ALSPAC participants only leaves 2396 respondents who listed 6961 friends (4572 unique individuals). We assigned individuals in our study sample to friendship networks based on an undirected edge list. An undirected edge list was used because, firstly, many ALSPACparticipating nominees did not complete the Friends questionnaire, making reciprocity impossible to ascertain, and, secondly, this maximized the number of ties included in the analyses. Two kinds of links between individuals were used: direct nomination links (A nominated B, vice versa, or both) and links though a third individual (A and B both have direct link to C). This procedure left 1115 individuals in 411 friendship networks with 2 or more members (rather than 446 individuals in 223 friendship networks if only reciprocated ties had been used). While some individuals belong to multiple friendship networks, we used only one, randomly chosen friendship network per respondent in the analyses that follow.
Schools. ALSPAC's anonymized secondary school identifier at Key Stage 4 (missing for approximately 18% of the core sample), when pupils are approximately 15 years old, acts as the school classification. Adolescents in the analysis sample attended 79 schools with a mean of 36.4 respondents per school (range: . Neighbourhoods. The aforementioned lower super output areas functioned as the neighbourhood classification. The LSOA data indicate where respondents were living on 1 January 2008, when they were, on average, about 17 years old. The respondents in the study sample resided in 563 areas, with a mean of 5.1 respondents per neighbourhood (range: [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Some residential mobility was evident in the sample: between 1 January 2001 and 1 January 2005, 457 of the 2,839 (16.1%) respondents for which the residential LSOA is known at both time points moved to a different LSOA in the Bristol area; between 1 January 2005 and 1 January 2008, 216 out of the 2868 (7.5%) respondents moved to a different LSOA in the Bristol area.

Modelling approach
We ran logistic multiple classification models [94] with individuals nested in schools, areas and friendship networks [95] to investigate the social clustering of sexual experience and its association with life history predictors.
First, we ran a set of models without predictors (bar age and sex) to assess the amount of variance located at different levels of the social structure, each model using a different combination of the three social structure classifications, from individual only to all classifications (making eight clustering-only models in total). While the full clustering-only model is of most interest, running the full set of social structure models may suggest ways in which models with a simpler structure lead to the misattribution of variation. For example, as friends often attend the same school, studies without information on friends, may erroneously attribute variation at the level of friendship networks to schools.
Next, we ran eight substantive models with different blocks of life history predictors: an SES model (maternal and paternal education, financial difficulties and home ownership status); a parental life history pace model (maternal ages at menarche and first birth, a binary variable indicating whether the adolescent's mother had had sexual intercourse before her 16th birthday, and lastly paternal age at index pregnancy); a parental investment model (female and male parenting scores and father absence before age 10); a neighbourhood deprivation model (IMD 2010); a personality model (Big Five personality factors); a pubertal development (pubertal timing measure); a model with all of the foregoing predictors; and, finally, a modified version of the full model which also included the participant's educational achievement. We did not include educational achievement in earlier models because it is arguably itself a life history trait and, moreover, strongly related to parental SES. Note that if adding a predictor leads to a reduction of the share of the total variance at a particular level of the social structure, it suggests that part of the similarity of members of the same group at that level is actually due to similarity of the added predictor [95].
The model estimates of the classification variance parameters from the logistic regressions were used to calculate residual intraclass correlation coefficients (ICCs) with a latent variable approach [96]. The ICC is a measure of the similarity of members of the same group, viz., the expected correlation in the outcome of interest between two randomly selected members of the same group. All analyses used Markov Chain Monte Carlo (MCMC) estimation methods, as implemented in MLwiN [97]. Statistical modelling was performed in MLwiN 2.30 [98] run from within Stata 12 [99] using the command runmlwin [100].
Models can be compared on the basis of the deviance information criterion (DIC), a measure that combines model fit and complexity [101]. Some rules of thumb for the interpretation of information criteria are as follows: when the difference in DIC between a particular model and the best model in the candidate set (lowest DIC) is less than or equal to 2, the model in question has 'substantial support', a model whose DIC is between 4 and 7 points higher than the best has 'considerably less support', and a model with a DIC that is 10 or more points higher has 'essentially no support' [102].

Missing data
The proportion of missing data varied between 19.4% for conscientiousness and 0% for sexual experience by approximately 17.5, sex, age and neighbourhood deprivation. For participants, the mean number of missing values in the final sample was 2.05, with a maximum of 14, although more than 80% of participants in the final sample had fewer than 5 missing values. We used multiple imputation, conducted in Stata 12, to avoid well-known problems associated with complete-case analysis (biased parameter estimates and loss of power) [103]. Prior to imputation, we standardized all age variables, neighbourhood deprivation, parenting scores and personality trait scores, and log-transformed neighbourhood deprivation, cubed the mother's parenting score, and squared the father's parenting score in order to approximate normal distributions. All analyses were performed on 20 imputed datasets. We used 'mi estimate' command in Stata 12 which calculates parameter estimates for each imputed dataset individually and then combines them according to Rubin's rules [104].
Data were more likely to be missing if collected later during the study. Family SES was negatively correlated with the proportion of missing data. For example, the mean number of missing values was 3.5 if mothers were in the lowest education category, which decreased with increasing educational level to 1.3 for the highest education category.

Sample description
Descriptive statistics for the study variables are given in table 1. Their pairwise correlations can be found in electronic supplementary material, S2.

Clustering
The first set of models explored the clustering of sexual experience across the social structure, without including substantive predictors (apart from the covariates age and sex). Note that we compare clustering for all combinations of social classifications (one combination per model). These clustering-only models were therefore not performed in any particular order and do not involve model selection. Table 2 lists the clustering models in order of goodness of fit, starting with the best-fitting model according to the DIC. The school-only model (DIC = 3643.90), neighbourhood-only model (DIC = 3664.82) and friendship-network-only model (DIC = 3610.51) all fitted the data better than the individual-only model (DIC = 3672.43). However, inclusion of the neighbourhood classification alongside either the school or the friendship-network classification (or both) does not improve the model fit compared with the same models without neighbourhoods, which suggests that the latter were 'borrowing' variance from schools and/or friendship networks in the neighbourhood-only model. Inclusion of the friendship network had the largest impact on model fit. Models without both the friendship network and the school classification received essentially no support ( DIC ≥ 10).  classification, however, received substantial support ( DIC ≤ 2), suggesting that the neighbourhood classification could be left out of the model without much of an impact on model fit. Table 3 provides model estimates of the (residual) variance associated with the different classifications, for the single-classification models and the combined model, and expresses these as ICCs. Even in the single-classification models without substantive predictors (apart from respondent age and sex), the residual intraclass correlation at the level of schools (0.029) and neighbourhoods (0.033) was very modest. In the combined model containing all social classifications, a residual ICC of 0.023 was found for schools while only 0.010 remained for neighbourhoods. A far larger residual ICC was found for friendship networks: 0.25 in the combined model.

Unadjusted models
The results for the unadjusted SES, parental life history pace, parental investment, neighbourhood deprivation, personality and pubertal development models are given in  Household SES was negatively associated with the probability of a young person having had sex. Young people were less likely to be sexually experienced if their parents were more educated (OR = 0.50, p = 0.003 for maternal degree versus no educational qualification or CSE; OR = 0.59, p = 0.01 for paternal degree versus no educational qualification or CSE) or if living in owned rather than rented accommodation (OR = 0.55, p = 0.04). Mother-reported financial difficulties did not independently predict whether adolescents had had sex. The SES model explained 7.6% of the variance, but the residual intraclass correlation for friendship networks was hardly affected by the addition of indicators of household SES (rICC = 0.233 versus 0.250 for the full clustering-only model).
Two parental life history variables were significant in the unadjusted parental life history pace model. Adolescents whose mother was older at first pregnancy were less likely to have had sex (OR = 0.80, p < 0.001). Those whose mother had an early sexual debut (less than 16) were more likely to have had sex (OR = 1.72, p = 0.001). Maternal age at menarche and paternal age at index pregnancy did not add predictive power to the model. The parental life history pace model explained 6.9% of the variance; the residual intraclass correlation for friendship networks was somewhat reduced (rICC = 0.216).
Parenting scores were not significantly associated with sexual experience in late adolescence, but adolescents who grew up in a father-absent household were more likely to have had sexual intercourse (OR = 1.87, p < 0.001). Higher levels of neighbourhood deprivation were associated with an increased probability of my measure of sexual experience (OR = 1.14, p = 0.02). The neighbourhood deprivation model explained 4.6% of the variance; neighbourhood deprivation did not lead to substantial reduction of residual clustering at the friendship-network level (rICC = 0.228).
Three of the Big Five personality factors were significant predictors of whether an adolescent had had sex by 17.5 years of age. Adolescents who score higher on extraversion (OR = 2.19, p < 0.001), lower on conscientiousness (OR = 0.83, p = 0.004) and lower on emotional stability (OR = 0.80, p = 0.001) were more likely to have had sex. The unadjusted personality model, with age and sex, explained 15.4% of the variance. Out of all the (sets of) predictors, only personality traits appeared able to account for part of the clustering at the level of friendship networks. Adding personality reduced the residual intraclass correlation at the level of friendship networks from 0.250 to 0.149, a finding that suggests that some of the similarity between friends in sexual activity may be due to similarity in personality traits.

Full model
In the full, adjusted model (table 5;
. Compared with the multiple classification model without substantive predictors (apart from age and sex), the residual ICC of schools decreased from 0.023 to 0.014, that of neighbourhoods from 0.010 to 0.014, and that of friendship networks by nearly roughly a quarter, from 0.250 to 0.188. The proportion of explained variance for the full model, without respondent's educational achievement, was 0.209.

Full model + adolescent educational achievement
Because, like sexual experience, adolescents' educational achievement possibly reflects their life history pace ( §1.1.2), it may represent a correlated outcome rather than a (potential) causal factor. Moreover, adolescent education could be on a causal pathway between parental education and sexual behaviour. For the purpose of clarity, we therefore ran the full model both without (results above) and with adolescent educational achievement. The higher the number of A or A * results a respondent achieved, the lower was the probability of him or her having had sex by 17.5 years of age (table 5). Adding educational achievement increased the explained variance, as a proportion of total variance, from 0.209 to 0.236. Adding the respondent's educational achievement to the full model markedly reduced the estimates of the effects of maternal and paternal education, neither of which retained statistical significance. The proportion of explained variance for an education-only model (with age and sex; electronic supplementary material, table S3) was 0.104.

Main findings
In a sample of contemporary British adolescents, experience of sexual behaviour by 17.5 years clustered at the level of friendship networks but not in schools or neighbourhoods. This clustering among friends was not due, for the most part, to clustering of life history factors that predict sexual behaviour. Friendship formation based on similarities in personality traits did appear to explain about a quarter of the clustering of experience of sexual activity among friends. Life history predictors such as SES and father absence before the age of 10 did account for some of the variation found in adolescent experience of sexual behaviour but no evidence was found for effects of parental care or neighbourhood deprivation, counter to predictions of some life history models of adolescent development.
Friends are far more similar than non-friend age peers going to the same school or living in the same neighbourhood. This suggests that differences between schools and neighbourhoods, in this sample, have little impact on adolescent experience of sexual activity by age 17.5. Nor do schools and neighbourhoods appear to possess differing normative climates resulting in different levels of       sexual activity. These results are consistent with a multiplier effect for individual-level interventions as behaviours may cascade through friendship networks [78], although this would require that similarity of friends is at least partly due to social influence, something we were unable to test directly.
In contrast with expectations based on life history theory, neighbourhood deprivation did not predict experience of sexual activity, consistent with results of previous multisystem studies of risky adolescent sexual behaviour [62][63][64], which did not find measures of perceived neighbourhood quality or exposure to violence to show the associations predicted by life history theory. On the other hand, other studies (e.g. [2,40,41]) did find that ecological context at the neighbourhood level-socioeconomic deprivation and life expectancy-predicted age at first birth. These differences might be explained by the fact that the studies mentioned used age at first birth as an outcome rather than experience with sexual intercourse. Moreover, the neighbourhoods in Wilson & Daly's study [40] showed an extraordinary range in homicide levels and (homicide-adjusted) life expectancy, which might result in more pronounced life history effects. It should be noted, however, that Nettle [2] did not control for household SES [2]; and the study by Wilson & Daly [40] was performed entirely at the neighbourhood level. A large study from Northern Ireland that did look at both individual and neighbourhood levels found individual SES to be by far the largest predictor of age at first birth, but neighbourhood characteristics to have small additional effects [41].
The fact that we found no school-level clustering to speak of is possibly due to our definition of the school classification, which was simply based on a school identifier. A school class classification might have revealed clustering consistent with school-peer influences (cf. [78,75,82]), but note that class composition differs by subject in UK secondary schools, making school class less meaningful than in primary schools.
Personality was a strong predictor of experience of sexual intercourse, in line with existing studies on sexual behaviour [5,[53][54][55] and reproductive success [56,105]. Our results suggest that a sizeable part, around a quarter in the sample, of the similarity of sexual behaviour among friends is due to clustering of personality traits in friendship networks. Thus, studies that do not take similarity in personality into account may be overestimating the effect of (descriptive) peer norms on adolescent sexual behaviour.
Our finding that extraverted individuals are more likely to have had sex is consistent with previous findings on extraversion and risky sexual behaviour [5,[53][54][55], and fits with Nettle's suggestion that extraversion evolved as a life history strategy premised on high mating success [5,55], characterized by an early sexual debut, unstable pair-bonds and a high number of sexual partners. The fact that lower conscientiousness predicted a higher probability of having had sex is also in line with previous studies [53,106], and the general short-term orientation of low-conscientiousness individuals [107]. Likewise, the negative relationship between emotional stability and sexual intercourse is in line with the existing empirical literature [53]. Unlike several previous studies (reviewed in [53]), which found a negative association between agreeableness and risky sexual behaviour, we did not find an effect of agreeableness on the probability of having had sex. This difference may be due to the nature of our outcome measure, which did not measure risky sexual behaviour. Moreover, agreeableness is a desirable attribute in a potential partner which, by increasing opportunities for entering romantic relationships, While we cannot test for it directly, our results indicate more scope for social transmission of sexual norms and attitudes between members of a friendship network, than between (non-friend) peers in school or neighbourhoods. And while our results are consistent with the notion of peer groups as key socialization sources [108], they cannot be used to infer a relatively minor role of parents-assocializers [108], as we did not test the latter hypothesis. Apart from social influence, assortment on the outcome, or similarity in unmeasured predictors thereof, could also account for clustering. The negative association of sexual experience with educational achievement suggests this may in part reflect tradeoffs made by adolescents in how they invest efforts between education and social and sexual behaviour. Friends (or their parents) may be influential in making such decisions [109] or indeed help each other achieve social or educational goals.

Strengths and limitations
This study has a number of notable strengths. Most importantly, it includes friendship networks, neighbourhoods and schools in a multiple classification modelling framework, allowing for simultaneous estimation of social clustering at each level while taking the others into account. The inclusion of friendship networks as an additional classification in a multiple classification multilevel model [95], alongside schools and neighbourhoods, is a novel feature. We were also able to include a wide range of theoretically interesting predictors, such as neighbourhood deprivation and personality, because of ALSPAC's comprehensiveness. The use of ALSPAC also ensured the availability of a relatively large sample.
Several limitations should be kept in mind when interpreting the results reported here. As noted earlier, our analysis sample is skewed toward families of higher SES in less deprived neighbourhoods (see electronic supplementary material, S1). It is possible that the effects of life history predictors largely or exclusively manifest themselves at their extremes. Perhaps behavioural life history traits may be found to cluster much less in friendship networks, after accounting for the effects of life history predictors, in more extreme (e.g. very harsh) settings, where environmental conditions might predominate in line with life history predictions. Future studies could address this sort of question by considering whether social clustering patterns vary across contexts.
If closer friends, who are more likely to reciprocate friendship nominations, are more influential [110], then the inclusion of non-reciprocated friendship nominations would bias (downwards) our estimate of friend similarity. When a study based on data from Add Health explicitly addressed this question in an investigation of peer influence sexual activity, it found no evidence for an interaction between the sexual activity of the closest friend and whether or not the friend nomination was reciprocated [77]. We also included individuals in the same friendship network if they share a friend, which might dilute clustering.

Conclusion
In a sample of British adolescents, experience with sexual intercourse clustered in adolescent friendship networks, but not in neighbourhoods or schools. Thus, the clustering we found in friendship networks was not due to friends experiencing similar socio-ecological circumstances at the level of neighbourhoods or schools. While life history predictors did explain some of the variation in sexual activity, they did not explain much social clustering, strongly suggesting that similarity of life history predictors among friends is not responsible for clustering of sexual experience in friendship networks. Instead, the social clustering of sexual behaviour among friends could be due to a tendency to associate with similar others, perhaps because of potential coordination benefits from similarity, or due to social transmission of norms of behaviour among friends, possibly reflecting conformism or prestige bias.
Ethics. Ethical approval for the Avon Longitudinal Study of Parents and Children was obtained from the ALSPAC Ethics and Law Committee and the Local Research Ethics Committees.
Data accessibility. Data used for this submission will be made available on request to the Executive (alspac-exec@bristol.ac.uk). The ALSPAC data management plan (available here: http://www.bristol.ac.uk/alspac/ researchers/data-access/documents/alspac-data-management-plan.pdf) describes in detail the policy regarding data sharing, which is through a system of managed open access.