Effects of sports intervention on aggression in children and adolescents: a systematic review and meta-analysis

Objective To explore the impact of sports on aggression in children and adolescents and analyze whether different conditions in the intervention, such as type of sports, or intervention duration, have different influences on the effect of interventions. Method The study protocol was registered in PROSPERO (CRD42022361024). We performed a systematic search of Pubmed, Web of Science, Cochrane library, Embase and Scopus databases from database inception to 12 October 2022 for all studies written in English. Studies were included if they met the following PICO criteria. All analyses were carried out using the Review Manager 5.3 Software. We summarized aggression, hostility and anger scores using SMDs. Summary estimates with 95% confidence intervals were pooled using DerSimonian-Laird random effects model or fixed effects model according to between-study heterogeneity. Results A total of 15 studies were deemed eligible for inclusion in this review. The overall mean effect size indicated that sport interventions was associated with lower aggression (SMD = −0.37, 95% CI [−0.69 to −0.06], P = 0.020; I2 = 88%). Subgroup analyses showed that non-contact sports were associated with lower aggression (SMD = −0.65, 95% CI [−1.17 to −0.13], P = 0.020; I2 = 92%) but high-contact sports were not (SMD = −0.15, 95% CI [−0.55 to 0.25], P = 0.470; I2 = 79%). In addition, when intervention duration <6 months, sport interventions was associated with lower aggression (SMD = −0.99, 95% CI [−1.73 to −0.26], P = 0.008; I2 = 90%) and when intervention duration ≥ 6 months, sport interventions was not associated with lower aggression (SMD = −0.08, 95% CI [−0.44 to −0.28], P = 0.660; I2 = 87%). Conclusion This review confirmed that sports intervention can reduce the aggression of children and adolescents. We suggested that schools can organize young people to participate in low-level, non-contact sports to reduce the occurrence of bullying, violence and other aggression-related adverse events. Additional studies are needed to determine which other variables are associated with aggression in children and adolescents, in order to develop a more detailed and comprehensive intervention programme to reduce their aggression.


INTRODUCTION
aggression. Kreager (2007) found that high-contact sports such as football and wrestling led to increased violence, while non-contact sports such as baseball and tennis did not. Zurita-Ortega et al. (2015) reported that the overt aggressiveness of teenagers who practiced sport regularly was higher than sedentary teenagers, because they began to compete with each other.
This systematic review aims to integrate the existing research on sports intervention and explore the impact of exercise on children and adolescents' aggression. According to existing research, analyze whether different conditions in the intervention, such as type of sports, intervention duration, have different influence on the effect of interventions.

METHODS
The study protocol was registered in PROSPERO (CRD42022361024).

Search strategy
We performed a systematic search of Pubmed, Web of Science, Cochrane library, Embase and Scopus databases from database inception to 12 October 2022 for all studies written in English. The search strategy was designed by Yahui Yang and Fengshu Zhu by an initial scoping review of the literature. Studies were identified by using all possible combinations of the following groups of search terms: (a) ''adolescent '' OR ''teens'' OR ''youth'' OR ''teenager'' OR ''juvenile'' OR ''young'' OR ''minor''; (b) ''physical training'' OR ''sport'' OR ''exercise'' OR ''athletics''; (c) ''intervention'' OR ''behaviour change '' OR ''prevention'' OR ''experiment'' OR ''program'' OR ''reduction'' OR ''evaluation'' OR ''strategy'' OR ''effect''; (d) ''aggression'' OR ''bullying'' OR ''violence'' OR ''assaultive behavior'' OR ''atrocity'' OR ''physical assault'' OR ''fighting''. The specific search was amended as necessary for each database to account for different search functionalities. The reference lists of retrieved articles and grey literature were searched to detect studies potentially eligible for inclusion.

Inclusion and exclusion criteria
Studies were included if they met the following PICO criteria: (1) included typically developing children and/or adolescents (Population); (2) examined different sports including school physical education programs (Intervention); (3) included anactive/inactive comparator (Comparison) and (4) examined associations with aggression (Outcomes). Studies were excluded if they focused on populations with develop-mental disorders (e.g., Down syndrome).

Study selection
Search results were imported into Endnote to remove duplicates. Yahui Yang and Hao Zhu screened the titles and abstracts of the retrieved articles independently to remove irrelevant articles. Then the same reviewers independently screened remaining articles in full to determine the final included studies. Disagreements were resolved by consensus or consultation with Fengshu Zhu.

Data extraction
One reviewer extracted specific characteristics from included studies, including country, study design, paticipants characteristics (age, gender), sample size, intervention programme characteristics (name, type, duration, frequency), comparison programme and outcome variables. Shachar et al. (2016) reported mean and standard deviation (SD) of baseline and change-from-baseline, the reviewer calculated the final mean and standard deviation according to Cochrane Handbook version 5.1.0 (Higgins & Green, 2011). Another reviewer confirmed the content.

Outcomes
The primary outcome was aggression scores. The secondary outcomes were other externalizing behaviors of aggression, including hostility, anger, delinquent acts (including crimes of varying severity levels such as gang fights and extortion, but also minor theft and nuisances), attitude towards violence (ATV) and provocation/bullying scores. If outcomes were reported for more than one time point, we extracted results closest to post-intervention (Fung & Lee, 2018). If two or more measurement tools were used, we referred to a previously described hierarchy of outcome measures (Fung & Lee, 2018). If physical aggression and verbal aggression scores were reported concurrently, we extracted the physical aggression scores (Trajković et al., 2020a;Trajković et al., 2020b;Shachar et al., 2016;Carraro, Gobbi & Moè, 2014;Reynes & Lorant, 2002).

Risk of bias assessment
Yahui Yang and Hao Zhu assessed risk of bias of randomised controlled trials (RCTs) using the Cochrane collaboration tool 2.0 (Sterne et al., 2019) and assessed risk of bias of quasiexperimental studies using the JBI Critical Appraisal Checklists for Quasi-Experimental Studies (The Joanna Briggs Institute, 2016) independently. Discrepancies were resolved by consensus or deliberation with Fengshu Zhu.

Data analysis
All analyses were carried out using the Review Manager 5.3 Software. We summarised aggression, hostility and anger scores using SMDs. Summary estimates with 95% confidence intervals were pooled using DerSimonian-Laird random effects model or fixed effects model according to between-study heterogeneity (DerSimonian & Laird, 1986). The heterogeneity was estimated using I 2 , considering I 2 values of <25%, 25-50, and >50% as small, medium, and large amounts of heterogeneity, respectively (Higgins & Thompson, 2002). Subgroup moderator analyses were conducted to determine whether results differed according to intervention duration and sport type. Sensitivity analyses were used to explore the impact of individual studies. A narrative synthesis of the results was carried out using descriptive statistics in order to summarize characteristics of the studies where data cannot be extracted (Ioannidis, Patsopoulos & Rothstein, 2008).
Six RCTs and nine quasi-experimental studies were all identified as ''moderate quality''. The assessment results are shown in Tables 2 and 3.

Harwood
Blomqvist (2020) compared effects of Mixed Martial Arts (MMA) intervention and Brazilian Jiu-Jitsu (BJJ) intervention on aggression. The results showed that there was no significant main effect of aggression as a result of training (P = 0.100). However, the interaction between aggression and sport was significant (P < 0.001). Whereas MMA practitioners slightly increased their levels of aggression, BJJ practitioners reduced theirs. Pels & Kleinert (2016) reported a significant reduction of aggressive feelings was found for participants exercising individually in the rowing condition compared with the individual combat exercise condition.

Impact of interventions on delinquent acts
Blomqvist (2020) indicated that both MMA and BJJ intervention groups reduced criminal behaviour moderately (P = 0.030). Fung & Lee (2018) found that Chinese martial arts group had light decrease in delinquent behavior than did the physical fitness training group, but there was no significant fixed effects of training were found in delinquent behavior (P = 0.760). Setty, Subramanya & Mahadevan (2017) and Hortiguela, Gutierrez-Garcia & Hernando-Garijo (2017) reported effects of sport intervention on adolescents' attitude towards violence. Setty, Subramanya & Mahadevan (2017) showed a significant change in both yoga and control groups in self-reported ATV, pre-and post-intervention ( p < 0.05). But the mean change in the yoga group is 39.59%, compared to 7.51% in the control group, indicating significant improvement. Hortiguela, Gutierrez-Garcia & Hernando-Garijo (2017) reported the results of two dimensions of ATV-unjustified violence and violence linked to self protection. The unjustified violence and the violence linked to self protection fell from high to medium in the judo and capoeira teaching units, significant difference with large effect sizes were found between the pre-test and the pos t -test in unjustified violence (P = 0.021) while there was no difference in the control group. Rosa et al. (2021) carried out judo and ball games among children and adolencents. A significant improvement in the domain of provocation/bullying was observed after the interventions, with judo increasing 18.1% and ball games increasing 4.1%. In other words, the participants felt safer and more confident about other people's negative attitudes.

DISCUSSION
This review evaluated the effectiveness of existing sports interventions to reduce aggression in children and adolescents. The overall results showed that sports intervention could reduce the aggression and hostility of children and adolescents and could not reduce the anger, while the evidence is indeterminate at the domain level for delinquent acts, attitude towards violence and provocation/bullying.
A strong relationship between sport and aggression has been reported in the literature. Pino-Juste, Portela-Pino & Soto-Carballo (2019) reported that the higher the index of physical activity is, the lower the level of aggressiveness is. A systematic review pointed out that physical education played an important role in the prevention of bullying (Jimenez-Barbero et al., 2020). Another review found that positive youth development Interventions with a physical activity component among pre-and early adolescents aged 8-14 years may lower bullying behaviors (Majed et al., 2022). Gråstén & Yli-Piipari (2019) indicated that violence among children and bullying reduced during the Physical Activity as Civil Skill Program according to teachers' written feedback. These are consistent with our results. The European Commission's White Paper on Sport (2007) pointed out that the social code implied in the sports include fair competition and team spirit, which can cultivate teenagers' social behavior patterns and reduce their aggressive behavior. Lorenz (1966) believed sport was a ritualized venting of aggression, which teaches people to consciously and responsibly control their fighting behavior. However, the results of sensitivity analysis indicated the lack of robustness of the meta-analysis. This may be because the small sample sizes of the studies included in the meta-analysis and different basic characteristics of the studies led to a large heterogeneity of the pooled results, which requires cautious interpretation.
Subgroup analyses showed that non-contact sports were associated with lower aggression while high-contact sports were not. Sofia & Cruz (2017) surveyed 141 athletes from different types of sport and found the same result: athletes from sports with higher levels of physical contact tended to be more aggressive than those from sports with lower levels of contact. This may be because self-control lies in the central role in the regulation of aggression in sport (Sofia & Cruz, 2015;Sofia & Cruz, 2016). High-contact sports mean strong competition and impulsivity. People who participate in non-contact sports could better control their aggressive behavior. In addition, the comparison between rowing and combat exercise also confirmed this opinion (Pels & Kleinert, 2016), the non-contact rowing can reduce aggression more than the high-contact combat.
Moreover, when the intervention duration ≥6 months, sport interventions was not associated with lower aggression. There is no study focusing on the influence of the duration of sport intervention on the effect of intervention currently. Stansfield (2017) confirmed that higher levels of participation in sports increased violence involvement. Méndez, Ruiz-Esteban & Ortega (2019) also indicated that students with high exercise frequency were more aggressive than those with low exercise frequency. Due to the large difference of intervention frequency among the included studies, we did not conduct subgroup analysis. According to the result, it could conceivably be hypothesised that whether the low level of sport involvement releases the aggressive impulse, and with the accumulation of exercise, the aggressive impulse rises again. As we all know, high level of exercise can improve muscle strength, and muscular strength may be an important predictor of aggression in bullying (Benítez-Sillero et al., 2021), this view also supported our hypothesis.
From the above mentioned, we have reason to believe that low level of non-contact sports involvement may be more conducive to the release of aggressive impulses, so then reducing aggressiveness of children and adolescents. Nevertheless, what kind of sport intervention frequency and duration can play the largest role in it needs further research.

LIMITATION
One limitation of this review was that the intervention programme of included studies were highly diversified with small sizes and varying assessment methods, which resulted in the high heterogeneity. Highly diverse sport intervention programs suggested that there might be other variables besides sport, such as the attention given to the participants, the approach of the coaches, the 'winning at all costs' philosophy of sport and cultures of different countries, that could also play a role in the results. Further studies are necessary to make clear which variables are actually factors contributing to aggression. Another limitation was that some of the included studies could not be meta-analysed due to the lack of standard control groups or the inability to extract data, so only descriptive statistics were made. Besides, the included studies were limited to peer-reviewed journals in English identified by the search strategy, potentially omitting other relevant studies.

CONCLUSION
This review confirmed that sports intervention can reduce the aggression of children and adolescents. We suggested that schools can organize young people to participate in low-level, non-contact sports to reduce the occurrence of bullying, violence and other aggression-related adverse events. Additional studies are needed to determine which other variables are associated with aggression in children and adolescents, in order to develop a more detailed and comprehensive intervention programme to reduce their aggression.