As with most personality traits, aggression has been demonstrated to be partly heritable. The contested question, however, is how heritable? On this front, studies have been largely inconsistent, with some finding close to zero environmental influence (taking into account both shared and non-shared environments).
Now, a new study, recently published in the journal PLOS One, provides more evidence that shared (familial) environments do matter after all.
To be sure, heritability was still found to play a key role in mean behaviour, accounting for approximately 68% of the variation in trait aggressiveness, but it was not the only relevant factor in children’s development.
Both pro- and re-active forms of aggression were found to correlate with each other, supporting the hypothesis that both forms are related to overt (physical) aggression, shown to exert strong genetic effects, especially in childhood.
The study was conducted on 223 sets of monozygotic twins (who share all of their genes) and 332 sets of dizygotic siblings. Children’s aggressive behaviour was assessed and reported by teachers at ages 6, 7, 9, 10, and 12.
Researchers then analysed the data on both types of aggression using a multivariate latent growth curve model to chart the changes.
Results showed that aggression in early life, up to 6 years of age, is mostly due to genetic factors, whereas aggressiveness evident after that have more to do with parental behaviour – neglecting or encouraging children to be aggressive may be partly to blame.
Authors of the study admitted their sample was not large enough to account for sex differences. Furthermore, teachers were the only people reporting on children’s behaviour, which, some researchers have suggested, can have an impact on the outcomes of the study.
Regardless of the limitations, however, the authors are enthusiastic about the usefulness of their work.
“Our findings also corroborate those of other studies, demonstrating that programs designed to prevent reactive aggression should focus on reducing experiences of victimisation, whereas those meant to counter proactive aggression should be based on the development of pro-social values,” said lead author Stephane Paquin.