Читать книгу The Wiley-Blackwell Handbook of Childhood Social Development - Группа авторов - Страница 51

Twin studies use comparisons on a particular trait between monozygotic (MZ, identical) and dizygotic (DZ, fraternal) twins. Because MZ twins are 100% genetically similar, whereas DZ twins share on average only 50% of their segregated DNA (the part of DNA that varies between people), the twin method allows for estimation of heritability – the proportion of individual differences of a particular trait in a specific population explained by genetic differences. Heritability is estimated as double the difference between MZ and DZ twin correlations for a particular trait. By comparing MZ and DZ twins, we can also estimate the effects of environmental influences (as explained in the section “The Role of Environments Factors in Social Development”).

Twin studies have been established in many countries across the world, and some of these studies have been following twins for many years, even decades. These data allow for longitudinal investigations of genetic and environmental effects. For example, in the United Kingdom, the Twins Early Development Study (TEDS), is one of the most impressive examples of a developmental twin study – a representative longitudinal study of more than 10,000 twin pairs followed for more than 20 years, from birth to date. The main aim of this project is the study of different aspects of psychological development, such as cognitive abilities, behavior, and learning abilities and disabilities (Haworth et al., 2013; Rimfeld et al., 2019). Other large‐scale twin studies also exist in the United States (the Colorado Twin Registry), Canada (the Quebec Newborn Twin Study), Denmark (the Danish Twin Registry), Netherlands (the Young Netherlands Twin Register), South Korea (the South Korean Twin Registry), and Russia (the Russian School Twin Register). For more information, see the online resource: Meta‐Analysis of twin correlations and heritability (http://match.ctglab.nl/#/home).

Twin studies of antisocial behavior have shown that genetic factors account for approximately 45% of the variance in this trait in childhood and adolescence (Burt et al., 2007). Antisocial behavior is a heterogeneous phenotype; some authors differentiate an aggressive disorder that manifests itself in early periods (childhood) and a delinquent disorder that manifests itself in later periods (adolescence) (Moffitt, 1993). Ratings of aggression correlate with a diagnosis of oppositional disorder, whereas scores on the delinquent behavior scale correlate with a diagnosis of conduct disorder (Hudziak et al., 2004). It is possible that the two types of antisocial behavior disorders have different etiologies. Genetic factors account for up to 75% of variance in aggression, and these genetic influences are shown to be stable across ages (Bartels et al., 2004; Burt & Neiderhiser, 2009; Van Beijsterveldt et al., 2003).

An analysis of a large, multinational dataset (42,468 twin pairs from five European twin cohorts) found a high level of overall heritability of aggression in children (~60%), with heritability estimates of ~64% in boys and ~58% in girls. Interestingly, the study also reported sibling interaction effects in the opposite‐sex twin pairs: an aggressive female had a positive effect on male co‐twin aggression, whereas more aggression in males had a negative influence on a female co‐twin (Luningham et al., 2020).

Delinquent behavior has been found to be less heritable (30–40%), with less stability in genetic influences across ages (Bartels et al., 2004; Burt & Neiderhiser, 2009; Eley et al., 2003). However, different types of conduct disorder may themselves have different etiologies. For example, conduct disorder, which is comorbid with ADHD, has been found to be more heritable than other forms of conduct disorder (Thapar et al., 2001).

In relation to the etiology of bullying, which can also be considered as a subset of antisocial or aggressive behavior, twin studies demonstrated that 60–70% of the variance in bullying behavior can be influenced by genetic factors, with the remainder due to nonshared environmental effects (Ball et al., 2008; Veldkamp et al., 2019). Similar high genetic influences were found for bullying victimization. These findings suggest that if siblings from one family bully other children or are bullied by other children, it is more likely to be due to shared genetic inheritance than the family environment (such as parenting style).