In addition to molecular genetic methods, family‐based studies can also be implemented to study genetic influences on complex psychological traits. These studies are based on comparisons of family members with a different degree of genetic relatedness. Two types of family‐based studies widely used in behavioral genetic research are twin studies and adoption studies.
Twin studies
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).
Adoption studies
Another family‐based approach that allows estimation of the effects of genetic and environmental influences on individual differences in a trait is the adoption approach. In adoption studies, phenotypic similarities (similarities on a specific trait) can be compared between adopted children and their biological and adoptive parents; and between siblings from biological families and siblings from adoptive families. Adoption studies are also a powerful tool to study interactions between genetic and environmental factors.
In line with twin studies, adoption studies have demonstrated substantial effects of genetic factors on the development of social behavior. Studies of externalizing problems in adopted children of age 10–15 years, and their biological and nonbiological siblings, showed that externalizing behavior is mostly explained by genetic factors (65%). Similarly to the results of twin studies, heritability of aggressive behavior (70%) was higher than heritability of delinquent behavior (39%) (Van den Oord et al., 1994). However, in the Colorado Adoption Project, which also compared biological and nonbiological siblings, heritability of aggression (24–49%) and delinquent behavior (17–36%), assessed by parental and teachers’ reports, was lower (Deater‐Deckard & Plomin, 1999). Another example of an adoption study is the Early Growth and Development Study (EGDS), a US nationwide, prospective study of birth parents and adoptive families (https://egds.la.psu.edu/). A study that used EGDS data from 346 linked triads (birth mother, adoptive parents, adopted child) in order to assess externalizing behavioral problems, such as aggression and defiance, when children were 18, 27, and 54 months of age (Kerr et al., 2013). The results of this study indicated that biological mothers’ histories of major depression and adult antisocial behavior were found to confer risk for child externalizing behaviors at age 18 months, consistent with the role of genetic influences on child behavioral problems.
With trends toward selective adoption and the diminishing frequency of adoptions in many developed countries, including the United Kingdom and the United States, adoption studies are becoming a less feasible method of genetically informative investigations. However, the increased rate of reconstituted families (families composed of both siblings and half siblings) may offer a new way to evaluate the role of genetic factors in the transmission of complex disorders (Risch et al., 2014).
To summarize, there is growing evidence, from both molecular genetic studies and family‐based (twin and adoption) studies, for the complex polygenic architecture of sociobehavioral phenotypes, such as antisocial behavior. Future studies are needed to uncover the role of specific genetic variants across the whole genome, and their complex interplay with various environmental factors.
The Role of Environments Factors in Social Development
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