BACKGROUND: Are individual differences in attachment security inborn or shaped by the social environment? In infancy and early childhood, the evidence points to a substantial role of the environment, but a large twin study in early adolescence showed considerable heritability. Here we examined the twin heritability of attachment in middle childhood. We hypothesized that in middle childhood some heritability would emerge. Furthermore, we expected a role for cognitive and language abilities in explaining variance in attachment in middle childhood, partly related to the measurement of attachment, and we therefore examined associations with IQ.
METHODS: This pre-registered study included 415 same-sex twin pairs (52% girls, 58% monozygotic) between 8 and 11 years old (M = 9.59, SD = 0.79). Participants were recruited from an experimental cohort-sequential study including two age-overlapping longitudinal cohorts. Secure Base Script Knowledge was assessed with the Middle Childhood Attachment Script Assessment . Zygosity of the twins was determined using DNA samples. In the younger cohort, cognitive development was assessed with the Dutch version of the Wechsler Preschool and Primary Scale of Intelligence . In the older cohort, the Dutch version of the Wechsler Intelligence Scale for Children was used .
RESULTS: Significant additive heritability (38%) was found in the absence of a common environment component. This result diverges from findings in infancy and early childhood and aligns with the results in early adolescence.
CONCLUSIONS: The gene-environment correlation hypothesis suggesting that older children more actively shape their experiences in social contexts may offer a plausible explanation for the heritability of attachment in middle childhood. In middle childhood this mechanism might tip the balance toward a larger role for additive genetics. Larger longitudinal twin studies are needed to replicate the heritability of attachment after preschool age.