Shared and non-shared influences on the development of attachment in twins

This thesis describes a study of the development of attachment in a sample of infant twins. The aim of the research was to investigate the causes of individual differences in attachment security from a behaviour-genetic perspective. Contemporary attachment research views the development of attachment as being mediated by working models of attachment. An important implication of the contemporary view is that attachment is caused entirely by shared environmental factors. By contrast, behavioural genetics research suggests that the majority of variability in behavioural development is caused by non-shared environmental factors. The current study aimed to test this shared environmental model in families of twins. The study consisted of a sample of 58 pairs of twins and their mothers. Assessments were carried out of maternal sensitivity at 9 months, parental security of attachment (Adult Attachment Interview) at 10 months and infant attachment security in Ainsworth's Strange Situation at 12 months of age. Consistent with the linear model of attachment twins were more likely to receive the same attachment classification than would be expected by chance. Furthermore, concordance for attachment for MZ and DZ twins suggested only environmental influences on attachment. Shared components of variance in maternal sensitivity were also associated with shared outcomes in attachment. In addition, parents who were classified as Secure-Autonomous in the AAI were more likely to be sensitive and responsive to both infants - consistent with the internal working models view. There was also strong evidence of non-shared environmental influences on attachment and these differences in outcome were related to differences in maternal sensitivity. Furthermore, significant differences were found between those families concordant for attachment and those who were not for a range of psychosocial factors. The findings are discussed in terms of the importance of the non-shared environment for future models of the development of attachment