Like cross-cultural studies, comparisons of concordance rates between mono-zygotic and dizygotic twin pairs can be helpful in understanding the extent to which genetic factors determine certain behaviours such as security of attachment. 

The research considered in this chapter points to a strong environmental influence, e.g. maternal sensitivity and responsiveness is a key determinant of attachment security. However, consider the results of the following MZ/DZ twin study by Pasco Fearon and colleagues (2014). They measured attachment security using semi-structured interviews with 551 pairs of fifteen year old twins. The concordance rate for MZ twins was 0.42. DZ concordance was significantly lower at 0.2. 

• What do the results of Fearon et al. (2014) tell us about the relative roles of nature and nurture in shaping attachment behaviour as children get older?
• Why should caution be exercised in drawing this conclusion?
• Think about how attachment security is measured in infancy and how it is measured in adolescence and adulthood.