When investigating the genetic influences on friends' behaviour it is possible that MZ twins share more friends than DZ twins. This would represent a violation of the ‘equal environments assumption’, which assumes that MZ and DZ twins are correlated equally in their exposure to environmental factors of importance for the trait under study and would result in an inflated heritability estimate for friends' alcohol use [40]. In addition, the extent to which friends' alcohol use is correlated with adolescent own alcohol use could lead to a greater resemblance between MZ twins and an inflated heritability estimate for this variable. Based on the significant correlation between increased sharing of best friends and peer alcohol use (r = 0.17, P < 0.0001) and increased sharing of best friends in MZ twins (Mann–Whitney U = 36804.5, P > 0.0001), we controlled for sharing of best friends when conducting the genetic analyses by allowing the threshold to vary as a function of the number of shared friends [41]. The threshold is modelled as a simple linear function: ti = t + shared best friendsi
