To estimate heritability, i.e. the relative contribution of genetic and environmental sources to individual differences in choice behavior, we performed a biometrical genetic analysis using linear structural equation model (SEM) fitting, a standard approach in twin genetic research (Neale and Cardon 1992; Rijsdijk and Sham 2002). SEM models were fitted using the Mx package, which was specifically developed to model genetically informative data (Neale et al. 2002). These models assume that phenotypic variance arises from the following factors: additive genetic influences (A), non-additive genetic influences (D), environmental influences shared by family members (C), and individually unique (unshared) environmental influences (E). It was not possible to test a model including both D and C with our data, thus we tested ADE and ACE models separately. It is important to note that A, D, and C increase intrapair twin similarity whereas E decreases it.
