There are several methods that have been used to study genetic influences on behavior, including family, adoption, and twin designs, each of which has its own strengths and weaknesses (Plomin et al., 2001). We focus here on twin designs, as this has been the “work horse” of behavior genetics. The relative frequency of twins, comprising about 3 in every 100 births (Hamilton et al., 2015), and the comparative ease of obtaining them through population-based (Kaprio et al., 2002) or records based (Anderson et al., 2002; Meyer et al., 1996) registries, has made this a ready design for studying genetic influences on behavior. The basic tenet of the twin design involves comparing the similarity of different types of twins who differ in their genetic relatedness. Monozygotic (MZ) twins result from a single egg fertilized by a single sperm and, accordingly, share 100% of their genetic variation and all of their shared environment when reared together. Dizygotic twins (DZs) result from two eggs, fertilized by two sperm, and therefore share, on average, just 50% of their genetic variation (as do ordinary siblings),
