The twin methodology can be applied to the study of virtually any behavior of interest, and probably has: a vast literature surrounds twin studies of psychopathology (Hewitt et al., 1997; McGue et al., 2006), personality (Littlefield et al., 2011; Viken et al., 2007), cognitive ability (Plomin and DeFries, 1998; Trzaskowski et al., 2013), as well as other behaviors that may seem more surprising, such as divorce (McGue and Lykken, 1992), voting behavior (Eaves et al., 1999; Hatemi et al., 2015), and well-being and life satisfaction (Archontaki et al., 2013; Sadler et al., 2011). This brings us to what has been called the first law of behavior genetics (Turkheimer, 2000): “all human behavioral traits are heritable”. A good rule of thumb is that if you have to guess to what extent something is genetically influenced, a good guess is that “it” is about 50% heritable, regardless of what the “it” is (Polderman et al., 2015).
