The foregoing studies involved two commonly examined genetic variants related to dopamine and serotonin regulation. These variants, associated with DRD4 and 5-HTTLPR, respectively, have been conceptualized as linked to general environmental sensitivity (see Belsky et al., 2007). When considering hypotheses about affiliations with peers and their potential impacts, a different biochemical process—oxytocin—and related genes may be more specifically relevant. Interacting with neurotransmitter systems, oxytocin is involved in the regulation of social behavior and social cognition (Donaldson & Young, 2008). Oxytocin blood levels and genetic variance related to oxytocin (i.e., the oxytocin receptor gene; OXTR) have been linked to a wide array of social behaviors and characteristics relevant to social behaviors in humans (e.g., Walum et al., 2012) and other mammals (e.g., MacLean & Hare, 2015). For example, social recognition is related to blood-level oxytocin (Feldman, Monakhov, Pratt, & Ebstein, 2015) and pair bonding to OXTR variation (Walum et al., 2012). Socially relevant characteristics including callus-unemotional traits (Cecil et al., 2014; Rice & Derish, 2015), orientations toward individualism vs. collectivism (Luo & Han, 2014), aggressiveness (Wu, Li, & Su, 2012), and
