Cooperation and physical similarity in the social interactions of virtual twins, best friends, and full siblings

This study examined cooperation and physical similarity in the social interactions of virtual twins (VTs), best friends (BFs), and full siblings (FSs). VTs are biologically unrelated siblings of the same age who have been raised together since early infancy (Segal & Hershberger, 2005). Such individuals share similar experiences in the home environment, which is a part of the "nuture" component of the "nature-nurture" equation for understanding individual differences in behavior. Videotaped interactions of participants in this study came from a comprehensive study of twins, adoptees, peers, and siblings (TAPS). Pairs were rated on a six-dimensional measure of cooperation that was previously used in a study of monozygotic (MZ) and dizygotic (DZ) twin pairs (Segal, 2002). VT pair interactions in this study received lower ratings of cooperation than BF pair or FS pair interactions. Physical similarity of partners was related to pair ratings on the six dimensions of cooperation for VTs, but not for BFs or FSs. Discussion of results is informed by behavioral-genetic and genetic similarity theories.