ABSTRACT
Biological siblings, despite sharing many common experiences, can be very different in personality and in intellectual traits. This difference may result partly from the socializing influence of siblings’ friendship cliques and close friends. In this chapter, we review the evidence that peers and friends operate as “nonshared environmental influences” on siblings’ behavioral differences. Siblings belonging to different peer groups possessed personality differences that match with their peers’ characteristics. However, we report data indicating that siblings often share the same friends, so that within family differences in peer exposure should be empirically assessed rather than merely assumed. We reviewed studies of the causes of friends’ resemblance and concluded that selection (“birds of a feather flock together”) is often more powerful than peer influence. Thus, because selection is at work, peers can serve to reinforce preexisting (and sometimes genetic) differences between siblings. Nevertheless, because some peer influence exists, peers can also be regarded as a nonshared environmental influence.
