The definitions of friendship presented in the previous section refer only to the most basic criterion or feature of friendship (i.e., mutual liking). Certainly, there is more to friendship than this fundamental defining characteristic. In this section, we show that the interactions of friends differ substantially from the interactions of nonfriends. The centerpiece of this discussion is well known, specifically, that reciprocity is a fundamental characteristic of friendship.
Beyond the simple reciprocity of liking discussed in the previous section, we show that reciprocity between friends can be seen in several domains, including affect and behavior. “Reciprocity” refers to the tendency of two persons to act in the same way, either simultaneously or in sequence (Hinde, 1979). Typically, reciprocity is directional in the sense that actions of friends are directed toward each other, such as when one friend does something and the other friend responds (Ross, Cheyne, & Lollis, 1988).