Evidence from ethnographic, observational, and experimental studies with humans converges to suggest that males affiliate more than females with unrelated, familiar same-sex peers, but this has never been examined directly. With this aim, we compared frequency of affiliation with a single, randomly chosen, familiar same-sex peer for the two sexes during early childhood. A focal child was brought to a room with three play areas--one containing a same-sex peer, one containing an adult, and one empty-and time spent with the peer was tabulated. Results demonstrated that boys visited the play area with the same-sex peer more frequently than girls did, and more boys than girls spent significant amounts of time with the peer. Human males' greater willingness to affiliate with randomly chosen familiar peers likely contributes to sex differences in a number of characteristics of humans' social interactions. (Contains 1 figure and 1 table.)