Children in kindergarten, second, and fourth grades were required to judge how much they trusted each of their peers (classmates). A same sex pattern of peer trust was found; boys trusted boys more than they trusted girls, and girls trusted girls more than they trusted boys. This pattern of peer trust was evident in fourth- and second-grade children but not in kindergarten children. It was proposed that the same sex pattern of trust serves to reinforce and maintain the same sex pattern of peer relationships in children.