Final answer:
Same-sex playmate preferences are reinforced through gender socialization from family, education, peers, and mass media, where peers significantly influence by rewarding or penalizing behaviors concerning gender expectations.
Step-by-step explanation:
The preference for same-sex playmates is reinforced by gender socialization, which is the process where individuals learn the cultural norms and expectations traditionally associated with their gender. This socialization comes from several agents, including family, education, peers, and mass media, which collectively influence and shape behaviors and attitudes towards gender roles. Peers, in particular, play a significant role by often rewarding behaviors that conform to normative gender expectations and sometimes penalizing those that do not, hence reinforcing same-sex playmate preferences among children.