Psychosexual personality is not exclusively a postnatal and learned phenomenon. Three biologically-based sex differences can be cited as having promoted status differentiation between males and females. These are (a) greater physical strength of males, (b) greater aggressiveness of males, and (c) child-rearing and nursing roles of females. The opposite sex may serve as the primary frame of reference for a person's self-concept. If this is so, masculinity should be defined as being unlike females, and femininity should be defined as being unlike males. Sex-role development for males is more difficult than for females. During early childhood, females have more freedom in choice of behaviors. Games and sports, however, have long been of value in aiding male sex-role development. Sports provide one area where there is no doubt about sexual differences and where biology is not obsolete. Athletics help reinforce male differentiation from females in a world where many male functions have come to resemble female functions. Games and sports are, however, becoming less sex-differentiated, and this is eroding one form of masculine identification, perhaps one of few left. We should restrict female participation to particular sports, thus leaving others to be identified as "masculine." By having developing males, particularly those exhibiting feminine characteristics, participate in these "masculine" sports, we could help them establish a masculine identity. (PB)