The overwhelming majority of play research concerns juveniles. However, a full understanding of the phenomenon requires knowledge of play and playfulness across the life spans of those animals, including humans, who play in adulthood. The authors investigate a theory of play based on Darwin's concept of sexual selection that may account for the existence of play among adult humans. The authors hypothesize that playfulness becomes a highly desired characteristic in potential long-term mates but also that the reasons for desiring playful mates differ for males and females. The authors suggest that for males, playfulness in females signals youth and, hence, fecundity; for females, playfulness in males signals nonaggressiveness. They test these hypotheses using mate-preference data. (Contains 3 figures.)