Human beings are unusual in the animal kingdom in continuing to play as adults. In other species playfulness generally disappears after the juvenile stage. Researchers have attempted to test the hypothesis that human playfulness is sexually selected.
According to a new study humans remain playful throughout life due to sexual selection. The authors' found:
The results we report here support Chick’s (1998, 2001) hypothesis that adult playfulness results from sexual selection and signals positive qualities to potential long-term mates.
The positive qualities, according to the study, are different for men and women. The authors argue that playfulness in a man signals to a woman that he is not aggressive and thus unlikely to harm her or her children. Whereas, playfulness in a woman, due to its association with juveniles, signals to a man her youthfulness and, by extension, fertility.
The significance of the study lies in the fact that human playfulness throughout life is so at odds with the behaviour of other animals. So much so that it is difficult the see anyway of reconciling this universal behavioural trait with natural selection. Nor could it, given its universality, be readily explained in terms of genetic drift. Thus, if the theory of evolution is correct, the most likely way in which the trait had evolved would be sexual selection.
On the basis of this theoretical understanding the authors surveyed undergraduates on traits they found most attractive in a potential mate. A statistical analysis of the responses showed "ample support" for the hypothesis.
There are, however, caveats to the validity of the study, which the authors do themselves discuss. Most significantly, the study is based on what the respondents claimed they found attractive in a potential mate. However, just because a person may seek a mate demonstrating certain traits does not necessarily translate to actual mating behaviour. This is a significant flaw in the study, as selection works, not via desires or intentions, but by actual reproductive behaviours. Indeed, as the authors note, in previous studies when stated preferences have been compared with actual choices significant discrepancies have resulted. In other words, cognitive processes for mate preferences may not accord with mate choice behaviours. The significance of this latter point for the study is that whilst the data may well be reliable, how valid the data are is an open question.