The social biology of human populations: Spontaneous group formation conforms to evolutionary predictions of adaptive aggregation patterns

Abstract

Two experiments tested whether city commuter and pedestrian groups conformed with ecological predictions of adaptive group size, age, and sex composition. It was predicted that individuals with the greatest reproductive potential would optimize mating contacts and minimize competitive interference by segregating into small, mixed-sex groups, whereas more vulnerable individuals, too young or old for optimum fertility, would tend to aggregate independently of gender in larger groups, which may offer advantages for foraging and predator avoidance. To maximize reproductive potential, mature males should prefer grouping with young women of high fertility, whereas older women should prefer younger men. In Experiment 1, 2,432 persons in three cities were observed on buses, automobiles, and sidewalks, in yoked observations. As predicted, reproductively immature preschool and grade-school children formed larger groups than mature adults ( p < 0.0001) and showed no sex differences in grouping. Elderly adults (with decreased reproductive potential) were also found in larger groups than younger adults ( p < 0.01) and also showed no sex differences. Sexually mature men and women grouped more with the opposite sex than in all-male or all-female groups ( p < 0.00001). Mature adult men were the least aggregative category ( p < 0.01): they tended to be alone or in pairs with women, and all-male groups were conspicuously absent ( p < 0.005). These findings were consistent across different socioeconomic levels, ethnicities, and subcultures in our samples (p = NS). In Experiment 2, 475 bus passengers in three cities were observed selecting seatmates in a naturalistic choice paradigm. Only sexually mature adults exhibited a significant preference for the sex of their seatmates ( p < 0.0005): Young women chose most often to sit with other women, whereas young adult men more frequently chose seatmates of the opposite sex. Young women were chosen more often as seat partners than all age/sex categories combined ( p < 0.0005). Young women chose older partners ( p < 0.05), while middle-aged women preferred younger women and men ( p < 0.0005) as companions. Results are explained in the context of canalized behaviors arising early in human evolution.