Sexual Selection and the Evolution of Human Physique
Charles Darwin proposed in The Descent of Man and Selection in Relation to Sex (1871) that traits which improve reproductive success, such as sexually attractive adornments, or weaponry that enhances fighting potential, have been selected for during the course of evolution. The field of evolutionary psychology has revitalized Darwin's hypotheses of sexual selection and human evolution through integrating the fields of anthropology, biology and psychology. In this thesis I investigate the potential for sexual selection to have acted upon sexual dimorphism in body composition and secondary sexual adornments in women and men. In women, body fat accumulation around the hips, buttocks and thighs can be measured using the waist-to-hip ratio (WHR). Female body fat and body weight are critical as energy reserves for gestation, pregnancy and lactation. Female body shape, body weight and breast morphology have been implicated in male judgments of female physical attractiveness. Men from New Zealand (NZ), China, Samoa and the highlands of Papua New Guinea (PNG) rated images of women with low WHRs as most attractive, independent of changes in body weight. In studies of male preferences for female breast morphology, married men from NZ, Samoa and PNG preferred large breasts whereas unmarried men preferred medium-sized breasts. Darkly- and medium-pigmented areolae were preferred in each culture, as were symmetrical breasts. However, male preferences for female areolar size varied considerably across these cultures. Eye-tracking techniques were used to measure attention to morphological traits as men made attractiveness judgments of female images that varied in WHR and breast morphology. In studies using full-length female images that varied in WHR and breast size, men look most often and for longer at the regions of female physique in which fat deposits are greatest (i.e. the breasts followed by the waist). However, attractiveness judgments were driven primarily by WHR rather than breast size. In eye-tracking studies using female torsos as stimuli, men looked most often and for longest at the breasts and areolae, irrespective of differences in breast size and areolar pigmentation. Men rated large and medium size breasts, and medium and darkly pigmented areolae as most attractive. These eye-tracking studies show a possible discordance in male visual attention for morphological traits that appear to drive attractiveness decisions. However, when men were shown full-length images of women posed in back-view as well as in front-view there were significant differences in their viewing patterns. Men spent more time looking at the midriff region of back-posed images than front-posed images and, irrespective of body-pose, rated images with low WHRs as most sexually attractive. Darwin suggested that the human male beard evolved via female choice as a highly attractive secondary sexual adornment. Other authors have proposed that the beard may augment aggressive displays and enhance perceptions of social dominance among males. To test these hypotheses I developed a new questionnaire that integrated facial expressions with the presence or absence of the beard. These questionnaires were administered in NZ and Samoa. The presence of a beard augmented male perceptions of aggressive facial expressions in Samoa and NZ. However, women in these cultures rated faces without beards as more attractive than bearded men. Men and women in both cultures rated bearded men as looking older and as having higher social status. The findings suggest that the beard plays a stronger role in intra-sexual competition rather than inter-sexual mate choice.