Sexual Selection and Human Breast Morphology
Authors:   , Duncan Melanie 
Published: November 2021

<p>Human females, uniquely among primates, develop prominent breasts during puberty, well before reproduction occurs. Adipose tissue in breasts forms part of the "gynoid"  fat distribution, involving the hips, thighs, buttocks and breasts of women. Breasts are thus characterised as secondary sexual characteristics, and their evolution may be due, at least in part, to the effects of sexual selection. This partial reversal of the usual pattern of male adornment may be related to high paternal care in humans. Breast morphology is complex, so that women vary not only in the size and shape of their breasts, but also in the size, shape and pigmentation of their areolae and nipples. These traits change with reproductive status and age. Breasts are more prone to fluctuating asymmetry than many other features of human anatomy and such asymmetry may be closely related to some measures of reproductive success. This thesis used digitally altered images to investigate the impact of morphological changes on perceptions of attractiveness and other qualities. Study 1 investigated the impact of four breast sizes and three areola colours on the perceptions of two hundred participants. Breast size significantly impacted all ratings. Attractiveness and health ratings were maximised at the intermediate breast size for the lightest and original coloured areola, and at the largest breast size for the darkest areola. Ratings of nurturance, sexual maturity and estimates of age increased stepwise from the images with undeveloped breasts to the images with the largest breasts. Areola colour interacted with breast size. Darker areola were judged less attractive, less healthy and less nurturing when paired with small or intermediate breasts, but increased these ratings when paired with large breasts. There was no strong effect of areola colour on ratings of images with undeveloped breasts or on ratings of sexual maturity and age. Study 2 investigated the effect of breast asymmetry on attractiveness and health ratings using data provided by two hundred participants. Increasing levels of asymmetry, created by modifying one breast to increase the apparent volume (four levels from 102.5% to 110% of the original) or position (four levels from 1%-4% of the length of the image) resulted in progressively lower ratings. The differences in ratings between the images with extreme levels in asymmetry (107.5% vs 110% and 3% vs 4%) were smaller. Images that had been modified in the models left (and so seen on viewer's right side) were given higher ratings than those modified identically but on the other side. This may be an expression of a phenomena known as pseudoneglect, where people appear to attend more to the left. In Study 3, a diverse selection of images, taken from previously published reports on human breast morphology and attractiveness, were compiled as a single questionnaire and shown to 37 participants. The purpose of this pilot study was to assess the impact of different image types on ratings of attractiveness and health. Photographic images were rated higher than line drawings or silhouettes. Photographs may be more ecologically valid, as they are more realistic and can be tailored to match the study population. The results presented in this thesis indicate that variations in human breast size, areola colour and breast asymmetry have measurable effects on the perceptions (of both sexes) of attractiveness and health. Breast size also has significant impacts on perceptions of nurturance, reproductive status and age, whereas areola colour has less effect on these ratings. Questionnaire studies employing photographs are likely to be more effective than more stylised images. Morphological changes in the human breast may signal mate value and fertility and therefore may have been subject to sexual selection, as well as natural selection, during human evolution.</p>