Sexual Selection and the Evolution of Human Physique

<p>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.</p>

Sexual Selection and the Evolution of Human Physique

<p>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.</p>

Sexual Selection and Human Breast Morphology

<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>

Sexual Selection and Human Breast Morphology

<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>

Selection of breast features for young women in northwestern China based on the random forest algorithm

In the research of breast morphology, numerous breast features are measured, whereas only a few parameters are adopted for classification. Therefore, how to extract the key variables from the multi-dimensional features in a rational way is an issue that is focused upon. This study aimed to reduce the complexity of the dimensionality reduction for further improving the objectivity and interpretability of the selected breast features. Since the random forest (RF) algorithm can quantify the feature importance during training, the method was adopted to determine the optimal breast features for classification and recognition in this paper. Firstly, the anthropometric data of 360 females from northwestern China aged from 19 to 27 years were measured by non-contact three-dimensional body scanning technology and the contact manual measurement method. Then, the k-means clustering was applied to categorize breast shapes, and the RF algorithm was utilized to quantify and rank the importance of 25 breast features. Finally, to verify the availability of the RF algorithm on breast feature selection, the t-distributed stochastic neighbor embedding method was adopted to visualize the distribution of breast shape clusters into two dimensions. Meanwhile, four neural networks were determined to recognize the breast morphology. The results demonstrate that fewer breast features can effectively increase the accuracy of breast shape classification and recognition. The best performance of breast shape classification and recognition is obtained when the number of breast features is 13. In this case, the average Hamming loss of four neural networks is the smallest (0.1136). Interestingly, the bust circumference and the horizontal curve of breasts across the bust points are found to be the most important of the 25 breast features in this paper. The importance of the breast curve features is higher than that of the breast cross-sectional features, while the breast positioning features have the lowest importance. Meanwhile, the RF algorithm is verified to be more effective than traditional dimensionality reduction methods, such as principal component analysis, hierarchical clustering, and recursive feature elimination. The approach developed in this paper can be generalized to the dimensionality reduction of other body morphology.