Alan F. Dixson

Human Physique and Sexual Attractiveness in Men and Women: A New Zealand–U.S. Comparative Study

Men and women living in New Zealand and California completed five studies regarding human physique and sexual attractiveness. In Studies 1–3, women rated images of male stimuli and, in Studies 4–5, men rated female stimuli. In Study 1, women in both countries rated mesomorphic (muscular) and average male somatotypes as most attractive, followed by ectomorphic (slim) and endomorphic (heavily built) figures. In Study 2, amount and distribution of masculine trunk hair (chest and abdominal) was altered progressively in a series of front-posed male figures. In both countries, the image lacking any trunk hair was rated as the most attractive, with a steady decline in attractiveness as hirsutism became more pronounced. Study 3 assessed attractiveness of front-posed male figures that varied only in the length of the non-erect penis. Five lengths were presented: The smallest penile size was rated as less attractive than three intermediate sizes. The largest penile size was not the most attractive, but received higher scores than the unaltered and smallest penile size. In Study 4, men rated the attractiveness of back-posed female images varying in waist-to-hip ratio (WHR) (from 0.5 to 1.0). The 0.7 WHR figure was rated more attractive in New Zealand and the 0.6 WHR in California. Study 5 measured the attractiveness of female skin color; men expressed preferences for lighter skinned female figures in New Zealand and California. Results indicate very similar preferences for sexually dimorphic physical traits among men and women of European extraction, living in two culturally and geographically different environments.

Luteinizing Hormone Releasing Hormone Enhances Proceptivity in a Primate

Luteinizing hormone releasing hormone (LHRH) administered to ovariectomized marmosets treated with a low dose of 17 beta-oestradiol significantly enhanced proceptivity within 2 h. Without oestradiol priming LHRH treatment did not produce this effect. The dose of LHRH given did not significantly alter plasma levels of cortisol or progesterone. The behavioural effect of this treatment is therefore likely to have been mediated via a direct central action rather than indirectly through stimulating adrenocortical steroid secretion. These results provide the first demonstration that LHRH can potentiate the proceptive behaviour of a female primate.

Eye Tracking of Men’s Preferences for Female Breast Size and Areola Pigmentation

Sexual selection via male mate choice has often been implicated in the evolution of permanently enlarged breasts in women. While questionnaire studies have shown that men find female breasts visually attractive, there is very little information about how they make such visual judgments. In this study, we used eye-tracking technology to test two hypotheses: (1) that larger breasts should receive the greatest number of visual fixations and longest dwell times, as well as being rated as most attractive; (2) that lightly pigmented areolae, indicative of youth and nubility, should receive most visual attention and be rated as most attractive. Results showed that men rated images with medium-sized or large breasts as significantly more attractive than small breasts. Images with dark and medium areolar pigmentation were rated as more attractive than images with light areolae. However, variations in breast size had no significant effect on eye-tracking measures (initial visual fixations, number of fixations, and dwell times). The majority of initial fixations during eye-tracking tests were on the areolae. However, areolar pigmentation did not affect measures of visual attention. While these results demonstrate that cues indicative of female sexual maturity (large breasts and dark areolae) are more attractive to men, patterns of eye movements did not differ based on breast size or areolar pigmentation. We conclude that areolar pigmentation, as well as breast size, plays a significant role in men’s judgments of female attractiveness. However, fine-grained measures of men’s visual attention to these morphological traits do not correlate, in a simplistic way, with their attractiveness judgments.

Men's Preferences for Women's Breast Morphology in New Zealand, Samoa, and Papua New Guinea

Sexual selection via mate choice may have influenced the evolution of women’s breast morphology. We conducted an image-based questionnaire quantifying and comparing the preferences of men from Papua New Guinea (PNG), Samoa, and New Zealand (NZ) for images of women’s breast size, breast symmetry, areola size, and areolar pigmentation. Results showed that men from PNG preferred larger breasts to a greater extent than men from Samoa and NZ, providing some support for the hypothesis that men from subsistence living cultures have a greater preference for morphological cues indicative of caloric reserves. Symmetrical breasts were most attractive to men in each culture. However, preferences were highest among NZ men, followed by men from Samoa, and were lowest among men from PNG. These results did not support the hypothesis that people living in higher pathogen environments have a greater preference for traits indicative of pathogen resistance and developmental stability. Large areolae were preferred among men from PNG, and to a lesser extent in Samoa, while in NZ men preferred medium-sized areolae. Thus, men’s preferences for women’s areolar size appear to be highly culturally specific. Darkly pigmented areolae were most attractive to men from Samoa and PNG, whereas men from NZ preferred areolae with medium pigmentation. These findings suggest that areolar pigmentation indicative of sexual maturity is preferred by men rather than lighter pigmentation, which may signal that a woman is in the early years of reproductive maturity. This study highlights the importance of cross-cultural research when testing the role of morphological cues in mate choice.

A quantitative description of copulatory and associated behaviors of captive marmosets (Callithrix jacchus)

Measurements of male and female copulatory behavior in pairs of common marmosets were made during 30-min mating tests (n = 170). For the male,both frequencies and latencies of mounting, pelvic thrusting, and ejaculatory behaviors were analyzed. For the female, frequencies of proceptive displays (tongue-flicking) and of sexual receptivity were measured. The sexual and associated behavior of the marmoset is compared to that of other primates and the suitability of this species for laboratory studies of neurohumoral mechanisms and sexual behavior is assessed.

The Role of Breast Size and Areolar Pigmentation in Perceptions of Women’s Sexual Attractiveness, Reproductive Health, Sexual Maturity, Maternal Nurturing Abilities, and Age

Women’s breast morphology is thought to have evolved via sexual selection as a signal of maturity, health, and fecundity. While research demonstrates that breast morphology is important in men’s judgments of women’s attractiveness, it remains to be determined how perceptions might differ when considering a larger suite of mate relevant attributes. Here, we tested how variation in breast size and areolar pigmentation affected perceptions of women’s sexual attractiveness, reproductive health, sexual maturity, maternal nurturing abilities, and age. Participants (100 men; 100 women) rated images of female torsos modeled to vary in breast size (very small, small, medium, and large) and areolar pigmentation (light, medium, and dark) for each of the five attributes listed above. Sexual attractiveness ratings increased linearly with breast size, but large breasts were not judged to be significantly more attractive than medium-sized breasts. Small and medium-sized breasts were rated as most attractive if they included light or medium colored areolae, whereas large breasts were more attractive if they had medium or dark areolae. Ratings for perceived age, sexual maturity, and nurturing ability also increased with breast size. Darkening the areolae reduced ratings of the reproductive health of medium and small breasts, whereas it increased ratings for large breasts. There were no significant sex differences in ratings of any of the perceptual measures. These results demonstrate that breast size and areolar pigmentation interact to determine ratings for a suite of sociosexual attributes, each of which may be relevant to mate choice in men and intra-sexual competition in women.

Venus Figurines of the European Paleolithic: Symbols of Fertility or Attractiveness?

The earliest known representations of the human female form are the European Paleolithic “Venus figurines,” ranging in age from 23,000 to 25,000 years. We asked participants to rate images of Paleolithic figurines for their attractiveness, age grouping and reproductive status. Attractiveness was positively correlated with measures of the waist-to hip ratio (WHR) of figurines, consistent with the “sexually attractive symbolism” hypothesis. However, most figurines had high WHRs (>1.0) and received low attractiveness scores. Participants rated most figurines as representing middle-aged or young adult women, rather than being adolescent or older (postmenopausal). While some were considered to represent pregnant women, consistent with the “fertility symbol” hypothesis, most were judged as being non-pregnant. Some figurines depict obese, large-breasted women, who are in their mature reproductive years and usually regarded as being of lower attractiveness. At the time these figurines were made, Europe was in the grip of a severe ice age. Obesity and survival into middle age after multiple pregnancies may have been rare in the European Upper Paleolithic. We suggest that depictions of corpulent, middle-aged females were not “Venuses” in any conventional sense. They may, instead, have symbolized the hope for survival and longevity, within well-nourished and reproductively successful communities.

Male infanticide and primate monogamy

In their recent paper on the evolution of monogamy in primates, Opie et al. (1) conclude that male infanticide played a pivotal role in the origins of monogamy, more important than paternal care or female ranging patterns. Male infanticides are rare events that are difficult to observe in free-ranging primates. Direct observations of infanticides exist for ∼6% of all primate species (2). In the absence of sufficient direct observations of infanticides, Opie et al. (1) use a presumptive correlate of “infanticide risk”: the ratio between gestation length + lactational length and the duration of lactation (L/G+L). This approach presumes that females of those species having relatively longer periods of lactation are at greater risk of males killing dependent infants.

Human sexual behavior and the origins of gonorrhea.

Although approximately 400 extant primate species are currently known to science, only one of them—Homo sapiens— acts as host to the virulent sexually transmitted infection (STI) called gonorrhea. From a comparative perspective, this is surprising, given that many non-human primates, such as macaques, baboons, chimpanzees, and bonobos, typically engage in multiple-partner copulations. Such mating patterns are expected to favor the genesis and spread of STIs (Anderson, Hessel, & Dixson, 2004; Nunn, Gittleman, & Antonovics, 2000). Thus, we are ledtoaskwhygonorrheashouldhavearisenonlyinhumanbeings. The hypothesis outlined here provides a possible explanation. Gonorrhea is caused by the gram-negative bacterium Neisseria gonorrhoeae, which infects the epithelial cells of mucous membranes, includingthosewhichlinetheurethra,vagina,pharynx,andrectum.Gonorrheamaythusbe transmittedviavaginal, anal, and oral sexualcontacts (Bancroft,2009;Holmeset al., 2008; Rogstad,2011).N.gonorrhoeaewasfirst isolatedin1879,byAlbert Neisser, but since that time scores of additional Neisseria species have been described (Bennett et al., 2012). The vast majorityof thesearenon-pathogenic,however,existingasharmless commensals in the nasopharynx (Liu, Tang, & Exley, 2015). Moreover, transfer of chromosomal genes has been shown to occur between some of these harmless Neisseria species and N.gonorrhoeae(MaynardSmith,Dowson,S Rogstad, Elengasinghe, & Powles, 2015). N.gonorrhoeae may thus be likened to an‘‘invasive species,’’ in the sense that its precursor was transferred from its originalhabitat in thepharynx tonovel sites in thegenitalia,where it is less well tolerated, and causes disease. Such transfers of the pharyngeal bacteria that were ancestral toN.gonorrhoeaemost likely occurred as a result of the practice oforalsex.Humanbeingsaremuchmorepronetoengageinsuch contacts thanarethenon-humanprimates.Althoughvariouskinds of genital investigation commonly precede copulation in the prosimians, monkeys, and apes (for review, see Dixson, 2012), recorded incidences of fellatio and cunnilingus are very rare, especially for animals living under natural conditions. Thus, human sexual behavior is unusual in this respect, which may explain why gonorrhea has arisen solely inHomo sapiens. Precisely when the disease originated is uncertain—whether in biblical times or in Europe during the Middle Ages (e.g., Boyd, 1955; Morton, 1977; Vertue,1953).Eitherway,itappearsthathumanityisstilladapting physiologically to this STI, and that N.gonorrhoeae, in turn, is rapidly evolving resistance to the antibiotics with which we attempt to curb its spread (Whiley et al., 2012).