Sexual selection in humans
Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. Sexual selection is a biological way one sex chooses a mate for the best reproductive success. Most compete with others of the same sex for the best mate to contribute their genome for future generations. This has shaped our evolution for many years, but reasons why humans choose their mates are hardly understood. Sexual selection is quite different in animals than humans as they feel more of the evolutionary pressures to reproduce and can easily reject a mate. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that sexual selection played a part in the evolution of the anatomically modern human brain, i.e. the structures responsible for social intelligence underwent positive selection as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was "more favourable" in humans.
- 1 General hypotheses
- 2 Darwin's sexual selection hypothesis
- 3 Sexual dimorphism
- 4 Selection preferences and biological drivers
- 5 Phenotype
- 6 Geoffrey Miller hypothesis
- 7 Opposing arguments
- 8 See also
- 9 References
- 10 Further reading
Some hypotheses about the evolution of the human brain argue that it is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs (a quarter to a fifth of the energy and oxygen consumed by a human). Current consensus about the evolutionary development of the human brain accepts sexual selection as a potential contributing factor but maintains that human intelligence and the ability to store and share cultural knowledge would have likely carried high survival value as well.
Sexual selection's role in human evolution cannot be definitively established, as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selection, and others pleiotropy. Richard Dawkins argued that
- "When you notice a characteristic of an animal and ask what its Darwinian survival value is, you may be asking the wrong question. It could be that the characteristic you have picked out is not the one that matters. It may have "come along for the ride", dragged along in evolution by some other characteristic to which it is pleiotropically linked."
Darwin's sexual selection hypothesisEdit
Charles Darwin described sexual selection as depending on "the advantage which certain individuals have over others of the same sex and species, solely in respect of reproduction". Darwin noted that sexual selection is of two kinds and concluded that both kinds had operated on humans: "The sexual struggle is of two kinds; in the one it is between the individuals of the same sex, generally the male sex, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners."
Charles Darwin conjectured that the male beard, as well as the hairlessness of humans compared to nearly all other mammals, were results of sexual selection. He reasoned that since the bodies of females are more nearly hairless, the loss of fur was due to sexual selection of females at a remote prehistoric time when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that contrasts in sexual selection acting along with natural selection were significant factors in the geographical differentiation in human appearance of some isolated groups, as he did not believe that natural selection alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan women "the posterior part of the body projects in a most wonderful manner" (known as steatopygia) implies sexual selection for this characteristic. In The Descent of Man, and Selection in Relation to Sex, Darwin viewed many physical traits which vary around the world as being so trivial to survival that he concluded some input from sexual selection was required to account for their presence. He noted that variation in these features among the various peoples of the world meant human mate-choice criteria would also have to be quite different if the focus was similar, and he himself doubted that, citing reports indicating that ideals of beauty did not, in fact, vary in this way around the world.
The effects on the human brain formation during puberty is directly linked to hormones changes. The effects of hormones have been studied and have a border understanding than how the direct actions of the sex chromosome gene. The mismatch timing between biological puberty and age of social maturity in western society has a psychological expectation on children.  With puberty, men are generally hairier than women, and Darwin was of the opinion that hairlessness was related to sexual selection; however, several other explanations have been advanced to explain human hairlessness, a leading one is loss of body hair to facilitate sweating. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits.
Indicating that a trait is under sexual selection can be difficult to prove through correlational methods, as characters may result from different selective pressures, some involving sexual selection, others natural selection, and some may be accidental and due to pleiotropy. For example, monogamous primates are known to typically exhibit little sexual dimorphism such as particularly large males armed with huge canines; however, powerful big-toothed males can provide protection against predators and may be bigger for that reason, rather than in order to win confrontations over females. Males and females differing in size can specialize in, and more fully exploit, different food resources while avoiding competing with each other; furthermore, body size can be useful in avoiding predators and may also be of assistance in securing a mate. This is further complicated by the consideration that with larger body size, the skeleton of mammals becomes much more robust and massive (relatively speaking). Bearing these caveats in mind, levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have shown the earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have become more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).
The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair, pubic hair and penis size. The breasts of primates are flat, yet are able to produce sufficient milk for feeding their young. The breasts of non-lactating human females are filled with fatty tissue and not milk. Thus it has been suggested the rounded female breasts are signals of fertility. Richard Dawkins has speculated that the loss of the penis bone in humans, when it is present in other primates, may be due to sexual selection by females looking for a clear sign of good health in prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health.
Homo has a thicker penis than the other great apes, though it is on average no longer than the chimpanzee's. It has been suggested the evolution of the human penis towards larger size was the result of female choice rather than sperm competition, which generally favors large testicles. However, penis size may have been subject to natural selection, rather than sexual selection, due to a larger penis' efficiency in displacing the sperm of rival males during sexual intercourse. A model study showed displacement of semen was directly proportional to the depth of pelvic thrusting, as an efficient semen displacement device.
Selection preferences and biological driversEdit
There are a variety of factors that drive sexual selection in humans. Current available research indicates that selection preferences are biologically driven, that is, by the display of phenotypic traits that can be both consciously and unconsciously evaluated by the opposite sex to determine the health and fertility of a potential mate. This process can be affected, however, by social factors, including in cultures where arranged marriage is practiced, or psychosocial factors, such as valuing certain cultural traits of a mate, including a persons social status, or what is perceived to be an ideal partner in various cultures.
Selection preferences in femalesEdit
Some of the factors that affect how females select their potential mates for reproduction include voice pitch, facial shape, muscular appearance, and height. Several studies suggest that there is a link between hormone levels and partner selection among humans. In a study measuring female attraction to males with varying levels of masculinity, it was established that women had a general masculinity preferences for men's voices, and that the preference for masculinity was greater in the fertile phase of the menstrual cycle than in the non-fertile phase. There is further evidence from the same study that in fertile stages of the menstrual cycle, women also had a preference for other masculine traits such as body size, facial shape, and dominant behavior, which are indicators of both fertility and health. This study did not exclude males with feminine traits from being selected, however, as feminine traits in men indicate a higher probability of long-term relationship commitment, and may be one of several survival strategies. Further research also backs up the idea of using phenotypic traits as a means of assessing a potential mate's fitness for reproduction as well as assessing whether a partner has high genetic quality.
Another factor affecting the selection process is the environment which the person inhabits. In biological terms, certain environmental conditions may bring about demands for or the disregarding of certain traits. One such example is a preference for males whose facial structure indicates certain hormonal ratios, such as testosterone-cortisol levels (sex and stress hormones). Research shows that, for example, in countries with varying Human Development Index (HDI) levels, females have different preferences for sex-stress hormone ratios, as expressed in the male's face. A Royal Society research showed a significant correlation between a measure of societal development and preferences for indication of lower testosterone levels, as manifested in facial features, and the interaction between preferences for testosterone and cortisol. It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.
Selection preferences in malesEdit
Like their female counterparts, males also use visual information about a potential mate, as well as voice, body shape, and an assortment of other factors in selecting a partner. Research shows that males tend to prefer feminine women's faces and voices as opposed to women with masculine features in these categories. Furthermore, males also evaluate skin coloration, symmetry, and apparent health, as a means by which the select a partner for reproductive purposes. Males are particularly attracted to femininity in women's faces when their testosterone levels are at their highest, and the level of attraction to femininity may fluctuate as hormone levels fluctuate. Studies on men have also been done to show the effects of exogenous testosterone and its effects on attraction to femininity, and the results concluded that throughout several studies, men have shown decreased preference for feminine female faces in the long-term context, when given exogenous testosterone, but this difference did not occur with placebo.
Common preferences in either sexEdit
Sexual selection preferences are general terms by which the mating and reproductive process are understood. As one article states, sexual selection is in essence a process which favors sexual displays for attraction, aggressiveness, dominance, size, and strength, and the ability to exclude competitors by force if necessary, or by using resources to win. Both male and female use voice, face, and other physical characteristics to assess a potential mate's ability to reproduce, as well as their health. Together with visual and chemical signals, these crucial characteristics which are likely to enhance the ability to produce offspring, as well as long term survival prospects, can be assessed and selections made.
Sexual selection has continued to be suggested as a possible explanation for geographical variation in appearance within the human species; in modern hypotheses, marriage practices are proposed as the main determinant of sexual selection. John Manning suggests that where polygyny is common, men face intense competition for wives and are more likely to be completely unsuccessful in reproducing, and the result is strong selection of males for traits which are adaptive for successful reproduction. He proposes a link to skin color through selection of males for testosterone-mediated traits which confer an ability to successfully compete for females. He suggests testosterone makes the human immune system less competent to resist pathogens. In this view the antimicrobial properties of melanin help mitigate the susceptibility to disease that polygyny induces by increasing testosteronization. According to this argument, the anti-infective qualities of melanin were more important than protection from ultraviolet light in the evolution of the darkest skin types. Manning asserts that skin color is more correlated with the occurrence of polygyny – explicable by it having an antimicrobial function – than the latitudinal gradient in intensity of ultraviolet radiation, and he points to the lack of very dark skin at equatorial latitudes of the New World and the relatively light skin of Khoisan people in Africa.
Research seems to contradict Manning's explanation about skin color. In 1978, NASA launched the Total Ozone Mapping Spectrometer, which was able to measure the ultraviolet radiation reaching Earth's surface. Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. There was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin. Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of chimpanzees and humans from various regions of the Earth. Rogers concluded that, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair. Over time human hair disappeared to allow better heat dissipation through sweating and the skin tone grew darker to increase the epidermal permeability barrier and protect from folate depletion due to the increased exposure to sunlight. When humans started to migrate away from the tropics, there was less-intense sunlight, partly due to clothing to protect against cold weather. Under these conditions there was less photodestruction of folate, and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. The genetic mutations leading to light skin may have experienced selective pressure due to the adoption of farming and settlement in northern latitudes.
Anthropologist Peter Frost has proposed that sexual selection was responsible for the evolution of pigmentary traits of women in Northern and Eastern European populations. He contends that the diversity of hair and eye color in Northeast European populations originated as a consequence of intense female-female competition, and is an adaptation for reproductive success in women.
Geoffrey Miller hypothesisEdit
Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that many human behaviors not clearly tied to survival benefits, such as humor, music, visual art, some forms of altruism, verbal creativity or the fact that most humans have a far greater vocabulary than that which is required for survival, Miller (2000) has proposed that this apparent redundancy is due to individuals using vocabulary to demonstrate their intelligence, and consequently their "fitness", to potential mates. This has been tested experimentally, and it appears that males do make greater use of lower-frequency (more unusual) words when in a romantic mindset compared to a non-romantic mindset, suggesting that vocabulary is likely to be used as a sexual display (Rosenberg & Tunney, 2008). All these qualities are considered courtship adaptations that have been favored through sexual selection.
Miller is critical of theories that imply that human culture arose as accidents or by-products of human evolution. He believes that human culture arose through sexual selection for creative traits. In that view, many human artifacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits. During human evolution, on at least two occasions, hominid brain size increased rapidly over a short period of time followed by a period of stasis. The first period of brain expansion occurred 2.5 million years ago, when Homo habilis first began using stone tools. The second period occurred 500,000 years ago, with the emergence of archaic Homo sapiens. Miller argues that the rapid increases in brain size would have occurred by a positive feedback loop resulting in a Fisherian runaway selection for larger brains. Tor Nørretranders, in The Generous Man conjectures how intelligence, musicality, artistic and social skills, and language might have evolved as an example of the handicap principle, analogously with the peacock's tail, the standard example of that principle.
The role of sexual selection in human evolution has been considered controversial from the moment of publication of Darwin's book on sexual selection (1871). Among his vocal critics were some of Darwin's supporters, such as Alfred Wallace, who argued that animals and birds do not choose mates based on their beauty or fine plumages, and that the artistic faculties in humans belong to their spiritual nature and therefore cannot be connected to natural selection, which only affects the animal nature. Darwin was accused of looking to the evolution of early human ancestors through the moral codes of the 19th century Victorian society. Joan Roughgarden, citing elements of sexual behavior in animals and humans that cannot be explained by the sexual-selection model, suggested that the function of sex in human evolution was primarily social.
Joseph Jordania suggested in 2011 that in explaining such human morphological and behavioral characteristics as singing, dancing, body painting, wearing of clothes, Darwin and proponents of sexual selection neglect another important evolutionary force, intimidation of predators and competitors with the ritualized forms of warning display, which uses the same arsenal of visual, audio, olfactory and behavioral features as sexual selection. According to Jordania, most of these warning displays were incorrectly attributed to the forces of sexual selection. Jordania proposed an aposematic model of human evolution, where most of the human morphological and behavioral features that had been considered by Darwin as the result of sexual selection, via female choice, are explained by the aposematic (intimidating) display.
- Vogt, Yngve (29 January 2014). "Large testicles are linked to infidelity". Phys.org. Retrieved 31 January 2014.
- Miller G. (2000). The mating mind: how sexual choice shaped the evolution of human nature, London, Heineman, ISBN 0-434-00741-2 (also Doubleday, ISBN 0-385-49516-1).
- Neoteny and Two-Way Sexual Selection in Human Evolution: A Paleo-Anthropological Speculation on the Origins of Secondary-Sexual Traits, Male Nurturing and the Child as a Sexual Image
- Sexual Selection and the Mind
- Fisher, R.A. (1930) The Genetical Theory of Natural Selection. ISBN 0-19-850440-3
- Edwards, A.W.F. (2000) Perspectives: Anecdotal, Historial and Critical Commentaries on Genetics. The Genetics Society of America (154) 1419:1426
- Andersson, M. (1994) Sexual selection. ISBN 0-691-00057-3
- Andersson, M. and Simmons, L.W. (2006) Sexual selection and mate choice. Trends, Ecology and Evolution (21) 296:302
- Gayon, J. (2010) Sexual selection: Another Darwinian process. Comptes Rendus Biologies (333) 134:144
- Fisher, R. A. (1915). "The evolution of sexual preference". Eugenic Review. 7 (3): 184–92. PMC 2987134. PMID 21259607.
- Schillaci, M. A. (2006). "Sexual selection and the evolution of brain size in primates". PLOS ONE. 1 (1): e62. Bibcode:2006PLoSO...1...62S. doi:10.1371/journal.pone.0000062. PMC 1762360. PMID 17183693.
- McElreath, Richard (May 2018). "Sizing up human brain evolution". Nature. 557 (7706): 496. doi:10.1038/d41586-018-05197-8.
- Richard Dawkins (2009). The Greatest Show on Earth: The Evidence for Evolution. Free Press. ISBN 1416594787, ISBN 978-1416594789.
- Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex. 1 (1st ed.). London: John Murray. p. 256.
- Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex. 2 (1st ed.). London: John Murray. p. 402.
- Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex. 1 (1st ed.). London: John Murray. p. 398.
- Charles Darwin (1882). The Descent of Man and Selection in Relation to Sex. London: John Murray. p. 578.
- Charles Darwin (1882). The Descent of Man, and Selection in Relation to Sex. AMS Press. p. 605.
The races of man differ from each other, and from their nearest allies, in certain characters which are of no service to them in their daily habits of life, and which it is extremely probable would have been modified through sexual selection
- Darwin, C. (1936) . The Descent of Man and Selection in Relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.
- Gluckman, P. D.; Hanson, M. A. (25 July 2006). "Changing times: The evolution of puberty". Molecular and Cellular Endocrinology. pp. 26–31. doi:10.1016/j.mce.2006.04.005.
- Jablonski, N. G. (2006). Skin: a natural history. Berkeley, CA: University of California Press. p. PP13.
- Jablonski, N. G.; Chaplin, G. (2010). "Human skin pigmentation as an adaptation to UV radiation". Proceedings of the National Academy of Sciences. 107 (Suppl 2): 8962–8968. Bibcode:2010PNAS..107.8962J. doi:10.1073/pnas.0914628107. PMC 3024016. PMID 20445093.
- Evolution and Human Behavior: Darwinian Perspectives on Human Nature, by John Cartwright.
- Principles of Human Evolution, by Roger Lewin, Robert Foley.
- Morris, Desmond (2007). "Breasts". The Naked Woman. ISBN 978-0-312-33853-4.
- Dawkins, Richard (2006) [First published 1976]. The Selfish Gene (30th anniversary ed.). p. 158 endnote.
It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male's health, and the robustness of his ability to cope with stress, from the tone and bearing of his penis.
- Dixson, A. F. (2009). Sexual selection and the origins of human mating systems. Oxford University Press. pp. 61–65. ISBN 9780191569739.
- Miller, G.F. (1998), "How mate choice shaped human nature: A review of sexual selection and human evolution" in Handbook of Evolutionary Psychology.
- In a theoretical paper published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture that, "A longer penis would not only have been an advantage for leaving semen in a less-accessible part of the vagina, but by filling and expanding the vagina, it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity." – "Secrets of the Phallus: Why Is the Penis Shaped Like That?", ScientificAmerican.com.
- Grammer, Karl; Fink, Bernhard; Møller, Anders P.; Thornhill, Randy (1 August 2003). "Darwinian aesthetics: sexual selection and the biology of beauty". Biological Reviews. 78 (3): 385–407. doi:10.1017/s1464793102006085. ISSN 1469-185X.
- Rhodes, Gillian; Chan, Janelle; Zebrowitz, Leslie A.; Simmons, Leigh W. (7 August 2003). "Does sexual dimorphism in human faces signal health?". Proceedings of the Royal Society of London B: Biological Sciences. 270 (Suppl 1): S93–S95. doi:10.1098/rsbl.2003.0023. ISSN 0962-8452. PMC 1698019. PMID 12952647.
- Price, Michael E.; Pound, Nicholas; Dunn, James; Hopkins, Sian; Kang, Jinsheng (2 January 2013). "Body Shape Preferences: Associations with Rater Body Shape and Sociosexuality". PLOS ONE. 8 (1): e52532. Bibcode:2013PLoSO...852532P. doi:10.1371/journal.pone.0052532. ISSN 1932-6203. PMC 3534680. PMID 23300976.
- Feinberg, D. R.; Jones, B. C.; Law Smith, M. J.; Moore, F. R.; DeBruine, L. M.; Cornwell, R. E.; Hillier, S. G.; Perrett, D. I. (1 February 2006). "Menstrual cycle, trait estrogen level, and masculinity preferences in the human voice". Hormones and Behavior. 49 (2): 215–222. doi:10.1016/j.yhbeh.2005.07.004. PMID 16055126.
- "Strategies for Animal Survival 2 (Animal Studies Series)". Chip Taylor Communications.
- Shaw, Fionna (2009). "Influence of female 2D:4D ratio on attractiveness of male vocal and facial masculinity" (Dissertation). Cite journal requires
- Moore, F. R.; Coetzee, V.; Contreras-Garduño, J.; Debruine, L. M.; Kleisner, K.; Krams, I.; Marcinkowska, U.; Nord, A.; Perrett, D. I. (23 June 2013). "Cross-cultural variation in women's preferences for cues to sex- and stress-hormones in the male face". Biology Letters. 9 (3): 20130050. doi:10.1098/rsbl.2013.0050. ISSN 1744-9561. PMC 3645036. PMID 23536442.
- O'Connor, Jillian J. M.; Fraccaro, Paul J.; Pisanski, Katarzyna; Tigue, Cara C.; Feinberg, David R. (31 July 2013). "Men's Preferences for Women's Femininity in Dynamic Cross-Modal Stimuli". PLOS ONE. 8 (7): e69531. Bibcode:2013PLoSO...869531O. doi:10.1371/journal.pone.0069531. ISSN 1932-6203. PMC 3729951. PMID 23936037.
- Welling, Lisa L. M.; Jones, Benedict C.; DeBruine, Lisa M.; Smith, Finlay G.; Feinberg, David R.; Little, Anthony C.; Al-Dujaili, Emad A. S. (1 November 2008). "Men report stronger attraction to femininity in women's faces when their testosterone levels are high". Hormones and Behavior. 54 (5): 703–708. doi:10.1016/j.yhbeh.2008.07.012. PMID 18755192.
- Bird, Brian M.; Welling, Lisa L. M.; Ortiz, Triana L.; Moreau, Benjamin J. P.; Hansen, Steve; Emond, Michael; Goldfarb, Bernard; Bonin, Pierre L.; Carré, Justin M. (1 September 2016). "Effects of exogenous testosterone and mating context on men's preferences for female facial femininity". Hormones and Behavior. 85: 76–85. doi:10.1016/j.yhbeh.2016.08.003. PMID 27511452.
- Puts, David A.; Jones, Benedict C.; DeBruine, Lisa M. (1 March 2012). "Sexual Selection on Human Faces and Voices". The Journal of Sex Research. 49 (2–3): 227–243. CiteSeerX 10.1.1.699.7560. doi:10.1080/00224499.2012.658924. ISSN 0022-4499. PMID 22380590.
- "How Does Evolution Occur?". necsi.edu.
- Manning, John (2009). The Finger Ratio. Faber & Faber. ISBN 978-0-571-21540-9.
- MacKintosh, J. (2001). "The antimicrobial properties of melanocytes, melanosomes and melanin and the evolution of black skin". Journal of Theoretical Biology. 211 (2): 101–113. doi:10.1006/jtbi.2001.2331. PMID 11419954.
- Kirchweger, Gina (1 February 2001). "The Biology of . . . Skin Color". Discover. Retrieved 31 March 2015.
- Rogers, Alan R.; Iltis, David; Wooding, Stephen (2004). "Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair". Current Anthropology. 45: 105–8. doi:10.1086/381006.
- Jablonski, Nina G.; Chaplin, George; Chaplin (2000). "The evolution of human skin coloration" (PDF). Journal of Human Evolution. 39 (1): 57–106. doi:10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 24 March 2003.
- Elias, Peter M; Menon, Gapinathan; Wetzel, Bruce J; Williams, John (Jack) W (2010). "Barrier Requirements as the Evolutionary "Driver" of Epidermal Pigmentation in Humans". American Journal of Human Biology. 22 (4): 526–537. doi:10.1002/ajhb.21043. PMC 3071612. PMID 20209486.
- Jablonski, N. G.; Chaplin, G. (2010). "Colloquium Paper: Human skin pigmentation as an adaptation to UV radiation". Proceedings of the National Academy of Sciences. 107 (Suppl 2): 8962–8. Bibcode:2010PNAS..107.8962J. doi:10.1073/pnas.0914628107. PMC 3024016. PMID 20445093.
- Juzeniene, Asta; Setlow, Richard; Porojnicu, Alina; Steindal, Arnfinn Hykkerud; Moan, Johan (2009). "Development of different human skin colors: A review highlighting photobiological and photobiophysical aspects". Journal of Photochemistry and Photobiology B: Biology. 96 (2): 93–100. doi:10.1016/j.jphotobiol.2009.04.009. PMID 19481954.
- Frost, P. (2008). "Sexual selection and human geographic variation" (PDF). Journal of Social, Evolutionary, and Cultural Psychology. 2 (4): 169–191. doi:10.1037/h0099346. Archived from the original (PDF) on 25 April 2012.
- Cunningham, M.R.; Roberts, A.R.; Barbee, A.P.; Druen, P.B.; Wu, C-H. (1995). "Their ideas of beauty are, on the whole, the same as ours": consistency and variability in the cross-cultural perception of female physical attractiveness". Journal of Personality and Social Psychology. 68 (2): 261–279. doi:10.1037/0022-35184.108.40.2061.
- Geoffrey Miller, The Mating Mind, p.111; published 2001
- Klasios, J. (2013). "Cognitive traits as sexually selected fitness indicators". Review of General Psychology. pp. 428–442.
- Roughgarden, Joan (2004). Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People. Los Angeles: University of California Press.
- Jordania, Joseph (2011). Why do People Sing? Music in Human Evolution. Logos. pp. 186–196.