Thursday, September 24, 2015

Macho Macho Men

 James M Sherlock

The science of why masculine men are sexy.

  It’s no secret that men and women differ from each other physiologically; in fact it’s rather obvious. However, why these differences exist in the first place is often less clear. One aspect of these physiological differences that I study is the evolution of male facial shape. In contrast to females, males tend to have physically larger heads, with exaggerated jaw lines, thicker brows and heavier set features. It’s not only the case that male faces differ from females, but also that male faces show tremendous variation. This continuum, broadly speaking, spans highly masculine and feminine features with some males sporting enormous lantern jaws, with thick, rugged brows while others have soft, plump lips, high cheek bones and large, neotenous eyes [1]. A great deal of this variation is presumed to have arisen as a result of female preferences acting via sexual selection [2].
'Tony Fischer/Flickr Creative Commons'
Source: 'Tony Fischer/Flickr Creative Commons'
Females show nearly as much variation in their preference for facial masculinity as males do in actually displaying masculinity and this is thought to reflect an evolutionary trade-off. Sexy, lumberjack types, with their heavy-set features, are thought to be genetically superior as their larger facial features are a by-product of testosterone. Far from being good for you though, testosterone actually hampers the immune system and as a result is considered a handicap. In evolutionary theory, displaying handicaps and costly traits (i.e. testosterone dependent facial features) can be a signal of underlying quality that only healthy individuals are capable of producing. The classic example of this system is the peacock's tail, full of bright, lurid colours which can have the downside of attracting predators and make negotiating the environment quite difficult [3]. Under this logic, highly masculine males are signalling how strong their immune system is by basically announcing how much of a burden they are capable of shouldering while still functioning. This might sound counterintuitive but there is some evidence to suggest that high testosterone males may have better health than others with some studies finding that masculinity is associated with health benefits including a lower incidence of self-reported illness [4, 5].
'Peacock/Brendan Spragg'
Source: 'Brendan Spragg/Flickr Creative Commons'.
High testosterone isn’t all positive, however, and more masculine males are perceived as being more aggressive and poorer long-term partners [6], who might be more likely to cheat or become violent. Neither of these traits is particularly desirable in a partner and as a result feminine males, who have been exposed to less pubertal testosterone, tend to be perceived as superior parents. In line with predictions about masculinity and genetic quality, female preferences for more masculine faces tend to align with mating opportunities that offer the greatest genetic benefit. For instance, females show less of a preference for feminine faces during ovulation when the chances of conception are greatest [7, 8]. Manly men are also preferred when women are asked to rate faces for a short-term, sexual relationship [8-11] such as a one-night stand. Finally, when women have been cued with the presence of diseases they also tend to prefer masculine faces [12-14]. On the other hand, feminine faces are preferred when women are cued to resource scarcity [15] and by women who consider themselves to be from a lower socioeconomic background [16].
Ebner, Riediger, & Lindenberger, 2010
Masculinized (left) vs. feminized (right) facial stimulus used in Zietsch et al., 2015
Source: Ebner, Riediger, & Lindenberger, 2010
Taken together, these studies do seem to suggest that women prefer masculine men when genetic benefits are likely, and conversely prefer feminine men when they’re unlikely or when less aggressive, more stable partners would be beneficial. However, the evidence is far from conclusive. For instance, two competing meta-analyses (i.e. large scale statistical reviews of the results from previous studies) have found conflicting support for the idea that women’s menstrual cycles influences facial preferences at all [17, 18]. Additionally, some studies have failed to find any associations with preferences for healthy faces and preferences for masculine faces [1, 19], which undermine the idea that females are selecting male faces based on perceptions of health. One possibility that had been neglected until recently is that female facial preferences are being shaped by genetic influences. A number of previous studies of preferences for multiple different traits have identified genetic influences that can account for as much as 50 percent of the variation in an individual’s desire for certain characteristics in potential mates [20, 21].
In a recent study published in Psychological Science (link is external), my colleagues and I investigated whether such genetic influences might underlie female preferences for masculine faces. By measuring these influences, we would also be able to compare their magnitude against the impact of the context-dependent factors suggested by the good-genes hypothesis such as ovulatory cycle and pathogen sensitivity. To do this, we used data collected from thousands of identical and non-identical twins. Because identical twins share all of the same genes while non-identical twins only share half, we can use statistical modeling to estimate genetic effects on many different traits.
We found that the combined effect of contextual factors accounted for less than 1 percent of variation in women’s preferences for facial masculinity. On the other hand, genetic effects accounted for nearly 40 percent of the variation. This means that any differences between two women’s preferences for masculine males is more likely to be due to their genes than any of the previously proposed evolutionary contextual factors.
This doesn’t mean that contextual factors are unimportant or that genes determine whom we’re attracted to though. The majority of variation in facial preferences (around 60 percent) was still unexplained in our study, leaving ample room for alternative explanations and other environmental influences.
What we have shown here is the relative importance of genetic variation in influencing romantic preferences, and more generally, highly complex traits. It’s currently unknown whether these preferences actually push us towards partners who carry these traits. Choice of romantic partner is one of the most important decisions we will ever make in our lifetime, and the idea that our genes could influence this process has enormous ramifications. The next challenge will be explaining how this genetic variation fits into the broader story of our evolution, including how it influences actual partner choice.
1.         Enlow, D.H., R.E. Moyers, and W.W. Merow, Handbook of facial growth. 1982, Philadelphia: Saunders.
2.         Little, A.C., B.C. Jones, and L.M. DeBruine, Facial attractiveness: evolutionary based research. Philosophical Transactions of the Royal Society B-Biological Sciences, 2011. 366(1571): p. 1638-1659.
3.         Zahavi, A., Mate selection: A selection for handicap. Journal of Theoretical Biology, 1975. 53(1): p. 205-214.
4.         Rhodes, G., et al., Does sexual dimorphism in human faces signal health? Proceedings of the Royal Society of London. Series B: Biological Sciences, 2003. 270(Suppl 1): p. S93-S95.
5.         Thornhill, R. and S.W. Gangestad, Facial sexual dimorphism, developmental stability, and susceptibility to disease in men and women. Evolution and Human Behavior, 2006. 27(2): p. 131-144.
6.         Boothroyd, L.G., et al., Partner characteristics associated with masculinity, health and maturity in male faces. Personality and Individual Differences, 2007. 43(5): p. 1161-1173.
7.         Penton-Voak, I.S. and D.I. Perrett, Female preference for male faces changes cyclically: Further evidence. Evolution and Human Behavior, 2000. 21(1): p. 39-48.
8.         Penton-Voak, I.S., et al., Menstrual cycle alters face preference. Nature, 1999. 399(6738): p. 741-742.
9.         Burt, D.M., et al., Q-cgi: New techniques to assess variation in perception applied to facial attractiveness. Proceedings of the Royal Society B: Biological Sciences, 2007. 274(1627): p. 2779-2784.
10.       Little, A.C., et al., Partnership status and the temporal context of relationships influence human female preferences for sexual dimorphism in male face shape. Proceedings of the Royal Society of London.Series B: Biological Sciences, 2002. 269(1496): p. 1095-1100.
11.       Waynforth, D., S. Delwadia, and M. Camm, The influence of women's mating strategies on preference for masculine facial architecture. Evolution and Human Behavior, 2005. 26(5): p. 409-416.
12.       DeBruine, L.M., et al., The health of a nation predicts their mate preferences: cross-cultural variation in women's preferences for masculinized male faces. Proceedings of the Royal Society B-Biological Sciences, 2010. 277(1692): p. 2405-2410.
13.       Little, A.C., L.M. DeBruine, and B.C. Jones, Exposure to visual cues of pathogen contagion changes preferences for masculinity and symmetry in opposite-sex faces. Proceedings of the Royal Society of London.Series B: Biological Sciences, 2011. 278(1714): p. 2032-2039.
14.       DeBruine, L.M., et al., Women's preferences for masculinity in male faces are predicted by pathogen disgust, but not by moral or sexual disgust. Evolution and Human Behavior, 2010. 31(1): p. 69-74.
15.       Little, A.C., et al., Human preferences for facial masculinity change with relationship type and environmental harshness. Behavioral Ecology and Sociobiology, 2007. 61(6): p. 967-973.
16.       Lee, A.J., et al., Human facial attributes, but not perceived intelligence, are used as cues of health and resource provision potential. Behavioral Ecology, 2013. 24(3): p. 779-787.
17.       Gildersleeve, K., M.G. Haselton, and M.R. Fales, Do women's preferences change across the ovulatory cycle? A meta-analytic review. Psychological Bulletin, 2014. 140(5): p. 1205-1259.
18.       Wood, W., et al., Meta-analysis of menstrual effects on women's mate preferences. Emotion Review, 2014. 6(3): p. 229-249.
19.       Boothroyd, L.G., et al., Facial masculinity is related to perceived age but not perceived health. Evolution and Human Behavior, 2005. 26(5): p. 417-431.
20.       Verweij, K.J.H., A.V. Burri, and B.P. Zietsch, Evidence for genetic variation in human mate preferences for sexually dimorphic physical traits. PLoS ONE, 2012. 7(11): p. e49294.
21.       Zietsch, B.P., K.J.H. Verweij, and A.V. Burri, Heritability of preferences for multiple cues of mate quality in humans. Evolution, 2012. 66: p. 1762-1772.

No comments:

Post a Comment