Original cartoon by Alex Martin
Source: Original cartoon by Alex Martin

Several studies  —  mostly for women and rarely for men  —  have attempted to identify body shapes that the opposite sex rates as attractive. A common aim is to identify specific features that possibly evolved as signals indicating mate breeding potential. But could such simple indicators really be keys to the complex process of human partner choice?

Courtship signals

I vividly remember behaviour lectures by my former mentor Niko Tinbergen fifty years ago. Particularly enthralling was his pioneering research into courtship in a humble fish, the three-spined stickleback. As the breeding season begins, an adult male establishes a territory in shallow water and builds a tunnel-like nest with scraps of vegetation over a small hollow. For any passing female with an egg-swollen belly, he performs a zig-zag dance, first swimming towards her and then guiding her to the nest. The female swims through the tunnel, depositing scores of eggs, and the male follows to fertilize them. Afterwards, he fans water through the nest round the clock to aerate the eggs.

This courtship sequence led Tinbergen to recognize the sign stimulus  —  a simple signal evoking a specific response. A male stickleback in his breeding territory develops a bright red colour on his breast, which both attracts females and triggers aggression from other males. Similarly, a female’s egg-laden belly is a sign stimulus eliciting male courtship. Using crude dummies replicating only essential features, Tinbergen showed that a red-throated dummy “male”, moved in a zig-zag fashion, attracts a female to a nest, while a swollen-bellied dummy “female” evokes male courtship. Indeed, Tinbergen showed that an exaggerated signal  —  a supernormal stimulus  —  could be even more effective. For instance, a dummy “male” with a brighter-than-normal red breast evoked stronger aggression from test males.

Releasing signals in women?

 Victoria and Albert Museum (This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1923.)
Woman wearing a wasp waist corset (1890)
Source: From: Victoria and Albert Museum (This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1923.)

Although human behaviour is vastly more complex, researchers have sought comparable signals in women. In a standard test subjects are asked to rate attractiveness of 2-dimensional images. Following two seminal papers by Devendra Singh in 1993, attention focussed on the ratio between waist and hip widths in a woman’s body outline, reflecting body fat distribution. Waist:hip ratios (WHRs) barely overlap between the sexes. Typical healthy ranges are 0.67-0.80 for premenopausal women and 0.85-0.95 for men. Noting that “all theories of human mate selection based on evolutionary principles assume that attractiveness provides a reliable cue to a woman's reproductive value  ……… ”, Singh’s initial studies indicated that men generally rated female figures with low WHR around 0.7 as more attractive than any with higher values.

Extreme exaggeration of the hourglass shape in infamous 19th Century “wasp-waist” corsets has been interpreted as a supernormal stimulus enhancing female beauty. Paradoxically, however, corpulent “Venus” figurines from the Palaeolithic  —  with WHR ratios around 1.3  — have been interpreted in a similar way.

//creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Venus of Willendorf, on display at the Naturhistorisches Museum, Vienna
Source: From: Figure from Wikimedia commons; author: Captmondo or By Don Hitchcock (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Subsequent studies broadly confirmed that men generally rate women’s body shapes with WHR between 0.6 and 0.8 as most attractive. Moreover, preference for low WHR is consistent across several different populations and cultures. In Primate Sexuality, Alan Dixson records preferred WHR values of 0.6 for Chinese university students and Hadza hunter-gatherers of Tanzania, 0.7 for Indians and Caucasian Americans, and 0.8 for men in Bakossiland, Cameroon. In a 2010 paper, Barnaby Dixson and colleagues used eye-tracking to assess men’s preferences for women’s WHR and breast size. They recorded initial fixations and dwell times for men viewing front-posed images of the same woman manipulated to differ in WHR (0.7 or 0.9) and breast size. Within 200 milliseconds of the start of each test, either the breasts or the waist evoked initial visual fixation. Images with a WHR of 0.7 were rated as most attractive, regardless of breast size.

Unpublished image from the study by Dixson et al. (2010) kindly supplied by Barnaby Dixson
Men’s attractiveness ratings based on images of a woman manipulated to have different waist:hip ratios.
Source: Unpublished image from the study by Dixson et al. (2010) kindly supplied by Barnaby Dixson

In a 1998 communication, however, Douglas Yu and Glenn Shepard reported that male preference for women with low WHR might not be culturally universal. Noting that “every culture tested so far has been exposed to the potentially confounding influence of western media”, these authors assessed preferences in a culturally extremely isolated population of the indigenous Matsigenka people of southeastern Peru. Matsigenka men preferred outlines with high WHR, describing this almost tubular shape as healthier. In tests of other villagers on a gradient of increasing westernization, WHR preferences progressively approached those reported for westernized countries. Yu and Shepard concluded that previous tests “may have only reflected the pervasiveness of western media”. But this study is problematic because men were asked to rate westernized outlines from Singh’s original studies rather than culturally more appropriate figures.

WHR versus body mass?

The widespread statistical problem of confounding variables is also an issue (see my July 12, 2013 post The Stork-and-Baby Trap). Some other factor might account for associations between low WHR and attractiveness ratings. It has, for instance, been proposed that the real driving influence is body mass index (BMI).

In 2011, Ian Holliday and colleagues used multivariate analysis of female bodies to construct computer-generated 3-dimensional images that differed according to either BMI or WHR. Attractiveness ratings by both sexes reportedly correlated with differences in BMI but not in WHR. Brain scans recorded with functional MRI during testing revealed that changing BMI modulated activity in parts of the brain reward system. It was concluded that body mass, not body shape, actually drives attractiveness.

Yet in 2010, a cross-cultural study reported by Devendra Singh, Barnaby Dixson, Alan Dixson and others had yielded contrasting results. These authors allowed for possible effects of BMI by using test photographs of women who had undergone cosmetic micrograft surgery to narrow waists and re-shape buttocks, directly changing WHR. In all cultures tested, men judged women with low WHR as more attractive regardless of increases or decreases in BMI.

Other grounds for caution

Interpretations of any simple indicator of women’s attractiveness such as WHR are questionable. Rudimentary 2D representations of the female body commonly used in tests are greatly oversimplified compared to complex 3D reality. Moreover, body outlines are mainly shown in frontal view. Little is known about men’s responses to rear or side views, let alone to the overall 3D reality.

In a 2009 paper, James Rilling and colleagues used a more comprehensive testing procedure involving 3D videos and 2D still shots of real female models rotating in space. Analysis indicated that abdominal depth and waist circumference were the strongest predictors of attractiveness, surpassing both WHR and BMI.

One prime candidate for frontal signalling  —  the tuft of pubic hair that develops at puberty and marks the transition to womanhood  —  is rarely considered. A notable exception is a recent study by Christopher Burris and Armand Munteanu of male undergraduate students that, among other things, assessed responses to marked variation in female pubic hair. Remarkably, complete absence of pubic hair was rated as most arousing overall. This was interpreted with a convoluted hypothesis linking expansive pubic hair in women to high testosterone levels and infertility and attributing higher ratings to men more positively disposed to female sterility. But a crucial, disturbing point passed unmentioned: In any realistic evolutionary setting, a complete lack of pubic hair must surely signal infertility due to immaturity. How might one possibly explain the popularity of Brazilian bikini waxing in evolutionary terms?

Regardless of details, we should be wary of any evolutionary explanation that reduces complex human interactions to the simple stimulus-response behaviour of sticklebacks.

References

Burris, C.T. & Munteanu, A.R. (2015) Greater arousal in response to expansive female pubic hair is linked to more positive reactions to female sterility among heterosexual men. Canadian Journal of Human Sexuality 24: DOI: 10.3138/cjhs.2783.

Dixson, A.F. (2012) Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings (Second Edition). Oxford: Oxford University Press.

Dixson, B.J., Grimshaw, G.M., Linklater, W.L. & Dixson, A.F. (2010) Eye-tracking of men’s preferences for waist-to-hip ratio and breast size of women. Archives of Sexual Behavior 40:43-50.

Holliday, I.E., Longe, O.A., Thai, N., Hancock, P.B. & Tovée, M.J. (2011) BMI not WHR modulates BOLD fMRI responses in a sub-cortical reward network when participants judge the attractiveness of human female bodies. PLoS One 6(11):e27255.

Rilling, J.K., Kaufman, T.L., Smith, E.O., Patel, R. & Worthman, C.M. (2009) Abdominal depth and waist circumference as influential determinants of human female attractiveness. Evolution and Human Behavior 30:21-31.

Singh, D. (1993) Adaptive significance of female attractiveness: role of waist to hip ratio. Journal of Personality and Social Psychology 65:293-307.

Singh, D. (1993) Body shape and women's attractiveness: the critical role of waist-to-hip ratio. Human Nature 4:297-321.

Singh, D., Dixson, B.J., Jessop, T.S., Morgan, B. & Dixson, A.F. (2010) Cross cultural consensus for waist-hip ratio and women's attractiveness. Evolution and Human Behavior 31:176-181.

Tinbergen, N. (1951) The Study of Instinct. Oxford: Clarendon Press.

Yu, D.W. & Shepard, G.H. (1998) Is beauty in the eye of the beholder? Nature 396:321-322.

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