More on BMI v. WHR

Piers Cornelissen has offered some pointed objections to concluding that WHR (waist-hip ratio) is more important than BMI (body mass index) in the evaluation of women’s body attractiveness. He posted these objections as a response to Caution: Curves Ahead. Since comments are easily overlooked on this blog due to the nature of the site’s formatting, I have decided to highlight his argument by reproducing it here.

A few points regarding Caution: Curves Ahead

1) BMI and WHR range effects.
Smith et al. (Smith, K.L., Cornelissen, P.L. & Tovée, M.J. (2007) Colour 3D Bodies and Judgements of Human Female Attractiveness. Evolution and Human Behaviour, 28, 48-54) used colour videos of women, who were rotated twice through 360 deg, as stimuli in an attractiveness rating paradigm. The relative range for WHR and BMI in these stimuli can be described by looking at the range of their z-scores: -2.55 to 2.72 and -1.70 to 1.89 respectively. In other words, there was more variability in WHR than BMI. However, the correlations between attractiveness and WHR / BMI in this study were -0.28 (p=0.06) and -0.73 (p<0.0001) respectively.

A similar result was reported in Tovee et al. (Tovée, M.J., Warren, T.T.L., Hancock, P. & Cornelissen, P.L. (2002). Visual cues to female attractiveness: Waveform analysis of body shape. Proceedings of The Royal Society, B Vol. 269, No. 1506., 2205-2213). In this study, using 2D gray level images in front view, stimuli were picked deliberately so that the WHR range outweighed the BMI range by a factor of ~3.

In conclusion, using videos / photos of whole bodies, we have repeatedly found that we can’t get WHR to work in explaining attractiveness ratings even when it has a bigger relative range than BMI.

2) False positives.
If Caucasian males were to rely primarily on WHR for mate choice when judging the bodies of potential partners, they would be prone to making false positive errors; sometimes they would pick women with amenorrhoea as partners who are infertile. As Fig. 1 shows in Tovee et al. (Tovée, M.J. and Cornelissen, P.L. (1999) Visual cues to female physical attractiveness. Proceedings of the Royal Society of London Series B-Biological Sciences, 266(1415), 211-218), it is perfectly possible to find individual females with optimally attractive WHRs but who have extremely low body fat, and who have amenorrhoea as a consequence. So, it is hard to see how WHR alone can be treated as a reliable signal.

3) Micrograft surgery BMI / WHR manipulations.
Clearly this is an elegant paradigm in principle. Indeed, based on their Fig. 2A, I would be hard pushed *not* to agree that the post-operative figures in Dixson et al. are more attractive (Dixson, B., Sagata, K., Linklater, W., & Dixson, A. (2009). Male preferences for female waist-to-hip ratio and body mass index in the highlands of Papua New Guinea American Journal of Physical Anthropology DOI: 10.1002/ajpa.21181). However, my reason for this decision would have to include the fact that the post-operative images are aesthetically more pleasing because they are rounder and smoother. This image property was neither quantified nor coded qualitatively in any way, and no such measures were included in the analyses either as outcome variables or covariates. Moreover, from an image analysis point of view, a smoothness metric could be obtained without having anything to do with WHR. Therefore, it would be useful to check that roundness / smoothness is not a confound in future research of this kind.

Secondly, even if the conclusion from these experiments is correct, we still have to explain why WHR fails as a cue when photos/videos of the *whole* body are available. Are we really suggesting that the answer lies in males *only* making their mate selection choice when they have a close up view – because that is the implication.

Oh snap!

Wayne Hooke

Testosterone is Not a Handicap

Many contemporary beauty researchers assume/conclude that attractive, sexually dimorphic features in men (strong jaws, increased lean muscle mass, etc.) are true signals of mate quality. This model is best illustrated in peacock tail feathers: the size and color of the train makes the male more sexually attractive to peahens. Rather than being just an attractive, but functionally useless ornament, this weighty handicap is an honest signal to peahens of the male’s quality – because this tail is so costly to possess. Any peacock that can thrive with this large disadvantage must be well-adapted – that is, possess good genes.

Testosterone inhibits immune functioning. Given this, the basic argument is that any potential mate with testosterone-related features must have “good genes” in order to thrive with the testosterone handicap. One big problem with the testosterone as a handicap hypothesis is that testosterone is not merely an immunity suppressant. Testosterone produces benefits as well:

The primary fitness benefits of testosterone are multifaceted, including support for optimal spermatogenesis, the development and maintenance of secondary sexual characteristics that augment male competitiveness and attractiveness, as well as libido. Other associations include competitive ability and possible relationships with social dominance. (Bribiescas & Ellison, p. 104)

Thus, testosterone’s effects involve trade-offs: there are pros and cons to having testosterone coursing through one’s veins.

The second big problem with this hypothesis is that there is no evidence for this phenomenon in mammals. Roberts, et.al. (2004) found evidence for the immunocompetence handicap hypothesis (what I am calling the testosterone as handicap hypothesis) in birds and reptiles, but not mammals. Nunn, et.al. (2008) subsequently corroborated this conclusion with regard to mammals.

Arguments which assume sexually attractive, dimorphic features in human males are costly handicaps – and thus true signals – are overlooking the complex influence of testosterone in human adaptations as well as overlooking the lack of evidence for applying this concept to mammals generally and humans specifically.

Wayne Hooke

Image courtesy of Rhodney Carter, Wikipedia Commons.

Richard G. Bribiescas & Peter T. Ellison (2008). How hormones mediate trade-offs in human health and disease. (77-94). Evolution in Health and Disease, Stearns & Koella (eds.).

Nunn, C., Lindenfors, P., Pursall, E., & Rolff, J. (2009). On sexual dimorphism in immune function Philosophical Transactions of the Royal Society B: Biological Sciences, 364 (1513), 61-69 DOI: 10.1098/rstb.2008.0148

ROBERTS, M., BUCHANAN, K.L., & EVANS, M.R. (2004). Testing the immunocompetence handicap hypothesis: a review of the evidence Animal Behaviour, 68 (2), 227-239 DOI: 10.1016/j.anbehav.2004.05.001

Static & Dynamic – or is it Static vs. Dynamic?

Most beauty research is done using static stimuli: static photographic images are used rather than, for example, dynamic video depictions. Since most real-world interactions with others do not involve static presentations, it is important to know how similar these two types of attractiveness ratings typically are.

Roberts, et.al. (2009) have recently reported finding a positive correlation between attractiveness ratings of static images (neutral photographs) and dynamic images (video recordings). Only a small number of studies have looked at this relationship, with mixed results:

A review of this table shows inconsistent findings in between subjects designs. The Roberts, et.al. study is the first to date to use a within subject design to examine this relationship. The table below shows the various correlations between ratings of static and dynamic stimuli utilizing a within subject design. As you can clearly see, there are strong correlations in this data set.

Strengths

• The use of a novel design for exploring this research question.

Cautions

• Stimuli and raters were from college student samples
• As the authors acknowledge, within subject designs – which involve repeated measures – are subject to carry-over effects (the alteration in the rating of the second stimulus (e.g., the video recording) that is a result of having seen the first stimulus (e.g., the neutral photograph).
• The nature of the task performed for the video recording can have an effect on the correlation. In this study, correlations were stronger between the static image and the dynamic images when the stimuli involved a hypothetical self-introduction in a bar setting than when the topic of the dynamic recording was a recent holiday.

In defense of the common use of static imagery in beauty research it should be emphasized that static depictions enable better control over extraneous variables and enable clearer comparisons of the influence of purely structural bodily/facial characteristics.

Wayne Hooke

Roberts, S., Saxton, T., Murray, A., Burriss, R., Rowland, H., & Little, A. (2009). Static and Dynamic Facial Images Cue Similar Attractiveness Judgements Ethology, 115 (6), 588-595 DOI: 10.1111/j.1439-0310.2009.01640.x

Caution: Curves Ahead

In my last posting, I found myself musing about how WHR would influence ratings of body attractiveness if BMI was held constant. Recent research comparing the relative roles of BMI and WHR have tended to support a more prominent role for BMI over WHR. That is, the total amount of body fat seems to matter more than how that body fat is distributed. One recent study (Cornelissen, et.al., 2009) claims to have resolved the debate, concluding:

that although WHR appears to be an important predictor of attractiveness, this is largely explained by the direct effect of total body fat on WHR, thus reinforcing the conclusion that total body fat is the primary determinant of female body shape attractiveness.

I have found 3 recent or in press publications that have in many ways addressed my question [Singh, et.al. (in press); Dixson, et.al. (2010); and Dixson et.al. (in press)] and each reaches the opposite conclusion from Cornelissen; WHR is more important than BMI in determining female body attractiveness. Each uses before/after images of micrograft surgery in which fat is removed from the waist and implanted in the buttocks/hips (producing results similar to the liposuction on the right). This cosmetic surgery minimally impacts BMI but does reduce WHR. Using this methodology, each study concludes that WHR has a greater influence on attractiveness ratings than BMI.

Strengths

• Novel methodology
• Results found in several cultures: China (Dixson et.al. (in press); Papua New Guinea (Dixson et.al. (2010); Samoa, Komodo Island, Cameroon, and New Zealand (Singh et.al. (in press)

Cautions

• Not all before/after stimulus images show that a reduced WHR is more attractive to raters. WHR does not explain all of the variation in ratings.

Dixson et.al. (2010) suggest that studies which have found BMI to be more important than WHR have used stimuli with a wide range of BMI’s and a relatively restricted range of WHR’s – which likely would have the effect of inflating the influence of BMI. These three studies in effect do the reverse: use an expanded WHR range and a reduced BMI range: not surprisingly, they find the reverse outcome. It looks like this debate isn’t resolved after all….

Wayne Hooke

Photo courtesy of Dr. Mordcai Blau and David A. Copeland 2009

CORNELISSEN, P., TOVEE, M., & BATESON, M. (2009). Patterns of subcutaneous fat deposition and the relationship between body mass index and waist-to-hip ratio: Implications for models of physical attractiveness Journal of Theoretical Biology, 256 (3), 343-350 DOI: 10.1016/j.jtbi.2008.09.

Dixson, B., Sagata, K., Linklater, W., & Dixson, A. (2009). Male preferences for female waist-to-hip ratio and body mass index in the highlands of Papua New Guinea American Journal of Physical AnthropologyDOI: 10.1002/ajpa.21181

Dixson, B., Baoguo, L., & Dixson, A. (in press).  Female waist-to-hip ratio, body mass index and sexual attractiveness in China.  Current Zoology.

Singh D, Dixson BJ, Jessop TS, Morgan B, Dixson AF. (in press). Cross-cultural consensus for waist- to-hip ratio and women’s attractiveness. Evol Hum Behav.

Why Do We Think We Like Hourglass Figures?

BMI – the ratio of body mass to height, typically correlates well with ratings of body attractiveness. WHR – a direct comparison of waist and hip measurements – also correlates with attractiveness. Recent research that compares the relative strengths of the two ratios generally finds that variation in BMI accounts for a greater proportion of variation in attractiveness ratings than does variation in WHR. The implication is that, at least in contemporary industrial/technological societies, levels of body fat matter more than how that body fat is distributed. I found myself reflecting on these ratios in relation to women’s body attractiveness today, and wondered how WHR would influence ratings of body attractiveness if BMI was held constant? My guess was that WHR would be more strongly correlated with attractiveness ratings when controlling for BMI in this way. (I couldn’t recall a study that explored this possibility and I also could not find one in the literature – if you know of one please post a link or citation.) My rationale was that if subjects are matched for BMI, then WHR variation would likely result from variation in estrogen efficacy. My hypothesis was that, other things being equal, curviness resulting from estrogen efficacy would more strongly influence attractiveness ratings.

So far my thinking has been pretty predictable. Then I reflected on estrogens’ role in developing the sexually dimorphic features that are found attractive in women’s faces (Smith, et.al, 2006). That’s when I realized that, to date, comparisons of WHR and BMI are done on ratings of body attractiveness alone. This practice is sensible, since cognitively, evaluations of faces and bodies are separate processes. But, since estrogens significantly influence both facial attractiveness and body attractiveness, these two ratings should be related. [There is some support for this relationship (Thornhill & Grammer, 1999).]

These musings leave me wondering: might WHR be a better predictor of overall attractiveness than BMI in women?

Wayne Hooke

Image of the 3rd century Bikini Girls mosaic from the Villa Romana in Sicily courtesy of Roundtheworld. Wikipedia Commons.

Law Smith, M., Perrett, D., Jones, B., Cornwell, R., Moore, F., Feinberg, D., Boothroyd, L., Durrani, S., Stirrat, M., Whiten, S., Pitman, R., & Hillier, S. (2006). Facial appearance is a cue to oestrogen levels in women Proceedings of the Royal Society B: Biological Sciences, 273 (1583), 135-140 DOI: 10.1098/rspb.2005.3296

Thornhill, R. (1999). The Body and Face of Woman One Ornament that Signals Quality? Evolution and Human Behavior, 20 (2), 105-120 DOI: 10.1016/S1090-5138(98)00044-0

Sexual Orientation, Sociosexuality, and Sexual Dimorphism

Using digitally manipulated levels of sexual dimorphism in human male and female faces (like the ones to the right), Glassenberg et.al. (2009) found that, compared to heterosexual women, homosexual women preferred greater masculinization in female faces [Brown-Forsythe t(303.38) = -2.92, p<.01] while heterosexual women preferred greater masculinization in male faces [t(375) = 6.77, p<.001]. Compared to heterosexual males, homosexual males preferred masculinization in both male and female faces [t(520) = -7.42, p<.001 and t(520) = -6.72, p<.001 respectively]. Calculations based on sociosexual orientation were mostly non-significant, though relatively small, significant, positive correlations were found in heterosexual males between unrestricted SO and a preference for feminization in female faces [R(125) = .20, p<.05] while homosexual males showed a positive correlation between unrestricted SO and a preference for masculinized male faces [R(259) = .17, p<.001]. These specifics aside, all raters preferred feminized female faces to masculinized female faces.

Strengths

• large sample

Concerns

• rated stimuli consisted of 3 face composites to ensure recognizable individuality. There was no effort to match stimuli for attractiveness prior to manipulating sexual dimorphism, so an attractiveness x dimorphism interaction would be missed in this design.

This study suggests that homosexuals’ preferences are neither identical to nor mirror-images of heterosexuals’ preferences. This data also suggests that researchers should control for sexual orientation when conducting attractiveness studies in which sex/gender are relevant variables.

Wayne Hooke

Glassenberg, A., Feinberg, D., Jones, B., Little, A., & DeBruine, L. (2009). Sex-Dimorphic Face Shape Preference in Heterosexual and Homosexual Men and Women Archives of Sexual Behavior DOI: 10.1007/s10508-009-9559-6

The Price of Beauty

Jessica Simpson will be hosting a documentary/reality series that explores cultural differences in beauty and fashion – apparently by highlighting individual women in different cultures around the world. The series is being broadcast in the US on VH1 and will first air on Monday, March 15th at 10:00pm. I can’t speak to the content, but the series might be interesting/worthwhile for readers of this blog.

Wayne Hooke