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BMI and Self-Rated Attractiveness

July 19, 2011 5 comments

Frederick et.al. (2006), utilizing survey data primarily from readers at msnbc.com found a clear relationship (plotted in the image to the right) between BMI and body satisfaction. In this study, women tended to feel best about their bodies when their BMI was between 17.5-20 and men tended to feel best when their BMI was around 23-24. Using the standard rules-of-thumb for categorizing BMI values, women prefer being slightly underweight to on the low-side of normal weight. Men, on the other hand, prefer being at the higher end of normal weight. While no single illustration can accurately depict BMI (a range of heights/weights/ body types can produce identical BMI scores), the following images (available from BMI-Club) might be helpful.

Optimal BMI range for women’s self-rated body satisfaction (above).

Optimal BMI range for men’s self-rated body satisfaction (above).

Approximation of the point at which each sex’s BMI score predictably results in a “My body is unattractive” self-evaluation (above).

Approximation of the non-optimal BMI level for both women’s and men’s self-rated body satisfaction (above).

Noteworthy observations:

  • men are more satisfied with larger BMI scores (not surprising)
  • optimal BMI scores for both sexes do not predictably produce an “I have a good body” self-evaluation in either sex

Cautions

Conclusions based on the data used in this study may not generalize well to the larger population.

  • web surveys are limited by demographic differences in access to and use of the internet, sampling frame problems, response rate problems, (for external validity) and controlling access (for internal validity) (see Wiersma for a brief, accessible introduction or, e.g., DOI 10.1108/10662240510590360
  • MSNBC.com is the most popular news site on the web; the age-distribution of web news consumers is improving, but is still skewed toward the young and the educated, sex differences exist in preferences for what types of news are pursued, partisan political/ideological differences result in use of different news sources; etc. (Pew Research Center Report)
  • a recent review comparing self-report with objective measures of height and weight found a trend of under-reporting for weight and over-reporting for height in self-reports; with significant levels of variation between studies and widely divergent methods of measuring or estimating height/weight (doi: 10.1111/j.1467-789X.2007.00347.x)
  • for women the mean BMI in this study is 24.2 while the mean self-reported BMI in the NHANES study is 27.2 (and the mean measured BMI in the same study is 28.0)
  • for men the mean BMI in this study is 26.6 while the mean self-reported BMI in the NHANES study is 27.6 (and the measured BMI in the same study is 28.0)

Wayne Hooke

ResearchBlogging.org
FREDERICK, D., PEPLAU, L., & LEVER, J. (2006). The swimsuit issue: Correlates of body image in a sample of 52,677 heterosexual adults Body Image, 3 (4), 413-419 DOI: 10.1016/j.bodyim.2006.08.002

How Much Does Symmetry Influence Attractiveness Ratings?

July 14, 2011 1 comment

Stefan Van Dongen has just published a very nice meta analysis of the relationship between attractiveness ratings and measures of asymmetry. The noteworthy findings include:

  • visible asymmetries are more important to attractiveness ratings than are non visible asymmetries F1,37=7.55 (p=.01)
  • funnel plot analyses indicate a substantial publication bias in the literature
  • studies with large sample sizes show a near zero relationship between attractiveness ratings and asymmetry F1,36=6.97 (p=.01)

Visible vs Not Visible Asymmetries

The distinction here is straightforward: if raters can see the measured asymmetry it is assumed that the asymmetry is visible. For example, if facial asymmetries are measured and faces are evaluated for attractiveness, then the study is categorized as visible. On the other hand, if the asymmetry is in the body and photos of faces are rated, the study is categorized as not visible. This visible/not visible distinction is relevant to why humans find symmetry attractive – for example in comparing a good genes interpretation of symmetry to a processing fluency interpretation. Van Dongen makes the point that the models underlying the good genes interpretation of the role of symmetry on attractiveness require that non-observable symmetry be significantly related to attractiveness ratings.

Publication Bias

Since the value of a comprehensive data analysis is only as good as the data it uses, a check on the quality of the included sources is highly desirable. One such check is a funnel plot. In a funnel plot, each study is plotted for effect size and sample size. Since variation resulting from chance is more likely to be larger in studies with small sample sizes, a visual inspection of the plotted data points for the included studies should show a symmetric distribution around the typical effect size, with more variation in effect size expected for studies using smaller samples (hence the name, funnel plot). The published studies exploring the relationship between attractiveness and symmetry do not reveal the expected funnel-like symmetric shape. While a number of causes for this undesirable result are possible, publication bias is the most likely. A statistical technique used to minimize the effect of publication bias is the trim and fill method (especially useful in cases of publication bias). Van Dongen uses this technique to more accurately estimate the actual effect size of symmetry on beauty ratings.

Sample Size

A meta analysis that shows a decreasing effect size in studies with increasing sample sizes is another indicator of publication bias. The rationale for this conclusion is that there is a preference for publishing research that has found support for a particular hypothesis over research that reports finding no relationship. A manuscript that supports the null hypothesis is typically more interesting when the sample size is larger and thus gets published. Manuscripts that do not show a relationship are left ‘in the file drawer.’

Conclusion

Van Dongen’s overall study results, after accounting for bias, found that there was a significant effect of visible asymmetry on visual attractiveness ratings (r=.15 with a 95% confidence interval of 0.07-0.23). This degree of effect size is typically categorized as small/medium. What that means is that a person of average attractiveness (left side of the highlighted area below – the 50th percentile) who suddenly became more symmetric (by the typical amount of variation in symmetry found in human faces naturally) would now be rated more attractive than 62 percent of other people (the right side of he highlighted area below).

An illustration of the typical impact of symmetry on attractiveness.

Another way to illustrate this degree of change is via a beauty rating scale. For ease of estimation, imagine an 8-point rating scale (from 0-8) that is normally distributed with 4 as the average, typical score. This degree of change would take the average person’s rating from a 4 to a 4.3. On this same scale, a person who is in all other respects rated average for attractiveness, but who had an exceptionally high degree of symmetry, would likely be rated a 5 rather than a 4 (this degree of symmetry is expected only in about 1/1000 people. It is important to note that for the illustrations above to hold, the changes in symmetry must be visible. Van Dongen’s meta analysis found no relationship between attractiveness ratings and the symmetry measures of features that are not visible to the person doing the rating.

Cautions

One significant limitation in the symmetry/attractiveness literature is that the data primarily come from western, college student samples – limiting our ability to generalize these conclusions to other populations.

Wayne Hooke

ResearchBlogging.orgVan Dongen, S. (2011). Associations between asymmetry and human attractiveness: Possible direct effects of asymmetry and signatures of publication bias Annals of Human Biology, 38 (3), 317-323 DOI: 10.3109/03014460.2010.544676

Categories: General, The Body, The Face

Can’t Fight the Feeling

May 3, 2010 Leave a comment

A recent study (Karremans, et.al. 2010) that compared the preferences of blind and sighted men for the shape of adjustable dress forms with one of two WHRs (.70 and .84) has been getting some coverage in the popular press. Nineteen blind from birth adult males (aged 27-72 with a mean of 45.5) and 38 sighted males (with similar age range, variance, and means) were included in the study. Nineteen of the sighted men were randomly assigned to a blindfold condition and 19 were in a sighted condition. After feeling the mannequin with their hands, while paying special attention to the waist/hip area, all subjects rated the two dress forms on a 1-10 scale for attractiveness (only men in the sighted condition saw the mannequin). While the authors do not report combined means, the mannequin with the .70 whr averaged about a 7.5 rating while the mannequin with the .84 whr averaged a rating of about 6.5.

While there were no statistically significant differences between any of the group ratings (all 3 groups preferred the smaller whr), the authors emphasize that there were substantially different effect sizes between the blind and sighted subjects with regard to the strength of whr preference. A significant portion of the discussion section elaborates on the implications of these differences in effect size. Problematically, these sorts of effect sizes can readily result from chance in samples of this size, suggesting to me that the discussion of what these effect size differences might mean is superfluous (Fan, 1999).

The authors do reasonably conclude that, pending replication, the visual channel does not seem required to establish a preference in males for a smaller whr in females. This result suggests two interesting possibilities:

  • arguments based on visual characteristics of the whr preference in males may require significant qualification
  • since whr preference is not sensory channel specific (that is, it is not limited to the visual channel) it seems less likely that a specifically evolved mechanism is behind this preference

Strengths

  • the authors offer 3 possible classes of explanation for their data and do not overly emphasize the possibility of an evolved disposition

Cautions

  • Only two whrs were used: .70 (a reasonable optimal) and .84 (a “typical” higher value)

Wayne Hooke

ResearchBlogging.orgKarremans, J., Frankenhuis, W., & Arons, S. (2010). Blind men prefer a low waist-to-hip ratio Evolution and Human Behavior, 31 (3), 182-186 DOI: 10.1016/j.evolhumbehav.2009.10.001

Categories: The Body

Brain Scans Confirm: Men Like Curves in Women

March 6, 2010 1 comment

Platek & Singh (2010) report that stimuli depicting optimal waist-hip ratios (~0.70) activate the “reward center” in men’s brains; while stimuli depicting body mass index do not. They conclude that BMI preferences are therefore more culturally determined and, by suggestion, that WHR preferences are the result of evolved psychological mechanisms. I will point-out at the outset: this conclusion is not supported by their data.

An example of the stimuli used in this research appears to the right (from Singh & Randall, 2007). The stimuli consist of partial body photographs of women who have had fat surgically removed from their abdomens and surgically implanted into their buttocks. The result is that each woman more-or-less maintains the same BMI while exhibiting a more attractive WHR. (For a criticism of over-interpreting the significance of the current studies using micrograft surgery, click here). In this study, fMRIs were taken while men viewed these images. Platek & Singh found that “reward centers” in the brain were activated by stimuli depicting more optimal WHRs; while stimuli depicting variation in BMI did not.

Cautions

  • No stimuli depicted an optimally attractive BMI (range – apparently – was from 21.13-26.36; optimally attractive BMI is ~18-19)
  • Stimuli consisted of only a limited area of each woman’s body
  • Activation of the “reward center” only indicates that the viewer finds the stimulus rewarding
  • Activation of the “reward center” does not imply innate preferences or the activation of an evolved psychological mechanism
  • Caution should be maintained in interpreting fMRI data involving “reward centers” in the brain as a recent study of test-retest reliabilities in these measurements revealed correlation coefficients (ICCs) ranging from -0.15-0.44) (Fliessbach et.al., 2010)
  • Remember that arguments – even weak ones – which utilize topographical depictions of brain activity can be unusually persuasive (McCabe, 2007)

Strengths

  • This study provides some objective confirmation that men’s attractiveness ratings correspond to the activation of “reward centers” in the brain

The methodology used here does not provide a convincing head-to-head comparison of the relative importance of BMI or WHR in attractiveness ratings. It also does not provide evidence of an innate preference for WHR nor of a cultural influence on BMI preference.

Wayne Hooke

ResearchBlogging.orgFliessbach, K., Rohe, T., Linder, N., Trautner, P., Elger, C., & Weber, B. (2010). Retest reliability of reward-related BOLD signals NeuroImage, 50 (3), 1168-1176 DOI: 10.1016/j.neuroimage.2010.01.036

MCCABE, D., & CASTEL, A. (2008). Seeing is believing: The effect of brain images on judgments of scientific reasoning☆☆☆ Cognition, 107 (1), 343-352 DOI: 10.1016/j.cognition.2007.07.017


Steven M. Platek, & Devendra Singh (2010). Optimal Waist-to-Hip Ratios in Women Activate Neural Reward Centers in Men PLoS ONE : 10.1371/journal.pone.0009042


Singh, D., & Randall, P. (2007). Beauty is in the eye of the plastic surgeon: Waist–hip ratio (WHR) and women’s attractiveness Personality and Individual Differences, 43 (2), 329-340 DOI: 10.1016/j.paid.2006.12.003


Categories: Neuroscience, The Body

More on BMI v. WHR

February 6, 2010 4 comments

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


Categories: Methodology, The Body

Caution: Curves Ahead

January 24, 2010 2 comments

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


ResearchBlogging.org

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.


Categories: Methodology, The Body

Why Do We Think We Like Hourglass Figures?

January 18, 2010 7 comments

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.


ResearchBlogging.orgLaw 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

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