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The multivariate evolution of female body shape in an artificial digital ecosystem

Abstract: Human bodies exemplify complex phenotypes, likely to be subject to complex evolutionary forces. Despite the importance of body shape to health, social interactions and self-esteem, our understanding of body evolution and integration remains simplistically focused on simple ratios like waist-hip ratio (WHR), and body mass index (BMI), or manipulations of one or a few traits. Evolutionary selection analyses give a multivariate perspective, but highly correlated body measures create multicollinearity problems. Here we develop an original approach mimicking Darwinian selection to study how clonal lines of bodies, allowed to vary in 24 attributes via a mutation-like process, evolve in a digital ecosystem over 8 generations. Ten of 24 traits changed by more than one |S.D.| over seven generations of selection. Analyses of selection within generations, change in population mean, and change within clonal family lines all implicate slenderness, particularly narrow waists and long legs as the most important dimension of body attractiveness. WHR did not offer any improvement on waist girth as a predictor of attractiveness. Within the most successful clonal lineages, selection favored greater shapeliness, including larger busts, in addition to slenderness. Our results reveal the complex, multivariate nature of attractiveness, and that the success of simple ratios like WHR and BMI in previous studies is probably incidental to the importance of waist girth and general slenderness. Our results also suggest that the integration of the entire body phenotype is at least as important as any one trait, and that more than one way exists to make an attractive body.

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Alex’s Notes: Two weeks ago we had the pleasure of learning that a man’s preference for a woman’s lumbar curvature is not the result of their butt; it is the result of some damn good vertebral wedging that will help make them less of a liability when pregnant. However, lumbar curving is only one aspect of female body figure that determines attractiveness.

It has been suggested that an hourglass physique characterized by a low waist-to-hip ratio (WHR) serves as a “first-pass filter” for males to signify the female’s youth, health, and fertility. In fact, across the world of industrialized countries, men generally prefer WHRs of 0.6-0.7, although several subsistence farming, horticultural, and hunter-gatherer groups report that men prefer women with more masculine body shapes (WHR > 0.8). Nonetheless, these relationships suffer from the fundamental flaw of overlooking the inter-connectedness of many body features.

While the approach of isolating variables is useful for generating statistical power, it is unrealistic and lacks the capacity to detect how various traits interact to shape the attractiveness of the body. Fortunately, fancy statistical stuff has been created to help overcome these dilemmas. This approach has previously been used to estimate the effects of penis size, torso shape, and height on men’s attractiveness to women.

The current study constructed CGI images of female bodies that had 24 traits experimentally manipulated and set off to “breed” based on attractiveness ratings by males, thus allowing the bodies to “evolve” over multiple generations and allowing for inferences of the most important traits in determining attractiveness to be made.

The researchers started with bodily measurements from 273 women living in the USA, from which they calculated the averages for the 24 traits of interest in the study. They then drew 20 of these women randomly to serve as “progenitor” females. From these, families of five daughter images per progenitor were created through random manipulation of the 24 traits. The magnitude of the change was small, ranging from zero to 0.2 standard deviations from the average. For each progenitor and its five daughters, CGI avatars were made and uploaded to a website where anyone could rate them.

The average attractiveness rating for each of the 120 bodies was calculated, with the 60 lowest getting thrown out and two daughters of the remaining most attractive 60 being made using the same manipulations stated earlier. Thus, a new generation of 120 images was made which was rated and allowed the process to repeat for seven generations.

Overall, over 59,000 individuals participated in rating, with residence coming primarily from Australia, the USA, and various countries within Europe.

Interestingly, men and women gave almost identical ratings for attractiveness for each model. As it so happens, the best statistical model for predicting attractiveness was based on three variables in the beginning of the study: narrower waist, longer legs, and wider hips. The most overweight families were entirely eliminated in the initial screening for attractiveness. Over the course of all generations, girths evolved to be smaller, while limb and/or trunk length evolved to be longer, indicating a universal preference towards more slender bodies.

In fact, both waist and hip girth evolved to be smaller. However, waist girth evolved at a faster rate, leading to a lower WHR. The overall WHR started at 0.81 and ended at 0.69. Once an “acceptable” slenderness was obtained, however, something interesting happened: larger busts, smaller vertical distance from waist to butt, and longer necks all started to become significant predictors of attractiveness. Statistical analysis did in fact reveal that these traits did nothing for attractiveness when entered into a model with waist girth, indicating that they are overruled by the WHR when it comes to attractiveness.

At least in Western cultures, the predominant evolutionary trend is toward greater slenderness, particularly narrower waists. This was accompanied by lengthening of the legs and a reduction in hip girth, resulting in taller and more slender physiques. Within the most successful (and slender) families, selection then favored larger breasts and thighs.

 

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