No Evidence of the Good Genes Hypothesis Found

Using photographs of real men, Peters, et.al (2009) found no evidence of a preference for either masculinized or symmetric male faces or bodies in ovulating women.

Previous studies that have found a relationship between ovulation and attraction to masculine features have used computer-morphed images that are weak in ecological validity. This study used photographs of actual men, like the ones below.

Masculinity, attractiveness, and symmetry ratings of the stimuli appear to approximate a normal distribution, strengthening the ecological validity of this study. The only noteworthy limitation in this design is that there were no objective measurements of masculinity or symmetry – only subjective ratings were used.

The authors were also careful to use precise measurements of ovulation to ensure that the ratings of women in the ovulatory phase were well-within the previously identified six-day long sexually active phase of the menstrual cycle.

Wayne Hooke

Marianne Peters, Leigh W. Simmons, Gillian Rhodes (2009). Preferences across the Menstrual Cycle for Masculinity and Symmetry in Photographs of Male Faces and Bodies PLoS ONE, 4 (1) DOI: 10.1371/journal.pone.0004138

Sexual Dimorphism In Human Male Faces May be the Result of Natural Selection

I’ve just spent the weekend mulling over the implications of a PNAS article: The feeding biomechanics and dietary ecology of Australopithecus africanus. Now this isn’t the usual sort of article that gets discussed on this blog, but I was intrigued by the following images:  

M. Fascicularis (left) and Australopithecus africanus (right).  

What intrigued me about these images was the connection between the areas of these skulls that are subjected to the highest stresses when biting hard objects (the warmer colors) and the areas of the human male face that are larger than predicted by ontogenetic scaling. What is immediately apparent is that the unusually large features in a human male skull are those that (likely) are subjected to the highest strains when biting hard objects. Additionally, human male – compared to female – skulls project farther forward, implying, perhaps, an adaptation to premolar loading.  

This extrapolation from Strait, et.al.’s analysis led me to speculate about the hypothetical circumstances in which these enlarged skull features could evolve in males only via natural selection. To begin, it seems likely that the adaptation would be for extreme circumstances rather than for a preferred food source. That is, there would have to be some compelling connection between the survival of human males and their ability to eat less desirable, harder to chew food. On the assumption that females gathered and males more often hunted, it seems likely that females would tend to stay in areas where foraging for optimal/desirable food would be successful. Assuming reasonable success with foraging: females would not need extremely powerful jaws. Males, on the other hand, might follow game – wherever the trail led. This would no doubt lead them outside of optimal foraging areas at times. If the hunt failed, or went-on for an extended period, males would be forced to eat what they could find along the way. If unsuccessful hunts occurred often enough, males with more versatile, powerful chewing capacity would perhaps survive when others died, simply by virtue of being able to chew through, for example the tough shells and husks of nuts.  

The implication of this speculation is that the larger facial structures of a human male are not clear signs of a successful hunter and powerful protector, but rather indicators that this one is merely more likely to make it back alive after the next failed hunt. The fact that these dimorphic traits exist suggest that hunting was, perhaps, at some point in our evolutionary past, a dangerous and often unsuccessful activity.

Wayne Hooke

David S. Strait, Gerhard W. Weber, Simon Neubauer, Janine Chalk, Brian G. Richmond, Peter W. Lucas, Mark A. Spencer, Caitlin Schrein, Paul C. Dechow, Callum F. Ross, Ian R. Grosse, Barth W. Wright, Paul Constantino, Bernard A. Wood, Brian Lawn, William L. Hylander, Qian Wang, Craig Byron, Dennis E. Slice, Amanda L. Smith (2009). The feeding biomechanics and dietary ecology of australopithecus africanus Proceedings of the National Academy of Sciences

Move Over WHR and BMI: Abdominal Depth Has Entered the Building

Using photographs and videos of actual women, like the one below, Rilling et.al. (2009) have found that abdominal depth (“the depth of the lower torso at the umbilicus”) and waist circumference are stronger predictors of attractiveness than either waist-hip-ratio or body-mass index. I can imagine the lead story in the science section of the local paper now: “Bodies that don’t look pregnant really are attractive.” As we all know, headlines can be misleading and popular presentations of research can gloss over subtle details. Pregnancy is not the only cause of abdominal swelling (there are actually 420 causes of abdominal swelling – none of which seem particularly desirable or healthy).

Abdominal depth measurement is illustrated below (the black line across the participant’s midsection)

while waist circumference is illustrated in the following image.

The stimulus set reveals strengths and weaknesses:

Strengths:

• Un-retouched photographs
• Three-dimensional views
• Blurred faces/heads

Generally, this stimulus set has excellent ecological validity.

Weaknesses:

• Stimuli skewed toward the unattractive end of a ten-point scale (mean of 4.9/10) – making it very hard to draw conclusions about what is actually attractive
• Leotards and raised arms will flatten breasts, making them appear smaller – possibly leading raters to over-rely on apparent waist thickness, especially in profile
• Relatively large mean waist circumferences in the stimuli – mean of 69.1 cm compared to 59.1 in Playboy centerfolds
• Stimulus set contains few figures with optimal waist-hip ratios – making it hard to assess the contribution of the preferred WHR.

In the end, this study does not get us much closer to resolving the WHR/BMI/???? debate. A quick inspection of measures that had statistically significant correlations with attractiveness ratings:  

• Abdominal depth
• BMI
• Chest circumference
• Chest/Underchest ratio
• Hip circumference
• Leg/Stature ratio
• Mid-arm circumference
• Pelvic width
• Stature
• Underchest circumference
• Waist circumference
• WHR  

suggests the perhaps obvious conclusion that raters consider proportionality and size when evaluating the attractiveness of female bodies.  

FYI: Table 1 has a nice summary of correlations between various bodily measures and health. This alone makes the article worthy of a look.

J RILLING, T KAUFMAN, E SMITH, R PATEL, C WORTHMAN (2009). Abdominal depth and waist circumference as influential determinants of human female attractiveness. Evolution and Human Behavior, 30 (1), 21-31 DOI: 10.1016/j.evolhumbehav.2008.08.007

Standardizing Facial Photographs via IPD May Introduce Error

Much of beauty research requires comparing facial measurements from photographs of different subjects. Optimally, the distance from the sensor in the digital camera (or the film plane) to each subject’s face should be identical. Identical head-camera distances ensure that measurements taken from the photographs retain identical proportionality to measurements taken directly from the faces themselves.

When researchers are unable to control the head-camera distance when making the photographs, it is common to standardize the photographs based on interpupillary distance (Little et.al., 2008). Most often, this sort of standardization is required for photographs obtained outside of a laboratory. To illustrate, imagine that the two images below were taken from different head-camera distances.

1. Copyright Christiaan Briggs/Wikipedia Commons/http://commons.wikimedia.org/wiki/File:Sabaa_Nissan_Militiaman.jpg

The image on the left could have been taken with a shorter head-camera distance than the one on the right. Under such circumstances, even though the actual face is identical, any measures made from these images would be different. Of course, any conclusion drawn from comparing these different measurements would be inaccurate.

The usual solution to this problem – standardizing on interpupillary distance – is illustrated below:

By enlarging the smaller image until the distance between the pupils in both images is identical, we ensure that any measures we take from these two images will be identical. Clearly, this procedure works on images like those above: when the image is identical. What happens, though, when head-camera distance is standardized via equating the interpupillary distances of different faces?

The answer depends on how similar the interpupillary distances are between the faces. Problematically, there are significant differences within and between groups (Dodgson, 2004). The data are reported in millimeters.

SEXUAL DIMORPHISM
Sex	Male	Female
Range	52-78	52-76
Mean	64.67	62.31
St. Dev.	3.708	3.599

 

Clearly, matching interpupillary distance between images of men and women will introduce bias and some degree of error. Male faces have a greater mean and standard deviation. Thus, for example, male faces might appear smaller or female faces might appear larger relative to one another. Error can also result from standardizing within sex. Any facial feature whose dimensions vary along with interpupillary distance will be similarly distorted. The sexual dimorphism apparent in this table suggests standardizing images for interpupillary distance can blur significant sex differences measureable in faces of both sexes.

ETHNIC/RACIAL GROUPS
Race	White	Black	Hispanic	Pacific Islander	American Indian	Other
Range	52-75	56-78	57-71	55-71	59-72	56-70
Mean	61.99	65.62	63.54	63.17	65.12	63.26
St. Dev	3.429	3.489	3.104	3.146	4.003	3.315

 

The same sorts of error can be introduced in data when equalizing interpupillary distance across ethnic/racial groups.

In my view, data gathered from images that have been standardized by means of equating interpupillary distance should be interpreted cautiously and tentatively. Further, I recommend that researchers avoid this practice.

 

Dodgson, N.A. (2004). Variation and extrema of human interpupillary distance. In Stereoscopic displays and virtual reality systems, XI. Proceedings of the SPIE, 18-22 January 2004. . A.J. Woods, J.O. Merritt, S.A. Benton & M.T. Bolas (eds.). San Jose California.

Anthony C. Little, Benedict C. Jones, Corri Waitt, Bernard P. Tiddeman, David R. Feinberg, David I. Perrett, Coren L. Apicella, Frank W. Marlowe (2008). Symmetry Is Related to Sexual Dimorphism in Faces: Data Across Culture and Species PLoS ONE, 3 (5) DOI: 10.1371/journal.pone.0002106