Abstract: This paper reports independent studies supporting the proposal that human standards of attractiveness reflect the output of psychological adaptations to detect fitness-relevant traits. We tested novel a priorihypotheses based on an adaptive problem uniquely faced by ancestral hominin females: a forward-shifted center of mass during pregnancy. The hominin female spine possesses evolved morphology to deal with this adaptive challenge: wedging in the third-to-last lumbar vertebra. Among ancestral women, vertebral wedging would have minimized the net fitness threats posed by hypolordosis and hyperlordosis, thereby creating selective pressures on men to prefer such women as mates. On this basis, we hypothesized that men possess evolved mate preferences for women with this theoretically optimal angle of lumbar curvature. In Study 1, as hypothesized, men’s attraction toward women increased as women’s lumbar curvature approached this angle. However, vertebral wedging and buttock mass can both influence lumbar curvature. Study 2 thus employed a forced-choice paradigm in which men selected the most attractive woman among models exhibiting the same lumbar curvature, but for different morphological reasons. Men again tended to prefer women exhibiting cues to a degree of vertebral wedging closer to optimum. This included preferring women whose lumbar curvature specifically reflected vertebral wedging rather than buttock mass. These findings reveal novel, theoretically anchored, and previously undiscovered standards of attractiveness.
Alex’s Notes: One would think that human civilization has evolved past primal instincts. In fact, compared to other species for which attractiveness is a measure of survival and reproductive ability, it is not far-fetched at all to believe that human standards of beauty are arbitrarily dependent on sociocultural aspects alone. But as we know by now, we are not so different from our ancestors and many if not all biological and psychological pulses still run through our blood.
So what does a man look for in a woman’s body? The answer may very well lie in the spine. Evolving into bipeds presented a unique adaptive challenge for females during pregnancy. The classic baby-bump causes a significant forward shifting of the female’s center of mass (COM). If this COM were not somehow moved back atop the hips, then ancestral women would have been subjected to a nearly 800% increase in hip torque during pregnancy, which would have left the women fatigued and sore. This in turn would have reduced their ability to gather food and put their family at risk of nutritional deficits.
Yet women possess a unique adaptation to deal with this problem: they possess wedging in the third-to-last lumbar vertebra that helps pregnant women shift their COM back over their hips. But moderation is critical, as both insufficient and excessive lumbar curvature are associated with low-back pain. Therefore, it only makes sense that evolutionary pressures and natural selection would have favored women who had sufficient wedging to shift their COM back over the hips during pregnancy, as well as sufficient skeletal and muscular reinforcement to prevent hyperlordosis, fatigue, and spinal injury.
Men who selected these women would have gained considerable benefits, including a mate less vulnerable to injury, better able to forage during pregnancy, and more likely to sustain multiple pregnancies without injury. But ancestral men had no recognition of the vertebra or spine, leaving only observable cues such as the curvature of the lower back to guide their mate preferences. Thus, in the spirit of evolutionary psychology, a first study was performed where 102 undergraduate men rated the attractiveness of female models varying in lumbar curvature.
Wedging in the lumbar vertebrae extends the tailbone relative to the rest of the spine. Anyone can do it by “sticking” their butt out; you should notice a contraction in the lower back musculature as you enter hyperlordosis. The researchers took 15 silhouettes of a women figure (imaged from the side) and generated five morphs of each, varying in lumbar curvature. The curve range was based on what is naturally occurring in the human population and ranged from 14° to 69°. All the images were presented to each male participant in random order and were rated on a ten-point scale for attractiveness.
Women are more attractive the closer to the optimal 45.5° angle their lumbar spine creates.
Is it coincidence that this optimal angle is the middle-ground of what is clinically defined as hyper- and hypolordosis? I think not, but this importance of lumbar curvature does not demonstrate that preference was for the vertebral wedging, per se. Another explanation could be a large booty that gives the impression of wedging through increased lumbar curvature. Thus, a second study was performed.
Using the same participants, side images of women silhouettes were once again viewed and rated, only this time, all the morphs had identical butt protrusion, with the difference being how the protrusion came to be. Twelve silhouettes, each of which consisted of three morphs of a single female were viewed and rated by every participant. The first morph exhibited a muscular butt associated with physical fitness. The second morph exhibited a fat butt associated with deposition of adipose tissue around the hips, a cue to fertility in women. The buttock protrusion of the third morph, however, reflected lumbar vertebral wedging. Moreover, the physical fitness and vertebral wedging morphs exhibited the same external lumbar curvature.
“Because all three morphs within each array exhibited the same protrusion of the buttocks, any differences in attractiveness between morphs could not be attributable to buttock protrusion. Further, because the physical fitness and vertebral wedging morphs within a given array possessed the exact same angle of external lumbar curvature, any differences in attractiveness between these morphs could not be attributable to curvature angle per se, but rather must be due to the underlying morphological cause (buttock mass vs. vertebral wedging).”
Once all these images were generated (using Adobe Photoshop no less), four conditions for each were created in which the angle of the lumbar curvature was 9° or 4.5° below or above optimum.
Men preferred women whose buttock protrusion reflected vertebral wedging, not butt mass.
Furthermore, direct comparison of the morphs possessing identical lumbar curvature revealed that men preferred the morphs for which this angle was attributable specifically to vertebral wedging, unambiguously demonstrating that men distinguish between women whose lumbar curvature reflects vertebral wedging and women whose external lumbar curvature reflects a muscular butt.
But wait! There’s more! When considering the women with below-optimal levels of lumbar curvature, men preferred the women whose lumbar curve reflected vertebral wedging over women whose (identical) lumbar curve reflected a larger butt. However, when the women exhibited above-optimal angles of lumbar curvature, men preferred the omen with a larger butt, which reflects a preference for vertebral wedging closer to optimum because if the angle is above optimum and you have a small butt, then is signifies the hyperlordosis to be the result of excessive vertebral wedging. Conversely, if the butt is large, then it is more likely that the vertebral wedging is closer to optimal and it is simply the large butt that causes hyperlordosis.
Thus, 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. Don’t you just love evolutionary psychology?