Mate choice decisions: the role of facial beauty. (PART 1)

For most people, facial beauty appears to play a prominent role in choosing a mate. Evidence from research on facial attractiveness indicates that physical beauty is a sexually selected trait mediated, in part, by pubertal facial hormone markers that signal important biological information about the displayer. Such signals would be ineffective if they did not elicit appropriate cognitive and/or emotional responses in members of the opposite sex. In this article, I argue that the effectiveness of these hormonal displays varies with perceivers' brains, which have been organized by the degree of steroid hormone exposure in the uterus, and activated by varying levels of circulating steroids following puberty. I further propose that the methodology used for examining mate choice decisions has general applicability for determining how cognitive and emotional evaluations enter into decision processes.



Introduction

For most human beings, mate selection is a complex real-world decision that is of paramount concern for their future happiness. Such decisions inevitably involve emotional and cognitive assessment of prospective mates, and the ability to integrate these evaluations into the decision process. Given the importance of our final judgment, it is curious that an apparently trivial and ephemeral quality, beauty, appears to play such a prominent role in the decision process. In this article, I examine evidence for the biological importance of facial beauty and its influence on mate choice decisions. Based on these findings, I argue that physical attraction arises from an interaction between a perceiver's brain and a perceived face, both of which have been modified, in a complementary manner, by the actions of steroid hormones. Finally, I propose that this analysis might shed light on how affect and cognition work together in real-world decision processes.



Women's facial attractiveness

In the early 1990s, Langlois and colleagues provided theoretical and empirical support for the hypothesis that the average female face in a population is the most attractive. However, investigators have noted that the overlay averaging procedure used in these studies could blur facial details, increase symmetry, and change proportions, all factors that could enhance attractiveness. To avoid such potential problems, Johnston and Franklin developed a computer program that allows individuals to ‘evolve’ their most attractive facial composite. This approach found that evolved attractive female faces are (i) judged to be about 25 years of age, but (ii) possess features and proportions that are systematically different from an average face of that age. Specifically, the lower jaw region is smaller and the lips are fuller than those of the average (Figure 1).

Figure 1. Differences between average and attractive female faces. An average female face (a) and an attractive face (b) generated by modifying only the lower jaw and lips of the average face, using the program of Johnston and Franklin. Note that the eyes seem larger and the cheekbones appear higher in the modified face.



Using a different methodology, Perrett et al. independently verified most of these findings and Cunningham et al. showed that female faces with small narrow chins, large eyes and fuller lower lips are rated highest in beauty across many different cultures. It appears that the average face within any population might be judged attractive, but the most attractive face differs from the average in a systematic manner. The significance of these differences appears to lie in their hormonal origin.



Boys and girls enter puberty with very similar proportions of muscle, fat and bone but exit puberty as reproductive adults with completely different body shapes and compositions. This metamorphosis is primarily a function of steroid hormones. Under the influence of high estrogen levels, a young woman gains about 35 pounds of fat, changing the shape of her breasts, hips, thighs and lips. By contrast, a young man acquires about one and half times as much muscle and bone mass, controlled by the complex action of androgens (and aromatized androgens) acting both directly and indirectly (via release of growth hormone) on bone and muscle tissues. As a result, the average adult male has a longer and broader lower jaw than that of a female, and brow ridge growth results in more sunken narrow eyes.



From this hormonal perspective, attractive female faces are displaying physical features indicative of higher levels of pubertal estrogens (full lips) and lower levels of androgen exposure (short narrow lower jaw and large eyes) than average females. This combination of hormones also appears to be responsible for the female body shape that has been found to be most attractive in industrialized societies and predictive of high fecundity. (The atypical preferences found in some non-industrial societies are believed to be a consequence of imminent threats such as famine or high parasite load). In the absence of contraception, female fertility reaches its maximum in the mid-twenties (which is the estimated age of evolved attractive composites), declines by about 20% in the mid-thirties, and then falls precipitously by a further 60% during the forties. The thinning of a female's lips parallels these steep declines in fertility and, in the modern world, it is not uncommon for females to use lipstick or collagen injections for maintaining or enhancing their facial attractiveness.



This evidence suggests that female beauty depends upon specific highly visible hormonal markers that indicate high fecundity. In other female primates, fecundity signals such as labial swelling, chest blisters, or face reddening are quite common and males who are attracted to such cues enjoy clear reproductive benefits. However, in contrast to the pronounced cyclical fecundity signals exhibited by non-human primates, a woman's physical beauty is continuously displayed throughout her entire reproductive years, although some subtle changes in attractiveness at ovulation have been observed. This continuous display of attractiveness might be an adaptation to the large parental investment that arises from prolonged human infant immaturity. A continuously attractive woman can choose from a larger number of high quality males, secure a male's support for a long period of time, and replace him if necessary. Her choice, however, is influenced by the attractiveness of her male suitors.

Sexual selection

Beautiful songs, elaborate mating dances, and brightly colored iridescent tails, are some of the lures used by male animals to the entice members of the opposite sex. The effectiveness of these seductive signals has been evaluated by experimental ‘plastic surgery’. For example, increasing the length of a widowbird's tail by adding additional feathers produces super-tailed males who enjoy more reproductive opportunities than their average-tailed competitors. In the absence of human intervention, however, mating with attractive males appears to have real biological benefits. Petrie has shown that peacocks with large tails have higher survival rates and perhaps more importantly, this enhanced survival is evident in his mate's offspring. From a peahen's perspective, it appears that a male's beauty is not just an empty promise. She can gain reproductive benefits by selecting males who exhibit these extraordinary secondary sexual characteristics. Of course all such elaborate testosterone driven displays require significant energy to produce and flaunt so they automatically attest to the physical health of a male suitor, but the demonstrated benefits to an attractive male's offspring suggests that there is more here than meets the eye.

The major threat to the health and welfare of all multi-cellular organisms is invasion by parasitic microorganisms like bacteria, viruses, protozoa and fungi. Given these circumstances, it would be beneficial for females to select males with excellent immune systems because their offspring would reap the benefits of his good genes. However, good immunocompetence genes are not directly visible but a male's secondary sexual traits might provide a female with an ‘honest’ signal of their presence. This arises because testosterone acts like a ‘double-edged sword’. It is required for the expression of all secondary sexual traits, (songs, dances, ornamental displays, etc), but it is also a powerful immunosuppressant that reduces the effectiveness of a male's immune system. A male's body must choose between the competing demands of parasite resistance and the display of secondary sexual characteristics, and only males with the best genetic resistance to parasites can afford the latter choice. Elaborate testosterone dependent displays can serve as ‘honest’ proxies for good genes because they are simply beyond the means of males with lesser quality immune systems.