Yesterday, I was involved in a discussion on Facebook about whether women wear high heeled shoes because it makes them look sexier to men, or because they think that men think it makes them look sexier, or because the media has convinced them it makes them look sexier, or whatever. We did not reach any definite conclusions, yet I began to reflect on just what characteristics make humans appear more attractive to the opposite sex--and why. I concluded that though much of what drives our choices in seeking mates is cultural, even after we strip away the cultural biases and fads, there are a few evolved core biological preferences in mate selection. And the genes that drive these preferences have a single algorhythm---grandchildren.
When a lovesick 15 year old boy thinks he has fallen in love and experiences the surge of all the various neurotransmitters and hormones which define that state, I'm sure that the prospect of grandchildren is not exactly what's on his mind. His fantasies are likely to be a bit further up the chain of causality. And yet, the genes that come into play here are going about the job of insuring their own replication in distant progeny, at least two generations out. I will not anthropomorphize them by saying that these genes are "scheming and plotting" to make the boy a grandfather---genes don't plot anything---they just code for protein. But if allowed to play out their coding scenario, these genes have evolved precisely to enhance the probability that he will indeed achieve grandfatherhood.
Why grandfatherhood and not just fatherhood? Well, you could have twenty children, but if none of them ever reproduced, then none of your genes would be replicated in future generations. But if you have a lot of children and your children have a lot of children (when my grandfather came to this country, he was the last of his line, but when he died he had over one hundred living descendents), then it's a safe bet that some of your genes will go on indefinitely. It's a kind of immortality. Is that what our genes want---immortality? They don't want anything---they're just numerical sequences. But if their practical effect was not to cause their own replication by causing our replication, then they wouldn't be here----and neither would we.
So if the 15 year old boy's genes are switching on a "bonding response" in the presence of one female, but not any others, what are the characteristics that the genes are programmed to look for? In general, it's the same as for any other species: health, strength, beauty, control of critical resources, and availability. Many of the things we class as beauty are actually indicators of health. Present health is often indicated by skin condition. In the days before lab tests, a physician's examination began by observing skin condition and noting rash, pallor, jaundice, ruddiness, etc. Healthy, well nourished people usually have healthy looking skin. Your long-term health record is your hair. If you have long, silky hair, that means that you are not only healthy now, but have been continuously so for as long as your oldest hair, perhaps four years. Symmetry is universally preferred over asymmetry, and it's about more than beauty. If one side of your face is a mirror image of the other side, then it means that both sides were probably copied accurately. And if parts of your face were copied accurately, then the parts of your heart and other organs were probably copied accurately as well. It's a health thing.
In mate selection, the questions that any female human will ask are no different from the questions a female penguin would ask: 1. Does this young male have good genes? Will he sire strong, beautiful offspring? 2. Is he competent enough to protect and provide for my offspring? 3. Will he have the resources needed? For penguins, this last question might boil down to, "Did he collect enough pebbles to make a good nest?" For cheetahs, it would be, "Does he dominate a large territory full of gazelles?" And for humans, it would be, "Does he have a good job?" But it's all the same question: Can he muster sufficient resources? Sometimes, in humans and in other creatures, the question of social standing may arise. But that is just another way of asking about control of resources.
Beauty may be defined in different ways in different settings, but some things remain constant. Healthy, symmetrical specimens are universally preferred over sickly, misshapen ones. As far as weight is concerned, this will vary according to circumstances. One of the oldest objects of art ever found is the Venus of Wallenberg. It's a small figurine of a female torso. She is depicted as pregnant and was carved to have huge thighs and breasts. By today's standards, she would be considered grossly obese. But in her time, she was a fertility goddess. During a famine, such a woman might be able to bring a child to term and even nurse it for a while without much food. If you were a man living in a world where food was usually scarce, this is the kind of female you'd be looking for. Today, it's been a while since the last global famine, so epigenetic processes have suppressed our appreciation for the larger ladies. But it's still in our genes, waiting for the next famine.
Then of course, there is the matter of sexual selection. In most species, it's the females that do the selecting. And even among humans, we are exactly what our female ancestors wanted us to be. So those ladies who throw up their hands and lament, " Why are men such brutes, (or such fools, or whatever), should remember that if we are brutes, it is because your great, great grandmothers, faced with a choice between the shy, sensitive poet and the brute, opted for the brute. Over time, the males of a species become whatever the females want them to become. Consider the tail of the African Widowbird.
The Widowbird is a bird about the size of a Robin, but the males, and only the males, have a tail 12 to 14 inches long. There is no natural selection advantage for such a huge tail. it just weighs them down and makes it harder for them to fly and harder to escape from predators. But the females refuse to mate with any males except those with the longest tails. How does this happen? Consider that the ability for males to grow a long tail is a heritable trait. There is a gene for it. But the tendency for females to choose only males with long tails is also heritable. And although only males grow long tails, the gene for it can be carried through both the male and female line. And the gene causing females to choose long tails can also be carried by either sex. And that's where it gets complicated.
When a female chooses a long tailed male, she has inherited the tendency to do that from her mother. But if her mother had the gene for choosing long tails, then her father probably had a long tail. So she inherited both the gene for choosing long tails, and the gene which, in males, will grow a long tail. Of course, the male has not only inherited the long tail gene from his father, he has also inherited the gene for choosing long tails from his mother, since, if his mother did not have such a gene, she would not have mated with his father. So the poor chick will inherit both the gene for growing long tails and the gene for choosing them, and will get both from all four grandparents. So once a sexual selection preference gets started, it has a positive feedback loop, and a runaway effect occurs till the result reaches absurd lengths.
Do humans have such a runaway selection loop going? It's too early to be sure, but I doubt it. But you can be sure that sexual selection is a strong factor, perhaps the main factor, in the direction human evolution is heading. When free choice of mates is allowed, then over time, each sex will become whatever the other sex wants it to be.
And with sexual selection, nothing succeeds like success. The most effective way to make a man more attractive to a woman is to convince her that he is already attractive to most other women. Any member of a group who suddenly becomes more popular with some of the women also starts looking better to most of the other women. Why? It's called the "swinging stud " strategy. While no one would consciously process it out this way, a woman might sub-consciously say, "Wow. I don't know what Larry's got, but It looks like women really go nuts over him. If I could have his baby, my son would have whatever Larry has, and women would throw themselves at him. He would have a lot of reproductive opportunities, and I would have scads of grandchildren." When genes are manipulating our behavior, this is the kind of thing they have us doing.
Whenever we find a specific physical characteristic that seems to be identified with sex appeal across several cultures, then we may have evolved a trait to be attracted to that characteristic. I read once that waist to hip ratio seems to be a universal marker for female sex appeal. If a woman's waist is smaller than her hips, then, as you see her in silhouette, and see the line curving inward down one side from arm pit to waistline and then curve outward over the hip, that line is what male humans are programmed to look for. I asked my daughter about this. (She has a degree in anthropology.) She said, "That line is nature's way of saying, 'I'm a female in childbearing years, and I'm not pregnant.' " But wide hips in themselves might be attractive because before the days before modern medicine, childbirth was pretty risky, and a wider pelvic opening might make it less so. Any characteristic that is associated with sex appeal, over time and across several cultures, probably has an evolutionary basis even if we have no idea what it is. But when we confront any mate selection preferences, these are the kinds of questions we should ask ourselves. When we see that we find something attractive, we should ask, "Is this a cultural trait, or have I evolved a preference for (X) ? And if it's evolved, does it enhance survival, or does it confer a reproductive advantage? Or is it like the tail on the Widowbird, conferring no advantage at all?