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Wednesday, May 30, 2007

NATURAL SELECTIONS

MATE SELECTION

Sex, morals and DNA


It wouldn't be surprising to see a message along the following lines on an Internet dating site: "SJF, 26, wants to meet kind, generous, romantic, honest man."

This is how Geoffrey Miller, an evolutionary psychologist at the University of New Mexico in Albuquerque, would interpret the message: "Single Japanese female, age 26, seeking a healthy male of breeding age with a minimal number of personality disorders that would impair efficient parenting in a sustained sexual relationship, and a minimum number of deleterious mutations in his genome, especially with respect to the genes that influence the development of brain systems concerned with advertising moral virtue."

Consciously or unconsciously, this is what many women will be looking for in a long-term partner.

Kindness, romance and honesty are all important in a long-term relationship. That's not just my feeling, nor simply what you would conclude from reading a clutch of romantic novels picked up at the airport; that's what research and psychological surveys tell us. Remember this while we think about the things that make some people more attractive than others.

First, physical traits. There are a range of physical traits that people will agree are important in an attractive partner: height in men, for example, and a certain waist-to-bust ratio in women; in both sexes symmetrical facial features are considered attractive.

The problem, from an evolutionary point of view, is that if there are such traits, then why don't the genes for them spread through the population? Why are we not all tall, with large breasts in women, and small waists, and symmetrical features? Humans, and other animals, should, according to this argument, be much less individual than we actually are.

This is known as the lek paradox, because it was first formulated from observations of bird species in which males display themselves to females on small patches of land called leks. If females select the most attractive males to be their partners — which they do — then soon all the males should be equally attractive, and there would be nothing left to choose between them. Yet males still vary in attractiveness.

Something must maintain the differences in attractiveness, despite the most attractive males getting the most females and leaving the most offspring.

Marion Petrie, of Newcastle University in northern England, thinks that the variation in attractiveness is down to mutations that arise in the DNA repair kits of cells. Individuals — people, as well as animals with less efficient repair kits — will have greater variations in their DNA. This can sometimes be harmful, because unrepaired DNA may cause cancers, but sometimes it can be useful, because variation in our disease defenses can help fight disease.

In a computer model published in the journal Heredity, Petrie has shown that even when mate choice reduces genetic diversity — because females tend to pick the same males — mutations affecting DNA repair kits help to boost genetic diversity.

That's all very well, and it may even be pivotal in solving the long-standing lek paradox. But something is missing when it comes to humans.

It's all very well to imagine an insect or a bird choosing a sexual partner on the basis of some aspect of their attractiveness that might signal that they have a good set of genes to help potential offspring resist disease . . . but in humans?

It just doesn't wash, does it. We don't just choose to make a long-term commitment on the basis of the length of someone's legs. We all know how maddeningly complicated courtship and mate selection can be in humans.

OK. Now remember what we said at the beginning: It's not unreasonable to suggest that many women are looking for a kind, generous, romantic and honest man as a life partner. Does the quality of our DNA have any influence on the quality of our morals? Do morally virtuous traits reflect good genes?

Promises rather than threats

According to psychologist Miller, the answer is yes.

"Moral virtues may function as good genes indicators by being difficult to display impressively if one has a high mutation load that impairs the precision of brain and body development," he says.

In the Quarterly Review of Biology, Miller suggests that a moral preference for romance over aggression may signal that a partner will seek to maintain a relationship through promises rather than threats. This is why, he says, beauty is in the eye of the beholder — we perceive beauty not just purely in how someone looks, but how he or she behaves.

Someone who shows moral virtues may also be unconsciously signaling freedom from the mutations associated with autism, schizophrenia and language impairment. In other words, Miller's idea, which is a development of what Charles Darwin said about sexual selection, may help explain how the human moral sense evolved.

Attractive bodies can elicit short-term desire, but attractive moral traits can inspire long-term love, he says.

It makes sense, and I'd like to think he's right. Both things are important — how someone looks, but also how they behave. It's not all about being tall and handsome, or having a perfect ratio of breast size to waist size. That's common sense.

But Miller has provided a service by arguing how sexual selection has contributed to the evolution of morality.

The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is "Hito wa ima mo shinka shiteru (The Evolving Human: How new biology explains your journey through life)"; price 1,500 yen.


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