The rise and fall of the third chimpanzee

and woman are in political views, religion, and personality, the smoother will be the negotiation. Hence the match in personality traits is on the average closer for married couples than for dating couples, closer for happily than unhappily married couples, and closer for couples who stay married than for those who get divorced. But this still does not explain spousal resemblance in ear lobe length, which is only rarely cited as a factor in divorce. The remaining factor deciding whom you will marry, besides propinquity and smoothness of negotiation, is surely sexual attraction based on physical appearance. That in itself is no surprise. Most of us are aware of our preferences in obvious visible features like height, build, and hair colour. What is initially surprising is the importance of so many other physical traits that we usually do not consciously notice, such as ear lobes, middle fingers, and interocular distances. Nevertheless, all those other traits contribute unconsciously to the snap decisions we make when we are introduced to someone and a voice inside tells us, 'She's my type! Here is an example. When my wife and I were introduced to each other, I instantly found Marie attractive and vice versa. In retrospect, I can understand why: we are both brown-eyed, similar in height and build and hair colour, and so on. But, on the other hand, I also had a sense that there was something about Marie that did not quite match my ideal, even though I could not figure out what exactly it was. Not until Marie and I first went to a ballet together did I solve the puzzle. I lent Marie my opera glasses, and when she passed them back to me, I found that she had pushed the eye-pieces so close together that I could not see through them until I had spread them apart again. I then realized that Marie has more close-set eyes than I do, and that most women whom I had pursued before had wide-set eyes like my own. Thanks to Marie's ear lobes and other merits, I have been able to make peace with my and her mismatched interocular distances. Nevertheless, the episode with the opera glasses made me appreciate for the first time that I have always found wide-set eyes a turn-on, even though

I had not been explicitly aware of it.

So, we tend to marry someone who looks like us. But—wait a minute. The men who look most similar to a woman are the men who share half of her genes—her father or brother!

Similarly, the best-matched mate for a man would be his mother or sister! Yet most of us obey the incest taboo and certainly do not marry our parent or sibling of the opposite sex.

Instead, I am saying that people tend to marry a person who looks like the parent or sibling of the opposite sex. Our actual behaviour is summed up by a popular song of the 1920s.

I want a girl
Just like the girl
That married dear old Dad…

The reason we tend to resemble our mates is that many of us are looking for someone who reminds us of our parent or sibling of the opposite sex, who in turn resembles us. As children, we already begin to develop our search image of a future sex partner, and that image is heavily influenced by the people of the opposite sex whom we see most often.

For most of us that is our mother (or father) and sister (or brother), plus close childhood friends.

At this point, you are probably turning to your spouse or Significant Other, pulling out your tape measure, and discovering a gross mismatch between your and his (or her) ear lobes. Or perhaps you have pulled out a photo of your mother or sister, and you detect not the faintest resemblance when you hold it beside your spouse. You may be about to throw away this book as patent nonsense. But if your wife is not a dead ringer for your mother, don't stop reading, and conversely don't get worried that you should see a psychiatrist about your pathological search image. After all, remember:

1. Studies consistently show that factors like religion and personally influence our choice of spouse much more strongly than physical appearance. All I am making is the obvious point that physical traits have some influence. In fact, I would predict much higher correlation coefficients for physical traits between casual sex partners than between spouses. That is because we can select casual sex partners solely on the basis of physical attraction, without regard to religion or political views. This prediction awaits testing.

2. Remember also that your search image could have been influenced by any of the people of the opposite sex that you regularly saw around you as you were growing up. That includes playmates and siblings as well as parents. Perhaps your spouse resembles the little girl next door, rather than Mother.

3. Finally, remember that lots of independent physical traits enter into our search image, so most of us end up with a mild average resemblance to our spouses in many traits, rather than with a very close resemblance in a few traits. This idea is known as the 'buxom redhead theory'. If a man's mother and sister were both buxom redheads, he might grow up to consider buxom redheads very exciting, but redheads are relatively rare, and buxom redheads still rarer. Furthermore, the man's preference even in a casual sex partner is likely to depend on some other physical traits as well, and his preference in a wife will certainly depend on her views about children, politics, and money. Hence, in a group of sons of buxom redheads, a few lucky ones will find a girl like Mother in those two respects, some will have to settle for buxom non-redheads, others for non-buxom redheads, and most for run-of-the-mill non-buxom brunettes.

You may also be objecting at this point that my argument applies only to societies where spouses pick each other. As friends from India and China are quick to remind me, that is a peculiar custom of the twentieth-century US and Europe. It was not true of the US and Europe in the past, and it is still not true of most of the world today, where marriages are instead arranged by the families involved. The bride and groom often are not even introduced until the wedding day. How could my argument possibly apply to such marriages?

Of course it couldn't, if one is talking just about legal marriages. But my argument would still apply to the choice of extramarital sex partners, who may father a non-trivial fraction of children, just as blood-group studies proved for American and British children (Chapter Four). In fact, I would expect that if extramarital fathering is frequent even in societies where a woman already exercises her sexual preferences in choosing a husband, it may be even more frequent in societies with arranged marriages, where a woman's choice can only be expressed extramaritally. It is not just the case, then, that Fore men prefer Fore women over Californian women, and vice versa: our search images are much more specific. However, these insights still leave questions unanswered. Did I inherit or learn my search image for someone like Mother? If I were offered the choice of sex with my sister or a strange woman, I would certainly reject the offer of my sister and probably my first cousin, but would I prefer my second cousin over a strange woman (because the cousin probably resembles me more)? There are some crucial experiments that would settle these questions—for instance, keeping a man in a large cage with his female first, second, third, fourth, and fifth cousins, counting how many times he had sex with each, and repeating the experiment with many men (or women) and their cousins. Alas, such experiments are hard to do with humans, but they have been done for several animal species, with instructive results. I shall give just three examples, the cousin-loving quail, and the perfumed mice and rats. (We cannot use our closest relatives the chimpanzees for these examples, since they are so unselective.)

Consider first the case of Japanese quail, which are either brown or white. Quail normally grow up with their biological parents and siblings. However, it is also possible to 'cross-foster' quail by switching eggs between quail mothers and their nests before the eggs hatch. In that way, a baby quail may be reared by foster-parents and grow up with 'pseudo-siblings'—that is, littermates among whom the baby hatched but to whom the baby is not genetically related. The preferences of male quail have been tested by putting a male in a cage with two females and observing with which female the male spent more time or copulated. It turns out that males preferred whichever colour of female they grew up with. Furthermore, when a brown-loving male was given a choice between brown females that he had never seen before (although some were his relatives from whom he had been separated before hatching), he preferred his first cousin to his third cousin or an unrelated female, but he also preferred his first cousin to his sister. Evidently, male quail as they grow up learn the appearance of their sisters (or mother) with whom they are reared, then seek a mate that is very similar but not too similar. In fancy technical language, biologists term this the Principle of Optimal Intermediate Similarity. Like other things in life, inbreeding seems to be good in moderation—a little inbreeding, but not too much. For instance, among unrelated brown females a male prefers an unfamiliar one over a familiar one with whom he grew up (a pseudo-sister', who pushes the male's not-too-much-incest button). Mice and rats similarly