The Rise and Fall of the Third Chimpanzee

by Jared Diamond

  We saw in Chapter Four that this ideal depiction of a human society based on monogamous couples coexists with a good deal of extramarital sex. That activity also involves selection of sex partners, with adulterous women tending to be more selective than adulterous men. Thus, selection of spouses and sex partners is another important piece of what defines humanity. It is as basic to our rise from chimpanzee status as is the remodelled pelvis discussed in detail in physical anthropology texts. We shall see in the next chapter that our sexual choosiness may be central to the origin of the most conspicuous visible variability in modern humans. That is, much of what we think of as human racial variation may have arisen as a by-product of the beauty standards by which we choose our sex partners.

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  In addition to this theoretical interest, the question of how we select our mates and sex partners is of much personal interest. It preoccupies most of us for much of our lives. Those of us who are still unattached spend daily hours dreaming about whom we will consort with or marry. The question becomes more intriguing when we compare what turns on different people within the same culture. Think of the men or women that you find sexually attractive. If you are a man, for instance, do you prefer women who are blonde or brunette, flat-chested or buxom, and with big or small eyes? If you are a woman, do you like men who are bearded or smooth-shaven, tall or short, and smiling or scowling? Probably you do not go for just anyone, only certain types attract you. Everyone can name friends who got divorced, then chose a second spouse who was the spitting image of the first. A colleague of mine went through a long series of plain, slim, brown-haired, round-faced girlfriends, until he finally found one he got along with and married her. Whatever your own preference, you will have noticed that some of your friends have completely different tastes.

  The particular ideal that each of us pursues is an example of what are called ‘search images’. (A search image is a mental picture against which we compare objects and people around us in order to be able to recognize something quickly, like a Perrier bottle amidst all the other bottled waters on the supermarket shelf, or one’s child at a playground with other kids.) How do we develop our private search image for a mate? Do we seek someone familiar and similar to us, or are we more turned on by someone exotic? Would most European men really marry a Polynesian woman if given the chance? Do we seek someone complementary to us so as to fulfil our needs? For instance, there undoubtedly are some dependent men who marry a mothering woman, but how typical are such pairings?

  Psychologists have tackled this question by examining many married couples, measuring everything conceivable about their physical appearance and other characteristics, and then trying to make sense out of who married whom. A simple numerical way of describing the result is by means of a statistical index called the correlation coefficient. If you line up 100 husbands in order of their ranking for some characteristic (say, their height), and if you also line up their 100 wives with respect to the same characteristic, the correlation coefficient describes whether a man tends to be at the same position in the husbands’ line-up as his wife is in the line-up of wives. A correlation coefficient of plus one would mean perfect correspondence: the tallest man marries the tallest woman, the thirty-seventh tallest man marries the thirty-seventh tallest woman, and so on. A correlation coefficient of minus one would mean perfect matching by opposites: the tallest man marries the shortest woman, the thirty-seventh tallest man marries the thirty-seventh shortest woman, and so on. Finally, a correlation coefficient of zero would mean that husbands and wives assort completely randomly by height: a tall man is as likely to marry a short woman as a tall woman. These examples are for height, but correlation coefficients can also be calculated for anything else, such as income and IQ.

  If you measure enough things about enough couples, here is what you will find. Not surprisingly, the highest correlation coefficients – typically around +0.9 – are for religion, ethnic background, race, socioeconomic status, age, and political views. That is, most husbands and wives prove to be of the same religion, ethnic background, and so on. Perhaps you also will not be surprised that the next highest correlation coefficients, usually around +0.4, are for measures of personality and intelligence, such as extroversion, neatness, and IQ. Slobs tend to marry slobs, though the chances of a slob marrying a compulsively neat person are not as low as the chances of a political reactionary marrying a left-winger.

  What about matching of husbands and wives for physical characteristics? The answer is not one that would leap out at you immediately if you just looked at a few married couples. That is because we do not select our own mates for their bodies as carefully as we select the mates of our show dogs, racehorses, and beef cattle. But we select nevertheless. If you measure enough couples, the answer that finally emerges is unexpectedly simple. On the average, spouses resemble each other slightly but significantly in almost every physical feature examined.

  That is true of all the obvious traits you would first think of when asked to design your ideal beloved – his or her height, weight, hair colour, eye colour, and skin colour – but it is also true of an astonishing variety of other traits that you probably would not have mentioned in your description of the perfect sex partner. Those other traits include ones as diverse as breadth of nose, length of ear lobe or middle finger, circumference of wrist, distance between eyes, and lung volume! Experimenters have made this finding for people as diverse as Poles in Poland, Americans in Michigan, and Africans in Chad. If you do not believe it, try noting eye colours (or measuring ear lobes) the next time you are at a dinner party with many couples, and then get your pocket calculator to give you the correlation coefficient.

  Coefficients for physical traits are on the average +0.2 – not so high as for personality traits (+0.4) or religion (+0.9), but still significantly higher than zero. For a few physical traits the correlation is even higher than 0.2 – for instance, an astonishing 0.61 for length of middle finger. At least unconsciously people care more about their spouse’s middle finger length than about his or her hair colour and intelligence!

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  Thus, like tends to marry like. Among the obvious explanations that contribute to these results is propinquity: we tend to live in neighbourhoods defined by socioeconomic status, religion, and ethnic background. For instance, in large American cities one can point to the rich neighbourhoods and the poor neighbourhoods, and also to the Jewish section, Chinese section, Italian section, black section, and so on. We meet people of the same religion when we go to church, and we tend to meet people of similar socioeconomic status or political views in many of our daily activities. Since we thus have far more opportunities to meet people like us than unlike us in these respects, of course we are more likely to marry someone of our religion, socioeconomic status, and so on. But we don’t live in neighbourhoods grouped by length of ear lobe, so there must be some other reason why spouses tend to be matched in that respect as well.

  Another obvious reason why like tends to marry like is that marriage is not just a choice; it is a negotiation. We do not go out searching until we find a person with the right eye colour and length of middle finger, then announce to that person, ‘You are marrying me’. For most of us, marriage results from a proposal rather than a unilateral announcement, and the proposal is the culmination of some sort of negotiation. The more similar a man 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.

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  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 personality 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.

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  It is not just the case, then, that Foré men prefer Foré 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 learn in childhood what to look for in a mate, but they choose by smell more than by appearance. When infant female mice were reared by parents sprayed repeatedly with Parma Violet perfume, the females on reaching adulthood sought out Parma-Violetscented males in preference to unscented males. (‘I want a boy, just like the boy, that smells like dear old Dad’.) In another experiment, infant male rats were reared by mother rats whose nipples and vagina were sprayed with lemon odour, then the male on reaching adulthood was put in a cage with a lemon-smelling or unscented female rat. Each such encounter was videotaped and played back to note the times of key events. It turned out that males with scented mothers mounted and ejaculated more quickly when placed with a scented female than with an unscented one, while the reverse was true for males with unscented mothers. For example, sons of scented mother rats were so excited by a scented sex partner that they ejaculated in only eleven-and-a-half minutes, while they took over seventeen minutes to ejaculate with an unscented female. But sons of unscented mother rats took over seventeen minutes with the scented partner and only twelve minutes with the unscented partner. Obviously, the males had learned to be sexually excited by their mother’s smell (or lack of smell); they did not inherit the knowledge.