Book Summary: “Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature” by Geoffrey Miller


Title: Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature
Author: Geoffrey Miller
Scope: 4 stars
Readability: 4 stars
My personal rating: 5 stars
See more on my book rating system.

If you enjoy this summary, please support the author by buying the book.

Topic of Book

Miller attempts to apply Darwin’s theory of sexual selection to explain the developments of the human brain and human nature that are not directly tied to survival: art, music, creativity, morality and humor.

My Comments

The first few chapters are outstanding, but then I feel like Miller’s argument tapers off a bit. I think Miller gives compelling evidence of the importance of sexual selection through mate choice, but he goes a bit too far in terms of using it as prime explanation for the emergence of the human brain and human nature.

Key Take-aways

  • Darwin’s theory of natural selection was not his only sweeping theory. His other theory, sexual selection, has been neglected despite strong evidence for its utility.
  • Natural selection is about survival, while sexual selection is about reproduction.
  • Under natural selection, species adapt to their physical environment. Under sexual selection, species adapt to themselves via mating choices. In particular, choices made by individual females as to who to mate with play a driving role. Females are effectively choosing which male genes are passed onto the next generation.
  • Because sexual selection is determined by human thought rather than random mutations, it can find effective solutions faster.
  • Our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could.
  • Traditional Darwinist explanations that the human brain evolved for survival run into three problems:
    • Big brains are present in very few other animals.
    • Human brain tripled in size for two million years: long before any known technological innovations or behavioral changes.
    • So much of human behavior confers no clear survival payoff
  • When a female chooses to mate with a male that possesses a certain characteristic, their children will tend to inherent both the characteristic (from their father and the preference for mating with those that possess that characteristic (from the mother). This creates a potential positive feedback loop.

Important Quotes from Book

Most people equate evolution with “survival of the fittest,” and indeed most theories about the mind’s evolution have tried to find survival advantages for everything that makes humans unique.

Ever since the Darwinian revolution, this survivalist view has seemed the only scientifically respectable possibility. Yet it remains unsatisfying. It leaves too many riddles unexplained. Human language evolved to be much more elaborate than necessary for basic survival functions. From a pragmatic biological viewpoint, art and music seem like pointless wastes of energy. Human morality and humor seem irrelevant to the business of finding food and avoiding predators. Moreover, if human intelligence and creativity were so useful, it is puzzling that other apes did not evolve them. This book proposes that our minds evolved not just as survival machines, but as courtship machines. Every one of our ancestors managed not just to live for a while, but to convince at least one sexual partner to have enough sex to produce offspring. Those proto-humans that did not attract sexual interest did not become our ancestors, no matter how good they were at surviving. Darwin realized this, and argued that evolution is driven not just by natural selection for survival, but by an equally important process that he called sexual selection through mate choice. Following his insight, I shall argue that the most distinctive aspects of our minds evolved largely through the sexual choices our ancestors made.

The human mind and the peacock’s tail may serve similar biological functions. The peacock’s tail is the classic example of sexual selection through mate choice. It evolved because peahens preferred larger, more colorful tails. Peacocks would survive better with shorter, lighter, drabber tails. But the sexual choices of peahens have made peacocks evolve big, bright plumage that takes energy to grow and time to preen, and makes it harder to escape from predators such as tigers. The peacock’s tail evolved through mate choice. Its biological function is to attract peahens.

The tail makes no sense as an adaptation for survival, but it makes perfect sense as an adaptation for courtship.

The human mind’s most impressive abilities are like the peacock’s tail: they are courtship tools, evolved to attract and entertain sexual partners. By shifting our attention from a survival-centered view of evolution to a courtship-centered view, I shall try to show how, for the first time, we can understand more of the richness of human art, morality, language, and creativity.

I do not think that natural selection for survival can explain the human mind. Our minds are entertaining, intelligent, creative, and articulate far beyond the demands of surviving on the plains of Pleistocene Africa. To me, this points to the work of some intelligent force and some active designer. However, I think the active designers were our ancestors, using their powers of sexual choice to influence—unconsciously—what kind of offspring they produced. By intelligently choosing their sexual partners for their mental abilities, our ancestors became the intelligent force behind the human mind’s evolution.

The human brain and its diverse capacities are so complex, and so costly to grow and maintain, that they must have arisen through direct selection for some important biological function. To date, it has proven very difficult to propose a biological function for human creative intelligence that fits the scientific evidence. We know that the human mind is a collection of astoundingly complex adaptations, but we do not know what biological functions many of them evolved to serve.

As we shall see, sexual selection is unusually fast, powerful, intelligent, and unpredictable. This makes it a good candidate for explaining any adaptation that is highly developed in one species but not in other closely related species that share a similar environment.

One difference is that sexual selection through mate choice can be much more intelligent than natural selection. I mean this quite literally. Natural selection takes place as a result of challenges set by an animal’s physical habitat and biological niche. The habitat includes the factors that matter to farmers: sunlight, wind, heat, rain, and land quality. The niche includes predators and prey, parasites and germs, and competitors from one’s own species. Natural selection is just something that happens as a side-effect of these factors influencing an organism’s survival chances. The habitat is inanimate and doesn’t care about those it affects. Biological competitors just care about making their own livings. None of these selectors cares whether it imposes evolutionary selection pressures that are consistent, directional, efficient, or creative. The natural selection resulting from such selectors just happens, willy-nilly.

Sexual selection is quite different, because animals often have very strong interests in acting as efficient agents of sexual selection. The genetic quality of an animal’s sexual partner determines, on average, half the genetic quality of their offspring. (Most animals inherit half their genes from mother and half from father.) As we shall see, one of the main reasons why mate choice evolves is to help animals choose sexual partners who carry good genes. Sexual selection is the professional, at sifting between genes. By comparison, natural selection is a rank amateur. The evolutionary pressures that result from mate choice can therefore be much more consistent, accurate, efficient, and creative than natural selection pressures.

As a result of these incentives for sexual choice, many animals are sexually discriminating. They accept some suitors and reject others. They apply their faculties of perception, cognition, memory, and judgment to pick the best sexual partners they can. In particular, they go for any features of potential mates that signal their fitness and fertility.

In fact, sexual selection in our species is as bright as we are. Every time we choose one suitor over another, we act as an agent of sexual selection. Almost anything that we can notice about a person is something our ancestors might have noticed too, and might have favored in their sexual choices.

A major theme of this book is that before language evolved, our ancestors could not easily perceive one another’s thoughts, but once language had arrived, thought itself became subject to sexual selection. Through language, and other new forms of expression such as art and music, our ancestors could act more like psychologists—in addition to acting like beauty contest judges—when choosing mates. During human evolution, sexual selection seems to have shifted its primary target from body to mind.

This book argues that we were neither created by an omniscient deity, nor did we evolve by blind, dumb natural selection. Rather, our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could. We have inherited both their sexual tastes for warm, witty, creative, intelligent, generous companions, and some of these traits that they preferred. We are the outcome of their million-year-long genetic engineering experiment in which their sexual choices did the genetic screening.

Apart from sexual selection being a special sort of evolutionary process, the adaptations that it creates also tend to show some special features. Adaptations for courtship are usually highly developed in sexually mature adults but not in youth. They are usually displayed more conspicuously and noisily by males than by females.

Sexual selection theory was neglected for a century after Darwin and why it was revived only in the 1980s. The century of neglect is important to appreciate, because virtually all of 20th-century science has tried to explain human mental evolution using natural selection alone.

There has been a renaissance of interest in sexual selection, with an outpouring of new facts and ideas. Today, the world’s leading biology journals are dominated by technical papers on sexual selection theory and experiments on how animals choose their mates. But this has been a secret renaissance, hidden from most areas of psychology and the humanities, and largely unrecognized by the general public.

My theory suggests that our most cherished abilities were favored by the most sophisticated minds ever to have emerged on our planet before modern humans: the minds of our ancestors. It doesn’t reduce psychology to biology, but sees psychology as a driving force in biological evolution. It portrays our ancestors’ minds as both products and consumers evolving in the free market of sexual choice.

This sexual choice theory did not start out as a way of Darwinizing the humanities or trying to explain human creativity.

The first problem is that really large brains and complex minds arose very late in evolution and in very few species.

But over 99 percent of animal species thrive with brains much smaller than a chimpanzee’s. Far from showing any general trend towards big-brained hyper-intelligence, evolution seems to abhor our sort of intelligence, and avoids it whenever possible. So, why would evolution endow our species with such large brains that cost so much energy to run, given that the vast majority of successful animal species survive perfectly well with tiny brains?

Second, there was a very long lag between the brain’s expansion and its apparent survival payoffs during human evolution. Brain size tripled in our ancestors between two and a half million years ago and a hundred thousand years ago. Yet for most of this period our ancestors continued to make the same kind of stone handaxes. Technological innovation was at a standstill during most of our brain evolution. Only long after our brains stopped expanding did any tradition of cumulative technological progress develop, or any global colonization beyond the middle latitudes, or any population growth beyond a few million individuals.

The third problem is that nobody has been able to suggest any plausible survival payoffs for most of the things that human minds are uniquely good at, such as humor, story-telling, gossip, art, music, self-consciousness, ornate language, imaginative ideologies, religion, and morality.

Linguistics textbooks do not include a good evolutionary theory of language origins, because there are none. Cultural anthropology textbooks present no good evolutionary theories of art, music, or religion, because there are none. Psychology textbooks do not offer any good evolutionary theories of human intelligence, creativity, or consciousness, because there are none. The things that we most want to explain in any evolutionary framework seem the most resistant to any such explanation. This has been one of the greatest obstacles to achieving any real coherence in human knowledge, to building any load-bearing bridges between the natural sciences, the social sciences, and the humanities.

Most theories of human evolution attempt to solve only one of these three problems. A few might solve two. None has ever solved all three. This is because the three problems create a paradox that cannot be solved by thinking in terms of survival of the fittest.

We get confused about the human mind’s biological functions because of a historical accident called human history. The courtship ornaments that our species happened to evolve, such as language and creativity, happened to yield some completely unanticipated survival benefits in the last few thousand years: agriculture, architecture, writing, metalworking, firearms, medicine, and microchips. The usefulness of these recent inventions tempts us to credit the mind with some general survival advantage.

Evolution found a way to act directly on the mental sophistication of this primate species, not through some unique combination of survival challenges, but through the species setting itself a strange new game of reproduction. They started selecting one another for their brains.

The intellectual and technical achievements of our species in the last few thousand years depend on mental capacities and motivations originally shaped by sexual selection.

Biology students now are usually taught that sexual selection is a subset of natural selection, and that natural selection is only loosely analogous to artificial selection by human breeders. This was not Darwin’s view: he saw sexual selection as an autonomous process that was midway between natural and artificial selection. Darwin was fairly careful about his terms. For him, artificial selection meant the selective breeding of domesticated species by humans for their economic, aesthetic, or alimentary value. Natural selection referred to competition within or between species that affects relative survival ability Sexual selection referred to sexual competition within a species that affects relative rates of reproduction.

Males usually compete to inseminate females. They do this by intimidating other males with weaponry and by attracting females with ornaments. Females exercise sexual choice, picking the stronger and more attractive males over the weaker and plainer. Over generations, male weaponry evolves to be more intimidating and male ornamentation evolves to be more impressive. There are two results. First, within each sexual species, males diverge from the female norm. Mature males become more strongly differentiated, compared with females, compared with young animals, and compared with their own ancestors. The other result is very fast divergence between species. The weaponry and ornamentation of one species can go off in a very different direction from the weaponry and ornamentation of a closely related species. Thus, Darwin’s sexual selection idea could explain three enigmas: the ubiquity across many species of ornaments that do not help survival, sex differences within species, and rapid evolutionary divergence between species.

His theory of sexual selection through mate choice was the crowning achievement of these investigations—yet it was the one most vehemently rejected by his contemporaries.

Sexual selection’s century of exile from biology had substantial costs for other sciences. Anthropologists paid little attention to human mate choice in the tribal peoples they studied for most of this century. By the time mate choice was accepted as an important evolutionary factor, most of those tribal peoples had been exterminated or assimilated. Psychologists had little evolutionary insight into human sexuality and their discipline was dominated for decades by Freudianism.

Following Marx, the social sciences saw a culture’s mode of production as more important than its mode of reproduction. Economists had no explanation for the importance of “positional goods” that advertise one’s wealth and rank in comparison to sexual rivals. In the other human sciences as well—archeology, political science, sociology, linguistics, cognitive science, neuroscience, education, and social policy—there was a blind spot where the theory of sexual selection should have been.

When these sciences did try to trace the evolutionary roots of human behavior, they have usually come up with theories based on “survival of the fittest” and “the goods of the species.” Mate choice was simply not on the intellectual map as an evolutionary force.

Today, evolutionary biology is proclaiming that the old map of evolution was wrong. It put too much weight on the survival of the fittest and, until the 1980s, virtually ignored sexual selection through mate choice. Yet in the human sciences we are still using the old map, and we still do not know where we came from, or where we are going.

Under natural selection, species adapt to their environments…  Under sexual selection, species adapt too, but they adapt to themselves.

What the physical environment is to natural selection, sexual preferences are to sexual selection.

With sexual selection, genes act as both the fashion models and the fashion critics… This creates the potential for the same kind of feedback loops that drive progress in high fashion and modern theology. These feedback loops are the source of sexual selection’s speed, creativity, and unpredictability.

Fisher’s key insight was that the offspring of choosy females will inherit not just longer tails, but also the genes for the sexual preference—the taste for long tails. Thus, the genes for the sexual preference tend to end up in the same offspring as the genes for the sexually selected trait.

Most human cultures have been overtly polygynous. In hunter-gatherer cultures the men who are the most charming, the most respected, the most intelligent, and the best hunters tend to attract more than their fair share of female sexual attention. They may have two or three times as many offspring as their less attractive competitors. In pastoral cultures the men who have the largest herds of animals attract the most women. In agricultural societies the men who have the most land, wealth, and military power attract the most women.

In the early 1970s, biologist Robert Trivers realized that, from this difference in “parental investment,” all else follows. Because eggs cost more for females to make than sperm costs for males, females make fewer eggs than males make sperm. But since each offspring requires only one of each, the rarer type of DNA packet, the egg, becomes the limiting resource. Thus, Trivers argued, it makes sense that males should compete more intensely to fertilize eggs than females do to acquire sperm, and that females should be choosier than males. Males compete for quantity of females, and females compete for quality of males. Trivers’ supply-and demand logic explained why in most species, males court and females choose.

In female mammals the costs of pregnancy and milk production are especially high, amplifying the difference between male competitiveness and female choosiness.

From the point of view of genes in any male body, the body itself is a sinking prison ship… The only deliverance for a male’s genes is through an escape tube into a female body carrying a fertile egg. Genes can survive in the long term only by jumping ship into offspring. This is why males of most species evolve to act as if copulation is the whole point of life. For male genes, copulation is the gateway to immortality. This is why males risk their lives for copulation opportunities.

For a female, too, the body is a sinking ship, but it has almost everything necessary to make more bodies: eggs, womb, milk. The only thing missing is a DNA packet from a male. But there are many willing donors… Quality becomes the issue… If she chooses an above-average male, her offspring get above-average genes, and are therefore more likely to survive and reproduce. It is for this reason that female mate choice evolved.

Almost all human pregnancies arise in sexual relationships that have lasted at least several months, if not years. Modern contraception has merely reinforced this effect.

Kenrick found that for one-night stands, women care much more about the intelligence of their partner than men do, but for marriage, men and women have equally high standards for intelligence. For almost every sexually desirable trait that has been investigated, men and women get choosier as relationships get “more serious.” For most couples, getting serious means having babies. Sexual selection works through the sexual choices that actually result in babies being born, not just the sexual choices that result in a little copulation.

Women quickly learn the difference between male short-term mating and long-term commitment. They know it is generally easy to get a man to have sex, but hard to get him to commit. Male mate choice is usually exercised not when deciding whether to copulate once, but when deciding whether to establish a long-term relationship. This is why sexual competition between women is usually competition to establish long-term relationships with desirable men, not competition to copulate with the largest number of men. Even polygynous men have limited time and energy, and so have high incentives to be choosy about their longterm partners.

Before sexual reproduction evolved, there were several ways for organisms to accomplish the evolutionary task of spreading their DNA around. There was the divide-and-conquer strategy: wrap DNA in single cells that busily eat nutrients until they grow large enough to split in half, leaving each half to grow and split in turn. Bacteria are the masters of this technique, capable of doubling their populations every few minutes, but vulnerable to mass extermination through perils such as toothbrushes and soap.

There was also the cloning-factory strategy: grow a body with billions of cells, and then assign the task of DNA-spreading to a privileged minority of those cells, which bud off to make new, genetically identical bodies. Many fungi reproduce this way, epitomizing the rustic virtues of simplicity and fecundity. Yet this strategy, though successful in the short term, stores up trouble for the long term. Once a harmful mutation arises, as it sooner or later will, there is no means of expunging it. This propensity to accumulate damaging mutations makes such asexual species quite unsuited to evolving much sophistication. This is because bodily and mental sophistication require a great deal of DNA, and the more DNA one has, the more trouble mutations cause.

In the last few hundred million years, an increasing number of species have turned to a third way of spreading their DNA around—the fashionable new method called sexual reproduction, with improved mutation-cleansing powers… parents. Of the 1.7 million known species on our planet, most engage in sexual reproduction. Sexual species include almost all plants larger than a buttercup and almost all animals larger than your thumb. It includes most insects, all birds, and all mammals, including all primates.

Sexual reproduction probably arose as a way to contain the damage caused by mutations. By mixing up your DNA with that of another individual to make offspring, you make sure that any mutations you have will end up in only half of your offspring. Your sexual partner will have mutations of their own, but they are almost certain to be different mutations on different genes. Because offspring have two copies of each gene, the normal version inherited from one parent often masks the failures of the mutated version inherited from the other parents. Incest is a bad idea because blood relatives often inherit the same mutations, which are not masked by normal genes when close relatives produce offspring.

By endowing the next generation with unequal numbers of mutations, sexual reproduction ensures that at least some offspring will have very good genes. They will preserve the genetic information that keeps the species working.

Sexual selection needs some way to connect the sensory abilities of animals to the mutation levels of the potential mates they are choosing between. Fitness indicators are the connection, for they are the traits that make fitness visible.

Perhaps the human mind’s most distinctive capacities evolved through sexual selection as fitness indicators. We could call this the “healthy brain theory,” in contrast to the runaway brain theory The healthy brain theory suggests that our brains are different from those of other apes not because extravagantly large brains helped us to survive or to raise offspring, but because such brains are simply better advertisements of how good our genes are.

From the viewpoint of an animal making sexual choices, fitness indicators are just proxies for good genes. But the sexual selection that results from mate choice does not just influence the genes for fitness. It shapes the fitness indicators themselves. These fitness indicators combine evolutionary fitness with physical fitness and mental fitness. That is the key. By trying to get good genes for their offspring, our ancestors unwittingly endowed us with a whole repertoire of very unusual fitness indicators which have come to form an important component of the human mind.

To traditional evolutionary psychologists, human abilities like music, humor, and creativity do not look like adaptations because they look too variable, too heritable, too wasteful, and not very modular. But these are precisely the features we should expect of fitness indicators. If a human mental trait shows large individual differences, high heritability, high condition-dependence, high costs, and high correlations with other mental and physical abilities, then it may have evolved through sexual selection as a fitness indicator. If we make an inventory of what the human brain can do, we find two general themes: very few of the ancient mental abilities that we share with other apes look like fitness indicators, but many mental abilities unique to humans do look like fitness indicators.

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