Why isn’t Everyone a Genius?

There is one argument against the claim that the distribution of intelligence is different by race or gender that I have very rarely seen and have not discussed here — the claim that since there is no advantage to being stupid there is no reason why evolution would produce any stupid people. The obvious problem with it is that, race and gender aside, stupid people exist. But that raises the question of this post. Being smart is a good thing, it is possible to be smart — some people are — so why isn’t everybody?

I see three possible sorts of explanation.

Biological Cost

The brain is expensive, uses a much larger fraction of energy than its fraction of body weight. I don’t know if there is evidence that the cost is larger for smarter people, don’t even know if there is a correlation between brain size and intelligence,¹ but both seem plausible. If so, there is a tradeoff between intelligence and nutritional requirements. Greater intelligence might have others sorts of biological cost, increase the risk of cancer say, or rate of aging. If, as seems likely, the reproductive advantage of intelligence faces diminishing returns, is less the more smart people there are, one would expect an equilibrium where the marginal benefit of increased intelligence was just balanced by the marginal cost.

Reproductive Cost

The reproductive advantages of increased intelligence are obvious: The smarter you are the better job you can do of producing food and evading predators. The smarter a man is the more able at persuading women to bear his children. The smarter a woman is, the more able to attract a high quality mate who will help to support the children she bears, avoid being seduced by males who won’t.

But there is also a reproductive cost — because there is a conflict of interest between me and my genes. They “want” me to produce and successfully rear as many children as possible. I want to have a long and happy life. They try to bribe me into obeying them by making sex pleasurable. I take the bribe and escape obedience by masturbation, non-reproductive sex, contraception. They try to bribe me by making me enjoy interacting with children. I get a cat or dog to interact with instead. The more intelligent I am, the more likely I am to perceive the conflict and act to achieve my goals, not theirs.

Construction Difficulties

High end sports cars go fast, handle well and, I am told — I have never owned one — spend a lot of time in the shop. In order to get a high level of performance you have to get everything exactly right. The more things you have to get exactly right the more likely you are to get something wrong.

It presumably works for humans as well. The higher the level of performance the genes are aiming for in building you, the more likely it is that something in the design that has to go exactly right will go wrong. They can aim to produce a genius every time — and end up with an idiot or a lunatic nine times out of ten.

For a race car the payoff to being just a little faster is that you win the race, which makes it worth paying the cost of a lot of repairs, risking something going wrong. For my Honda Odyssey the payoff to being a little faster is much lower. It pays Honda to buy reliability at some cost in performance — and they do.

What about humans? A sufficiently successful man can father a hundred children, an unsuccessful man may father none — because no woman is willing to bear them for him. A successful woman is limited to the number of children she can bear. The scarce input to reproduction is womb space; even a less successful woman has it. With fewer resources she cannot bring as many children to adulthood as the successful woman can but she has the resources needed to produce them, may be able to trade the use of her womb to bear them for help bringing them up.

It follows that reproduction is a higher stakes game, the payoff varies more, for men than for women. We are designed by our genes for reproductive success. From the standpoint of the genes, it is worth paying a higher cost, accepting a greater risk of failure, in designing men. That is at least a plausible explanation for the observation that male IQ has a wider spread than female, more geniuses and more morons.

Explaining Diversity

My first two explanations of why we are not all geniuses showed why people are not as smart as they could be. It did not show why people, all of whom are trading off costs of being smart against benefits to maximize their reproductive success, do not all end up equally smart. The existence of reproductive costs, biological or behavioral, to intelligence implies that the optimal design does not maximize intelligence but why isn’t the optimum the same for everyone?

My third explanation provides one answer: Morons were designed to be geniuses but something went wrong. There is a different explanation that works for the first two as well. The benefit from an ability, including intelligence, depends on how many people have it.

Consider giraffes. A giraffe faces a tradeoff between the advantage of a longer neck, the ability to reach higher leaves, and the costs, the need for more food to sustain the larger body, higher blood pressure to get the blood up to the brain. If other giraffes were only twelve feet tall there would be a large advantage to being eighteen feet tall, lots of leaves unreachable by the others to eat. The more tall giraffes there are, the less that advantage. With enough, the gain just balances the cost, and a slightly shorter giraffe does just as well, the loss in access to a little more food balanced by the gain of the lower cost of a shorter neck. With enough slightly shorter giraffes, a still shorter one does equally well, and so on down. In equilibrium, the higher leaves are just enough less browsed to pay the cost of having a long enough neck to reach them.

The same logic applies to humans. The more smart people there are, the more of the things that can only be done by smart people are being done and the less the benefit to being one of the people able to do them. In equilibrium, if everyone has been optimized by evolution (ignoring construction difficulties — we are in the world of the first two explanations), all levels of intelligence that exist have the same reproductive payoff.² Hence, even without construction difficulties, humans will vary in intelligence. And height. And strength. And …

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1. A reader links in the comment thread to evidence that it does.

2. The same logic explains why all the checkout lanes at the supermarket take about the same time to go through, all careers have the same net advantage — and when they don’t. For an explanation see Chapter 1 of my Price Theory, starting at the subhead “Supermarket Lanes.”