How Did General Intelligence Evolve? - What Is Intelligence?

The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012

How Did General Intelligence Evolve?
What Is Intelligence?

As I discuss in Chapter 1, evolutionary psychology contends that the human mind consists of evolved psychological mechanisms. Evolved psychological mechanisms are domain-specific. It means that these evolved and innate solutions to adaptive problems each operate only in their own specific narrow domains of life.

For example, the cheater detection mechanism, which was among the very first evolved psychological mechanisms to be discovered,16 operates only in the domain of social exchange; it helps us detect potential cheaters when they try to cheat us out of a fair exchange. But the cheater detection mechanism does not help us, nor is it operative, in any other domains of life. It does not help us, for example, learn our native language. It does not help us decide how to allocate limited parental resources among our children (in other words, which of our children to favor unconsciously—yes, parents do have favorites among their children17). And it does not help us recognize familiar faces of the people we know.

By the same token, the language acquisition device, which helps us learn our native language from our mothers when we are small children,18 does not help us detect cheaters in social exchange, decide how to allocate parental resources, or recognize familiar faces. In fact, it does not help us do anything except for the one task of learning our native language. It does not even help us learn second and third languages as adults, which is why learning a foreign language is so much more difficult than learning to speak our native language as a child, which comes very naturally and easily to us (because we have the innate ability to do so).

Evolved psychological mechanisms are domain-specific because adaptive problems, which they are designed to solve, are domain-specific. All problems of survival and reproduction happen in specific domains; there were no general problems that did not happen in a specific context for our ancestors to solve. Evolution did not give us a domain-general solution like a computer because there were no domain-general problems like an IQ test in the ancestral environment.

However, if the contents of the human brain are domain-specific, how can evolutionary psychology explain general intelligence, which is seemingly domain-general, not domain-specific? Isn't general intelligence a domain-general solution?

General intelligence thus posed a significant theoretical problem for evolutionary psychology for a long time. How can evolutionary psychology explain the evolution of general intelligence? I believe that what is now known as general intelligence may have originally evolved as a domain-specific adaptation to deal with evolutionarily novel, nonrecurrent problems.19 Now what does that mean?

The Pleistocene Epoch (between 1.6 million and 10,000 years ago), during which humans evolved, was a period of extraordinary constancy and continuity. Nothing much happened for more than a million years. Our ancestors were hunter-gatherers on the African savanna all of their lives. Their grandparents were hunter-gatherers on the African savanna all their lives. Their parents were hunter-gatherers on the African savanna all their lives. Their children were hunter-gatherers on the African savanna all their lives. Their grandchildren were hunter-gatherers on the African savanna all their lives.

This kind of constancy is difficult for us to fathom. In our grandparents’ generation, most people were farmers; now very few people are. In our parents’ generation, most people were factory workers; now very few people are. Now most of us, regardless of our specific occupation, conduct our business and trade at least partly on our computer, a device that did not exist in our grandparents’ or even our parents’ generation.

It is against the backdrop of the extreme stability of our ancestors’ environment during the Pleistocene Epoch that all of our psychological adaptations evolved. For instance, those who had a taste for sweet and fatty food during the Pleistocene Epoch lived longer and reproduced more successfully, by acquiring more calories, than those who did not have such a taste for sweet or fatty food.20 Or those who preferred a certain landscape for their habitat lived longer and reproduced more successfully, by avoiding potential predators in hiding, than those who did not have such a preference.21 The evolution of psychological mechanisms—or any adaptation, physical or psychological—assumes a stable environment. Because evolution usually takes place very, very slowly, over tens and hundreds of thousands of years, solutions cannot evolve in the form of psychological mechanisms if the problems keep changing.22 The fact that we have so many evolved psychological mechanisms today is testimony to the extraordinary stability and constancy of the ancestral environment.

Technically, the speed of evolution depends on the strength of selection pressure—how crucial it is for survival and reproduction to solve a given adaptive problem. The rate of evolution of a trait is proportional to the adaptiveness of the trait—the correlation between possessing the trait and being able to reproduce. For example, if cosmic rays from an explosion of a nearby supernova render all but redheads sterile, then in just one generation everyone on earth will be a redhead because no one else will be able to reproduce. But selection pressures are usually much weaker (for example, the cosmic rays will allow redheads to reproduce at a 5% greater rate than everyone else), so evolution of most traits take many, many generations.

Because adaptive problems in the ancestral environment remained more or less the same generation after generation, our evolved psychological mechanisms were sufficient for our ancestors to solve them. In this sense, our ancestors did not really have to think in order to solve their adaptive problems. They didn't have to think, for instance, what was good to eat. All they had to do was to eat and keep eating what tasted good to them (sweet and fatty foods that contained high calories), and they lived long and remained healthy. People who preferred the wrong kind of food—like brightly colored mushrooms or an entirely vegetarian diet—died off before leaving too many offspring, and we did not inherit our psychological mechanisms from them. All the thinking had already been done by evolution, so to speak, which then equipped our ancestors with the correct solutions in the form of innate domain-specific psychological mechanisms. For the most part, our ancestors never had to figure out problems on their own.

Even in the extreme continuity and constancy of the ancestral environment, however, there were likely occasional problems that were evolutionarily novel and nonrecurrent, which required our ancestors to think and reason in order to solve. Such problems may have included, for example:

1. Lightning has struck a tree near the camp and set it on fire. The fire is now spreading to the dry underbrush. What should I do? How could I stop the spread of the fire? How could I and my family escape it? (Since, as they say, lightning never strikes the same place twice, this is guaranteed to be a nonrecurrent problem!)

2. We are in the middle of the severest drought in as long as anyone can remember. Nuts and berries at our normal places of gathering, which are usually plentiful, are not growing at all, and animals are scarce as well. We are running out of food because none of our normal sources of food are working. What else can we eat? What else is safe to eat? How else can we procure food?

3. A flash flood has caused the river to swell to several times its normal width, and I am trapped on one side of it while my entire band is on the other side. It is imperative that I rejoin them soon. How could I cross the rapid river? Should I walk across it? Or should I construct some sort of buoyant vehicle to use to get across it? If so, what kind of material should I use? Wood? Stones?

To the extent that these evolutionarily novel, nonrecurrent problems happened frequently enough in the ancestral environment (a different problem each time[23]) and had serious enough consequences for survival and reproduction, then any genetic mutation that allowed its carriers to think and reason would have been selected for, and what we now call “general intelligence” could have evolved as a domain-specific psychological mechanism for the domain of evolutionarily novel, nonrecurrent problems, which did not exist in the ancestral environment and which there are no dedicated psychological modules to solve.

From this perspective, general intelligence may have become universally important in modern life24 only because our current environment is almost entirely evolutionarily novel. In the ancestral environment, general intelligence might have been no more important than any other domain-specific evolved psychological mechanism, like the cheater detection mechanism or the language acquisition device. General intelligence helped our ancestors only in the very narrow domain of evolutionary novelty—evolutionarily novel problems were by definition few and far between in the ancestral environment—just as the cheater detection mechanism helped them only in the very narrow domain of social exchange and the language acquisition device helped them only in the very narrow domain of the native language acquisition. General intelligence became more important than other evolved psychological mechanisms only because our environment has changed so radically in the last 10,000 years and most of the problems we face today are evolutionarily novel. The importance of general intelligence may itself be evolutionarily novel.

This theory suggests that more intelligent individuals are better than less intelligent individuals at solving problems only if they are evolutionarily novel. More intelligent individuals are not better than less intelligent individuals at solving evolutionarily familiar problems that our ancestors routinely had to solve. The theory suggests that the performance of general intelligence, as but one domain-specific psychological mechanism, is independent of the performances of all the other domain-specific psychological mechanisms. I review some of the evidence for this theory of the evolution of general intelligence in the next chapter.