Intelligence is hard to define. We think of it as one thing, but it has so many different parts: verbal ability, spatial ability, memory, pattern recognition, logic, social judgement, artistic ability, lateral thinking, intuition, musical ability…the list goes on and on. Some scholars, like Howard Gardner, famously argue that there’s not one intelligence, but many.

But as a teacher once told me, if you want a single umbrella to cover ALL of intelligence, then it’s the ability to adapt; to take whatever situation you’re in and turn it around to your advantage. That’s really what intelligence is, he asserted. I like this definition because it integrates all the different parts of intelligence into a coherent system. It also has common sense appeal. If you turn a situation to your advantage (negotiate a good business deal) people will praise you for being clever. If you turn a situation to your disadvantage (drive your new car into a tree) people will immediately call you an idiot.

We even have sayings like “if you’re so smart, why aren’t you rich?” and “a fool and his money are soon parted.” These sayings make the point that smart people turn life to their advantage; dump people do the opposite. Further, if a scientist invents a cure for a disease or some new technology, he has provided a great advantage to all of humanity and it praised as a major genius.

Finally, brain size roughly tripled in the last 4 million years of human evolution, so intelligence was clearly an adaptive advantage in even the evolutionary sense of the term.

The ability to adapt situations to our advantage is why humans are considered the most intelligent animal. Despite having so many disadvantages (we lack fur, strength, speed, claws, sharp teeth, wings) we were able to adapt the world to our advantage. We didn’t have fur, so we created fur coats. We didn’t have claws, so we created knives. We couldn’t run fast, so we created cars. We didn’t have wings, so we invented airplanes etc. We were able to use plants to our advantage (agriculture) and animals to our advantage (domestication) and subdue and capture animals like gorillas who are many times our size and strength.

So despite being such a weak disadvantaged animal, our freakishly large brains allowed us to become the most powerful and prosperous animal on the planet.

However there are problems with the above definition of intelligence. For one, if you define intelligence as “turning situations to your advantage” it sounds like you’re saying sociopathic opportunists are intelligent and martyrs are stupid. As comic Bill Maher recently said “to cowards, courage always looks like stupidity.”

My rebuttal to this criticism is that if you’re a good person, then being selfish is not to your advantage, because, to quote the Bible “For what shall it profit a man, if he shall gain the whole world, and lose his own soul?” The problem is unless you know what motivates another person, it’s sometimes hard to judge whether their behavior is adaptive or maladaptive. The cost/benefit analysis would vary from person to person making this definition abstract and subjective.

Another problem with this definition (as commenter “Swanknasti” stated) is that in biology, the term adaptive specifically means advancing your genes, while in the context of defining intelligence, adaptive behavior is probably behavior that advances your goals. The two definitions are related though, because we evolved feelings (fear, hunger, love, tribalism, jealousy, etc) that motivate us to pursue goals that advance our survival and the survival of our family and race, but many smart people lack these primitive motivations causing their behavior to appear maladaptive to a biologist.

A third problem with this definition, as commenter “godslayer” pointed out is that a lot of unintelligent animals (i.e. cockroaches) are highly adaptive so it seems wrong to equate intelligence with adaptability. While cockroaches are well adapted physically, they lack a mental ability to adapt their behavior.

But godslayer exposes the need to be more precise in how intelligence is defined. Perhaps the most precise definition of intelligence was offered by scientist Arthur Jensen who wrote (emphasis mine):

“The term ‘intelligence,’ then, would apply only to the whole class of processes or operating principles of the nervous system that make possible the behavioral functions that mediate the organism’s adaptation to its environment, such as stimulus apprehension, perception, attention, discrimination, stimulus generalization, learning, learning-set acquisition, remembering, thinking (e.g., seeing relationships), and problem solving

Even though Jensen seemed to agree that intelligence was about adaptation, he wanted psychologists to stop using the word “intelligence”, writing:

I find it pointless to talk about intraspecies individual differences in “intelligence” as I have defined it. …the word ‘intelligence’ in this broad generic sense causes confusion in discussing individual differences in humans, as I hypothesize that all biologically normal humans possess the same intelligence in the sense in which I have defined it, but they show quantitative differences in these functions, which are best described behaviorally in terms of independent latent variables, or factors.

Other scientists that emphasized adaptability when defining intelligence include Stephen Hawking who said “intelligence is the ability to adapt to change” and scholar Robert Sternberg defined intelligence as “the ability to adapt to the environment, or modify the environment, or seek out and create new environments.”

Along similar lines, scholar David Wechsler said “intelligence is the capacity to understand the world and the resourcefulness to cope with its challenges.”

Abstract reasoning

Not all scientists emphasize adaptability when defining intelligence. Some emphasize abstract reasoning. But these are related. Abstract is the opposite of specific, which means if you can reason on a very abstract level, you can adapt to a wide range of environments instead of only one specific type. A major difference between humans and other animals is that animals have instinct which is useful in specific environments while intelligence is general enough to adapt to almost any situation.