Traveling on a separate path, however, intelligence researchers have amassed just as much evidence that individual differences among many disparate cognitive abilities are correlated with one another. This suggests the possibility of causal forces that influence performance on most cognitively complex cognitive tests, regardless of the content. Recently intelligence researchers have proposed two possible causal forces: (a) deleterious mutations or developmental abnormalities that influence many different cognitive mechanisms or (b) cognitive mechanisms that are utilized to some extent in most or all complex cognitive tasks.

Scientists such as Matthew Keller, Geoffrey Miller and Ronald Yeo and others have done important research on (a). I have tended to focus on (b), as have other scientists such as Christopher Chabris, Andrew Conway, Jeremy Gray, Nicholas Mackintosh, and Han L.J. van der Maas. Of course, the two causal forces aren't mutually exclusive. It is most certainly an interaction of multiple cognitive mechanisms, all of which are affected by developmental instability, that causes the general factor of intelligence (or g for short) to emerge. It is highly unlikely that g is the result of a single process.

Regardless of what comprises general intelligence, at first blush the mere existence of a general intelligence factor seems incompatible with the strong modularity view of the mind-- g is domain-general rather than domain-specific, since it is associated with performance on cognitive tasks in a multitude of different contexts.

To be fair, in Leda Cosmides and John Tooby's original formulation of the evolution of the human mind, they acknowledge the existence of such domain generality, calling such forms of reasoning improvisational intelligence. They argue that this form of reasoning is employed whenever a module doesn't exist to solve a particular problem. They didn't get too much into individual differences in domain-general cognition, though, and the field of evolutionary psychology in general has tended to focus on what Leda Cosmides and Tooby refer to as dedicated intelligences that exist, in some degree, in nearly every member of the human species. Evolutionary psychologists have tended to assume that for any trait important to fitness, selection pressure would reduce variance around an optimal level of the trait, with individual differences being random noise.

Recently, a number of researchers, including Lars Penke, Geoffrey Miller, and Satoshi Kanazawa, have attempted to unite evolutionary psychology with differential psychology. To further explore this tension between evolutionary psychology and the theory of general intelligence, I recently teamed up with a superb team of scientists (Colin DeYoung, Jeremy Gray, and Deidre Reis) to examine individual differences in a paradigm that has been used extensively by evolutionary psychologists to provide evidence that cognitive abilities are domain-specific rather than domain-general: the Wason four-card selection task.

On the Wason card selection task, participants are presented with four cards. Participants are told that each card has a letter on one side and a number on the other side. Their task is to decide which cards (and only those cards) need to be turned over to find out whether a given statement is true or false. Here is a common item (try it yourself!):

If you're like most people, you probably chose A or you choose A and 3. The correct answer is A and 7. It's important to select the 7 card in order to actively try to falsify the statement-- the hallmark of good scientific reasoning. Bad scientific reasoners only search for theories ('cards') that confirm their preconceived ideas. In this example, if we turn over the 7 card and there is an A on the other side, we know we have violated the rule. Only about 10-20% of people choose both A and 7.

Try another version of the same task:

If you're like most people, this version of the task was much easier to solve. Now over 75% of people solve this version of the problem. What a stark difference! But why? what is it about contextualizing the task that makes it so much easier to solve? Various proximal psychological theories have been proposed to explain this effect, but here I want to focus on evolutionary explanations.

According to evolutionary psychologists such as Leda Cosmides and John Tooby, reasoning about realistic scenarios such as social exchange and precautions is supported by dedicated information processing modules that result from evolutionary selection pressures exerted by situations involved in social exchanges or physical danger. These sort of situations involve if-then reasoning concerning violation of rules of specific content. Since no such pressure has been exerted by situations that weren't reoccurring themes in our evolutionary ancestry, the human mind doesn't have cognitive modules for more decontextualized forms of reasoning or reasoning using arbitrary rules. In those situation, performance is expected to be much worse.

This hypothesis doesn't leave much room for general intelligence in domain-specific forms of reasoning that were important in our evolutionary ancestry. I very much wondered, though, whether this is really true. It seemed entirely possible to me that there could be both domain-general and domain-specific contributions to any reasoning task. The prior research on this matter has been mixed, with some studies finding a stronger relation between cognitive ability (sometimes measured using SAT scores) and abstract reasoning relative to contextualized reasoning, another study finding the opposite, and another finding intelligence to be similarly associated with both types of problem.