This comment was in response to my earlier post which argued that researchers should try to discover the genetic causes of mental illness instead of trying to decipher intelligence. The commenter makes some excellent points, although I still believe that untangling the (incredibly) complicated genetic underpinnings of mental illness has far more social value than connecting the dots between IQ and race.
It’s far from clear that schizophrenia is a less complex phenomenon than intelligence. In fact, the opposite may be true. In the first instance, schizophrenia isn’t an ‘unambiguous personality trait’ (it’s not really a personality trait at all); it’s a severe, pervasive, and extremely heterogeneous mental disorder. Some individuals exhibit positive symptoms (e.g., hallucinations, delusions, etc.), others exhibit only negative symptoms (flat affect, apathy). Some people have a single psychotic break and make a full recovery; others are doomed to an institutionalized life. One of the biggest problems schizophrenia researchers face is deciding just how to categorize all the different manifestations of schizophrenia. It’s not even clear yet whether it’s a spectrum disorder, a qualitatively different state, or a collection of disorders of unrelated etiology that happen to have overlapping phenotypic expressions. In general, if you were to ask for a ‘simple’ psychological phenomenon, I think schizophrenia would be one of the last items on most researchers’ lists.
Contrast that with intelligence. The many manifestations of intelligence, which reliably load very highly on a single underlying factor (the so-called ‘G’), popular notions of multiple intelligences notwithstanding. You can take almost any two measures intuitively thought to tap intelligence and odds are they’ll be positive correlated. Typically, fluid G accounts for anywhere between 30-70% of the variance in zero-order measures. That’s huge, and suggests that there are likely to be a relatively small number of factors contributing to variations in general intelligence. For example, brain volume alone (and particularly prefrontal volume) accounts for about 10% of the variance in gF scores. Now I’m certainly not suggesting intelligence is simple; but I do happen to believe (as I suspect plenty of other researchers do) that intelligence will prove to be easier to understand than a disorder like schizophrenia. If we were to take it one step at a time, we’d never get anywhere. It’s good for scientists to keep in mind that the issues they deal with are complex, but only up to a point.