Low IQ is a risk factor for developing schizophrenia, though the mechanism behind this association is somewhat unclear. A new study sheds a little light on this subject, and suggests the link might be genetic. The gene in questions is neuregulin 1, about which little is known. They find, first, that a regulatory SNP is associated with the development of psychotic symptoms in a particularly at-risk population (see part a above– each bar is the percentage of subjects developing symptoms for a given genotype). They also find lower levels of activity in certain part of the brain in the patients with the TT genotype (see parts b and c above).
Further, here are the means and standard deviations of the IQ distributions of the different genotypes:
CC: 101.9 (8.4)
CT: 100.4 (9.4)
TT: 94.3 (6.9)
So this regulatory polymorphism could explain some of the natural variation in IQ.