Proceedings of the Royal Society B has a paper out which reports evidence of positive selection on genes which seem to have some relationship to the heritability of schizophrenia (Nature has a good summary). The authors imply that the genes in question were likely selected for reasons totally unrelated to schizophrenia, but that the new variants increase susceptibility toward the disease. This is understandable, rapid selective bursts can result in the increase in frequency of alleles which might have negative side effects that are on the balance erased by the benefits which they confer (a natural outcome of pleiotropy). An engine which is pushed to its limits can accelerate faster and maintain a higher constant velocity, but it is also more likely to overheat or malfunction in some manner. Over time one assumes that engineers can design systems which can produce all the positive effects while masking or eliminating the negative one, but such improvements take time. And so it is with evolution. It seems that our species (H. sapiens that is) has been subject to very powerful bursts of selection of late. A possible consequence of this is that a host of deleterious phenotypic outcomes which have yet to be purged by the reshaping hand of selection may also emerge concomitantly. Adaptive evolution wins in the long run, but over the shorter time scale the noise of maladaptive correlated response is something that needs to be remembered. Note: Yes, I know schizophrenia is a complex disease, and genes do not tell the whole story in all likelihood.