A recent short article published in the Archives of General Psychiatry by McGrath et al. (2010) highlights that abnormal vitamin D status in newborn babies is a predictive factor for developing schizophrenia later in life. The study was conducted on 424 patients and 424 controls, all born in Denmark since 1981. Presumably the population the data set was constructed from was very large if they were able to find 424 babies who later developed schziophrenia.
The key finding is that the risk of schziophrenia has a U-shaped curve with neonatal serum concentration, which is illustrated in figure 2 of the article.The findings were statistically significant with a maximum relative risk of 2.1 for having very low vitamin D status compared to the optimum level.
One figure that's sadly lacking from the publication is a histogram of the entire population for vitamin D serum concentration. Since Figure 2 is only given in percentiles, we cannot evaluate what the actual optimal vitamin D concentration is, nor what is too high. I think this is a major oversight in this article.
The potential links between the general family of autism, schizophrenia, and bipolar disorder and vitamin D deficiency have been hypothesized before, such as in this article from the Vitamin D council. Obviously, with a relative risk of 2.0 vitamin D isn't the whole story here, but it likely plays a role in the regulation of brain development. The question is how? Is it a precursor hormone to a development hormone? McGrath referenced an earilier article (on which he was also an author), Eyles et al. (2005) which suggests that vitamin D plays a direct role as a paracrine hormone in the mammalian brain.