The reason for a decrease in total variance over development, particularly that due to environmental factors, is not obvious. It may seem more logical that variance because of environmental factors should increase with age as opportunities for exposure to unique environmental events extend. For the early-developing cortical regions in which environmental variance does increase during the age range of our study, this may be the case. However, for the later developing areas, the heritable phenotype is something that itself is being created over time. As described earlier, there is an increasing body of literature regarding changes in gene expression continuing over the course of postnatal development. If genes with particularly strong effects come on line, they may potentially substantially impact the variance because of genetic factors. The interaction of genetic with environmental factors over time may also increase heritability.
