The mechanisms underlying the increasing genetic contributions to variation for some traits over development are not well understood. One likely factor is changes in gene expression, for example as triggered by pubertal processes. The presence of developmental changes in gene expression in the brain has been well established in animal models (Stead et al. 2006; Sun et al. 2005). A growing number of genes whose expression in the brain changes over development is being documented in humans as well, including genes related to the dopamine transporter (DAT), BDNF, its receptor, tyrosine kinase B (trkB), and others (Webster et al. 2006; Weickert et al. 2007).
