Lamarck wasn't all wrong. A parent's experiences can be passed onto a child. In particular, a mother's drug use, even if discontinued before pregnancy, can "raise the risk among both their children and grandchildren for addiction or other psychiatric disorders." Presumably, the mechanism is epigenetic. Mouse models have shown that this effect can happen independent of genetics, as it can arise in previously healthy mice who are intentionally given drug addictions. Similar mouse models have shown that obesity propensity may have similar sources.
Researchers have shown directly what the corollation between advanced parental age and a variety of mental illnesses including mental retardation had long suggested. A significant share of mental retardation cases arise from mutations that either arise first in a child, or arise first, for example, in a mother in the case of a mutation that only manifests in men.
Progress has been made in identifying a mechanism for schitzophrenia that may explain why it manifests in adolescence instead of early childhood. A certain pair of cells called pyramid cells and chandelier cells may be atrophied. Excess neutral connections in children provide reserves of neural capacity that mask the effect. But, excess connections are apparently thinned out in adolescence, causing the symptoms to manifest. Identifying a mechanism may lead to more effective treatments and may make early diagnosis more important. For example, a treatment might be designed to prevent the thinning of neural connections that manifests in adolescence, leaving patients with tween minds as opposed to adult schitzophrenic minds. A better understanding of the mechanism involved may also shed more light on underlying how biochemical and genetic traits could give rise to the condition. From here.