Depression:Hardcoded?

Researchers found that violence in adults could result from the interaction between an abusive childhood and two different versions of a DNA switch that controls a gene called monoamine oxidase. Both childhood abuse and a particular version of the switch are needed to give a high chance of a person's becoming violent. In other words, then, it is nature with nurture that is the key, and not nature versus nurture.

Causes of depression:
The story starts with a gene called 5-HTT that controls the uptake of a chemical called serotonin into nerve cells. Serotonin carries signals between nerve cells and helps to regulate a person's mood. Persistently low levels of it are thought to cause depression. Several antidepressants, including Prozac, are thought to work by blocking the uptake of serotonin into nerve cells. If it hangs around longer, it produces a stronger signal.

People can inherit different versions of a region of DNA—known as a promoter region—that is responsible for switching 5-HTT on and off

5-HTT-promoters come in two flavours, known as “short” and “long”. Since people inherit one version of this promoter from their mother and one from their father, they can have two shorts, two longs, or one of each

Severe stress alone will not cause depression; the sufferer must have the short promoter variant. And the two-thirds of the population that have this must suffer severe stress before they are likely to become depressed.