The Genetic LinkEdit
I have not brought up the genetic link up till now, but it is without doubt one of the primary risk factors. Depression seems to run in families, and even after the environmental effects are taken into account, the genetic link is still clearly there . Some studies have shown that approximately one out of every three people have a genetic predisposition to develop a depression. However, like in many other cases, the interplay between genes and environment is also relevant for depression: only about half of those with the genetic predisposition will actually develop the illness. In any case, should you have cases of depression among close blood relatives, do take it as a warning that you too might be at risk.
Note: A person is most closely related to their siblings, their parents, and their children. In either case, you share with them approximately 50% of your genes (for which there is variance among the breeding population). Grandparents, grandchildren, aunts, uncles, nephews, and nieces are next: the shared portion is approximately 25%. In these cases, the conditional probability of having a depression knowing that your relative had a depression is higher than the above mentioned absolute probability of about one third. For relatives farther beyond the genetic proximity measure (cousins, etc.), the conditional probability approaches the absolute probability for the general population, and is therefore not quite as relevant as an indicator.