  That is why alternative etiologies can complement, rather than compete with, each other, as in Meehl’s (1977) example of an elderly man, genetically somewhat at risk for depression, who develops it after his spouse dies. It seems wrong to call this a genetically caused episode of depression: the genes needed an unusual environment, so that in most other circumstances the man would not be depressed. But he would not have developed clinical depression had his genes been different. Both factors are causally relevant.