Esta pointed to this WaPo article identifying a potential genetic cause of post-traumatic depression; if you lack the gene, the studies’ authors say, “traumatic experiences are like falling off a bicycle, but genes determine whether the person is wearing a helmet.”
I can’t say that I find that analogy especially compelling, though I will note that during depressed times I find my brain function slowed, almost as if I’ve been trepanned. No, that’s not quite right; more like someone has put a felt barrier between my thoughts and myself.