Boy, this is an age-old discussion...and it always seems like science is trying to "prove" human behavior is purely genetic. And there's periodically  this "aha! how we know" statement made. But if it were true, you would think we would have "won" the war on cancer along time ago, yes?

But...Well, humans are just too complex to prove anything causal...the best we can do is show possible significant relationships, and that involves much speculation, as it is. I have worked with a lot of people over the years as a psychologist, and there's a few where you could say maybe, "yes, this looks like there might be a genetic or other physiological involvement"...and if that is the case, I'd get medical consultation. I've come to believe that "bi-polar affective disorder" is pretty much physiological, though there always seems to be more going on than that for a person. But depression? That's too complicated to be just written off as "genetic" (imho)...I once worked with person who went off meds for depression, and substituted those for a good diet, vitamins, daily exercise, sitting under a light daily and meditation...and they were actually feeling better...though they really had to stay in this regime, for otherwise they started getting depressed again. Long story short...what causes happiness is not that simple of a question...

"Once in awhile we get shown the light, in the strangest of places, if we look at it right" - Hunter/Garcia