Personally, I don't know whether being tanned keeps me happy (as "melanin science" would suggest), but getting tanned sure lifts my mood for at least a few hours. What is the biochemical mechanism behind this?
Further, there seems to be a very rough but real relationship between latitude and attitude, with hotter climes correlating with hotter moods. This is a consistent theme through most literature at least since Shakespeare, with his hot-blooded Italians and melancholy Danes.
I read an article about how Tromso, Norway, the farthest Northern small city in the world, has no higher rates of seasonal depression than elsewhere. But, it appears from reading the article that Tromsonites have evolved a culture of self-therapy emphasizing near-mandatory conviviality during winter and bright artificial lights.
Further, self-selection is no doubt going on with people who can't stand the winters getting out of Tromso and others who don't mind them migrating in. If there is a genetic component to Seasonal Affective Disorder, this self-selection of darkness-likers will accumulate over the generations.
There's no doubt a big cultural component in this latitude-attitude correlation. For example, a culture is more likely to develop the charming tradition of shooting guns off in the air to celebrate (e.g., more Baghdadites were killed by falling bullets during peace celebrations at the end of the Iran-Iraq war than were killed by Iranian missile attacks during the eight year war) if it's not 20 below outside. In places where it's too cold to go outside, a culture will emphasize developing the kind of self-restraint that keeps you from blasting holes in the ceiling. There is probably also a biological component, but it's not clear if it's hereditary or environmental. In other words, when Jimmy Buffett sings that changes in latitude mean changes in attitude is he correct for within an individual, or just across ethnic groups?