But even this explanation only goes so far. If one controls for household income, Republicans still hold a significant edge: that is, poor Republicans are happier than poor Democrats; middle-income Republicans are happier than middle-income Democrats, and rich Republicans are happier than rich Democrats.Pew research used multiple regression analysis to control for all the available factors and found that, other things being equal (i.e. controlled for), such as health, income, faith, and marital status, Republicans are still happier than Democrats.
Polls, of course, cannot address causality. So, they cannot answer whether being happy make a person more likely to be a Republican, or whether being a Republican make one more likely to be happy.
One can speculate on other affects. For example, it seems reasonable to me that depressed people are more likely to be believe in the truth of disaster scenarios than happy people. This may explain why depressed Democrats have much stronger convictions that, for example, Iraq, Afghanistan, and global warming are all disasters.