Republicans have better average mental health
From Gallup: "Republicans are significantly more likely than Democrats or independents to rate their mental health as excellent...This relationship between party identification and reports of excellent mental health persists even within categories of income, age, gender, church attendance, and education."
This isn't surprising. Let's reasonably assume that most voters generalize from their own experience and estimate that the average American is about as mentally healthy as they are. On economic issues, Republicans tend to want the government to leave them alone whereas the Democrats want the government to protect them. The lower your estimate of the average Americans' mental health the more you will think that Americans need protection and so the more you will politically favor the Democrats.
Republicans think that the way to reduce poverty is to create incentives for poor people to act in ways that cause them to be less poor. This approach implicitly assumes that poor people are mostly capable of acting rationally. The Democratic approach to reducing poverty of transferring large amounts of resources to the poor doesn't rely on this implicit assumption.
Finally, college professors are one of the most solidly Democratic groups in America. What does this Gallop report indicate about our average mental health?
