It is easily noticed that a very large percentage of atheists are liberals. What I want to try to do is explain where this correlation comes from. One can say that atheists are smarter than theists, liberals are smarter than conservatives, therefore the correlation exists because atheism and liberalism are for smarter people. This is the kind of response your would probably hear from a liberal. And I think this response is completely wrong.
The reason being to why I think that response is wrong is that I do not think that liberals become liberals because of economic issues. In the same manner, conservatives are not conservative for economic issues. Economic issues are often the last thing on the mind of a liberal and a conservative. I mean, of course, the typical average liberal and the typical average conservative. If we pick a liberal at random and a conservative at random the chances would be that they have the views that they have not because they carefully studied the economic questions but because these parties are more in line with their social values.
It is not economic questions that make a typical average liberal a liberal and a typical average conservative a conservative. It is really social values that these people have. There are liberals and conservatives who really are concerned about economic questions but that is not very common in both parties. So what really separates the two parties are social values.
I speak from experience here. I know that when I started to leave Judaism I thought of myself as a liberal (Democrat). Politics was not something I thought much back then (I still never think much of politics, there is a difference between politics and political philosophy, what I am very interested is political philosophy not politics, back in those days I did not even think anything about political philosophy). The reason why I considered myself a liberal was simply because of social issues. I never thought about economic issues. What I saw was that the liberal party and the conservative party had conflicting social values. Because I was no longer religious I did not share with the standard social values of the conservative party. I had no problem with gay marriage. I had no problem with people smoking marijuana. I had no problem with women having more opportunity. I did have a problem with religion mixed with government, so I supported separation of church and state. I did not think that just because a person was different from me I have to treat him any less. And so forth. These are liberal values, the opposite are conservative values.
Thus, I called myself a liberal by simply looking at social issues and seeing which ones suited me more. The problem with such an identification is that it was groupthink. Because I called myself a liberal what I did was that I supported the liberals in what they did. Thus, whatever economic views they had I supported not because I had any economic opinion but more simply they represented a group that I supported. The religious conservatives do the same thing. Take a typical conservative and ask him about why he is a conservative. He will talk about social values. He would be opposed to gay marriage, or to marijuana, or to abortion, or to illegal immigrants, his religious views are more in line with his social values and so he identifies himself as a conservative. Conservatives also groupthink. They identify themselves to a party and they support the economic decisions of this party even though they themselves might not have much to say on economic issues. Indeed, from my interaction with conservatives I realized that conservatives are not fiscal conservatives. They are not opposed to government spending if the government spending is going into something they want. This is not very fiscally conservative of them to do. The point is that your typical conservative does not care about economic issues but social issues.
This is why such a large percentage of atheists are liberals and why such a large percentage of (more fundamentalist) theists are conservatives. There are liberals who are strongly concerned about economics and there are conservatives who are too but those are the exceptions to the rule.