There are at least two separate questions here: 1) why does the US have only two major parties; and 2) given such, why do people often pick one and stick with i

There are at least two separate questions here: 1) why does the US have only two major parties; and 2) given such, why do people often pick one and stick with it for all issues? The first has been studied to death. It has to do with more with how the US government is set up and less with the psychology of its people. There are other countries with different setups that are not two-party dominant. One viable way out of such dominance is some form of party-list proportional representation (see http://en.wikipedia.org/wiki/Party-list_proportional_representation http://en.wikipedia.org/wiki/Party-list_proportional_represe...) . The second is all about psychology, it most certainly has several components at play. I posit the most prevalent are attitudes of commitment given the initial choice of party and some form of path of least resistance. The former is common in psychology. It is a natural instinct to think your group is better as soon as you join it, for whatever reason, even an arbitrary one. This even happens in psychology experiments when groups are assigned randomly. The latter reason is simply most people don't research any issues nor do they want to. They go with perhaps the simplest method to decide, which is listen to what the group told me before. There is some real trade-off here. To be fully informed is perhaps impossible. You need some sort of proxy for most policy issues. Blindly following one of two major parties (on the other extreme) is probably not the answer either of course.