Why are there not more science majors?

There is a new paper (pdf) by Ralph Stinebrickner and Todd R. Stinebrickner on this topic, and here is their bottom line conclusion:

We find that students enter school quite optimistic/interested about obtaining a science degree, but that relatively few students end up graduating with a science degree. The substantial overoptimism about completing a degree in science can be attributed largely to students beginning school with misperceptions about their ability to perform well academically in science.