We find that 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.
An intermediate step in reaching this conclusion involves examining the process by which students update their beliefs about grade performance in each major. We again find evidence that science deviates from other majors.
By and large, the primary way to learn about one’s ability in science is to take science classes, while, for majors j other than science, students tend to collect much of their information about their ability in j from classes taken outside of the major.
Full paper from Stinebrickner & Stinebrickner.