Self-selection is a term used to indicate any situation in which individuals select themselves into a group. It is commonly used to describe situations where the characteristics of the people which cause them to select themselves in the group create abnormal or undesirable conditions in the group.
Self-selection makes it difficult to determine causation. For example, one might note significantly higher test scores among those who participate in a test preparation course, and credit the course for the difference. However, due to self-selection, there are a number of differences between the people who chose to take the course and those who chose not to. Arguably, those who chose to take the course might have been more hard-working, studious, and dedicated than those who did not, and that difference in dedication may have affected the test scores between the two groups. If that was the case, then it is not meaningful to simply compare the two sets of scores. Due to self-selection, there were other factors affecting the scores than merely the course itself.
Self-selection causes problems for research about programs or products. In particular, self-selection makes it difficult to evaluate programs, to determine whether the program has some effect, and makes it difficult to do market research.