Self-selection
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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.
For example, if one state offers significantly better benefits to the poor than a neighboring state, some of the poor may migrate from the state with worse benefits to the state with better benefits. Thus, the state which gives greater benefits will find itself burdened by more and more demand for those benefits, while the state which gives less benefits will find that it is burdened by less. This could result in a race to the bottom, in which states continually lower their benefits to force the poor out of their state and lower their fiscal burden.
Another problem posed by self-selection is determining 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.
A third problem posed by self-selection is skewed polling. For example, a poll taken of residences during the day will poll an artificially high number of homemakers and unemployed individuals, because those who are working are not at home to pick up the phone.
Self-selection causes problems for research about the 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.
Self-selection is a major problem is research in sociology, psychology, economics and many other social sciences.