What is Drift hypothesis?

Drift hypothesis, concerning the relationship between mental illness and social class, is the argument that illness causes one to have a downward shift in social class. The circumstances of one's social class do not cause the onset of a mental disorder, but rather, an individual's deteriorating mental health occurs first, resulting in low social class attainment. The drift hypothesis is the opposing theory of the social causation thesis, which says being in a lower social class is a contributor to the development of a mental illness.