Researchers from the Netherlands and Sweden seek to update the hypothesis that the five major risk factors for schizophrenia (urbanicity, migration, childhood trauma, los intelligence, and drug abuse) can be explained by sensitization of the mesolimbic dopamine system (DA) resulting from the experience of social defeat (SD). A review of the literature finds a connection between SD and schizophrenia for migration and childhood trauma, and that while evidence for mesolimbic DA sensitization is lacking (due to a lack of studies), the evidence for this in animals is strong. The study was published by Schizophrenia Bulletin.
Selten, J., van der Ven, E., Rutten, B., Cantor-Graae, E.; The Social Defeat Hypothesis of Schizophrenia: An Update. Schizophrenia Bulletin. Online September 23, 2013
According to the social defeat (SD) hypothesis, published in 2005, long-term exposure to the experience of SD may lead to sensitization of the mesolimbic dopamine (DA) system and thereby increase the risk for schizophrenia. The hypothesis posits that SD (ie, the negative experience of being excluded from the majority group) is the common denominator of 5 major schizophrenia risk factors: urban upbringing, migration, childhood trauma, low intelligence, and drug abuse. The purpose of this update of the literature since 2005 is to answer 2 questions: (1) What is the evidence that SD explains the association between schizophrenia and these risk factors? (2) What is the evidence that SD leads to sensitization of the mesolimbic DA system? The evidence for SD as the mechanism underlying the increased risk was found to be strongest for migration and childhood trauma, while the evidence for urban upbringing, low intelligence, and drug abuse is suggestive, but insufficient. Some other findings that may support the hypothesis are the association between risk for schizophrenia and African American ethnicity, unemployment, single status, hearing impairment, autism, illiteracy, short stature, Klinefelter syndrome, and, possibly, sexual minority status. While the evidence that SD in humans leads to sensitization of the mesolimbic DA system is not sufficient, due to lack of studies, the evidence for this in animals is strong. The authors argue that the SD hypothesis provides a parsimonious and plausible explanation for a number of epidemiological findings that cannot be explained solely by genetic confounding.