In a longstanding debate in the sociology of mental illness, “social causation” means low socioeconomic status (SES) increases risk of mental illness, while “social selection” means that the mentally ill decline in SES as a consequence of their illness (Hudson 2005). If genes transmitted from parent to offspring increase the chance of mental illness, then the lower SES of mentally ill for both parent and offspring may be a consequence not a cause of the mental illness of both (Kendler and Baker 2007).
It is difficult methodologically to distinguish the two kinds of pathways in a given data set—how much consequence, how much cause? Indeed, as the figure indicates, there are more than two causal links to investigate.
Moreover, there is an asymmetry in the contrast: heritability (link 1 in the figure) points to unknown, possibly heterogeneous genetic factors while the environmental factors are measurable and open to intervention (links 2, 3, and 4).
Adapted from page 123 ofTaylor, P. J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.
Hudson, C. G. (2005). “Socioeconomic status and mental illness: tests of the social causation and selection hypotheses.” American Journal of Orthopsychiatry 75(1): 3-18.
Kendler, K. S. and J. H. Baker (2007). “Genetic influences on measures of the environment: a systematic review.” Psychological Medicine 37(5): 615-626.
(Introduction to this series of posts)