Violating the Monotonicity condition for instrumental variable—Dimorphic patterns of gene–behavior association

I document the dimorphic patterns for the association between DAT1 9R9R genotype and measures of risky behaviors including drinking, use of marijuana, cocaine, other drugs, and number of sexual partners. The associations between DAT1 9R9R genotype and risky behaviors are often in opposite signs for females and males, in the National Longitudinal Survey of Adolescent Health (Add Health). This article sensitizes the researcher on the sexual dimorphic pattern of gene–behavior association, and its risk to violate the Monotonicity (no-defiers) condition in applications that use genetic markers as instrumental variables.