The Effect of Obesity on Mental Health
This paper contributes to the literature by using a novel instrumental variable as well as investigating the mechanism through which obesity affects mental health. An index of genetic risk for high BMI (body mass index) constructed by the National Longitudinal Study of Adolescent to Adult Health (Add Health) serves as the instrument for obesity. The index is based on single nucleotide polymorphisms (SNPs) determined by geneticists to be predictive of BMI.
The BMI genetic risk score meets the two main criteria of instrumental variables, strength and validity. The instrument is strongly correlated with BMI and waist circumference as well as binary measures of obesity. The instrument is uncorrelated with many baseline characteristics, including socioeconomic status based on maternal and paternal education and adolescent household income.
I find an effect of obesity on suicidal ideation and injury from suicide attempt for the full sample; on suicide attempts and counseling for some subsamples; and no effect on an index of depression for any group. In contrast to the previous literature, I do not find that the effects are concentrated in white females. The group with the largest estimated effect is different for each outcome.
Using IV probit, at the mean of all covariates I find a one unit increase in BMI causes a 5 percentage point increase in probability of suicidal ideation for the full sample, a centimeter increase in waist circumference increases the probability of suicide attempt by 5.6 percentage points for Blacks and injury from suicide attempt by 5.6 percentage points for the full sample.
Based on respondent and interviewer opinions of respondent attractiveness, social stigma based on interviewer opinion is a mechanism by which obesity affects mental health for white women while poor self-image plays a larger role for Blacks.