Abstract:The biological and social costs of body mass cannot be conceptualized in the same way. Using semiparametric methods, we show that the association between body mass index (BMI) and socioeconomic outcomes such as wages, being married, and family income is distinctly shaped by gender, race, and cohort rather than being above a specific threshold of BMI. For white men, the correlation between BMI and outcomes is positive across the "normal" range of BMI and turns negative near the cusp of the overweight range, a pattern that persists across cohorts. For white women, thinner is nearly always better, a pattern that also persists across cohorts. For black men in the 1979 cohort, the association between BMI and wages is positive across the normal and overweight ranges for wages and family income and inverted U-shaped for marriage. For black women in the 1979 cohort, thinner is better for wages and marriage. By the 1997 cohort, however, the negative association between body mass and outcomes dissipates for black Americans but not for white Americans. In the social world, "too fat" is a subjective, contingent, and fluid judgment that differs depending on who is being judged, who does the judging, and the social domain.Keywords: obesity; BMI; gender; race; wages; marriage N UMEROUS studies link obesity to poorer socioeconomic outcomes, including lower wages, family income, education, marriage rates, and spousal earnings (Averett and Korenman 1996;Cawley 2004;Conley and Glauber 2007;Glass, Haas, and Reither 2010). But does the medical definition of "obese" apply to the social world? Obesity is a condition defined as having excessive body fat. Because body fat is difficult to measure directly, obesity is usually approximated by body weight adjusted for height using standard cutoffs along the body mass index (BMI) (Ogden et al. 2007). The continuous BMI scale is divided into four major intervals (underweight, normal, overweight, obese) with subclassifications of mild, moderate, and severe within these groupings. 1 Although more accurate measures of body fat exist (e.g., waist-to-hip ratio, waist circumference, body volume index), these are far less commonly used in the social science research literature.BMI is a flawed measure of actual body fat because it is not sensitive to differences in mass that are due to fat versus muscle or bone, and it misclassifies more African Americans as having excessive fat than white Americans (Burkhauser and Cawley 2008). However, it is a simple and standard measure that allows medical and public health studies to examine the biological links between this measure of corpulence and health-related outcomes. The BMI cutoff of 30 is used internationally as a standard medical definition of adult obesity. Once we move from biological to social outcomes, however, the definition of obesity becomes far more subjective. What counts as being excessively fat in the social world? A sizable literature demonstrates that the body is socially constructed and that our perceptions of fat bodies are tied...