with access to playgrounds, proximity to fast-food restaurants, or neighborhood crime level .
Finally, it has not been tested whether exposure to the toxic environment is related to genotype. That is, individuals with obesity-predisposing genes may be particularly responsive to the effects of such a “toxic” environment. In addition, certain individuals may be more likely to seek out or expose themselves to aspects of the toxic environment. The topic of gene-environment correlations as a topic for additional research is discussed further in Section 7.
Several studies (e.g., [15-17]) have documented an inverse relationship between SES and obesity in previous years. In a recent review, Ball and colleagues  examined 34 articles to test the hypothesis that persons from lower SES strata are at increased risk of weight gain. Their hypothesis was supported for predominantly non-African American samples, but not for African American samples. Reviewing relevant studies, they found little support for a relationship between SES and weight gain among African Americans. In contrast, depending on the particular indicator for SES that was used (i.e., occupational status, education, and income), they found that lower SES was associated with an increased risk of weight gain in non-African American individuals. Specifically, the authors found an inverse association between occupational status and weight gain for men and women. When SES was assessed using education as the indicator, the relationship became less strong (particularly among men). Using income level as the particular indicator for SES, findings for associations between weight gain and SES were inconsistent for both men and women. Finally, the authors noted a differential rate of weight gain by SES and attributed that finding to an early onset of weight gain in a person’s life, when parental SES may still be influential.
Prospective analyses of the National Longitudinal Survey of Youth  found that children from lower SES families were more likely to have been overweight during the prior year than children from higher SES families. Negative associations between obesity status and household income and parental education were found even when controlling for ethnicity and other demographic variables.
Several mechanisms could underlie the link between low SES and obesity. Factors such as limited access to resources, poor knowledge of nutrition and health, increased exposure to fast-food outlets, and limited physical activity due to deprived or unsafe neighborhoods [20, 21] have been suggested to influence energy intake and energy expenditure and, consequently, body weight. For instance, in an ecological study of 267 postal districts in Melbourne, Australia, families living in the poorest SES strata