Obesity related to over-nutrition is investigated in a sample of 219 Mexican children from affluent families, ages 6–12 years. Defined as weight-for-age at or above the 95th percentile, obesity rates in middle childhood are very high in this population, being 24.2% of children (29.4% of boys and 19.1% of girls). Binary logistic regression shows that children are more likely to be obese if they are boys, from small households with few or no other children, and have more permissive, less authoritarian parents. Diet at school and activity patterns, including television viewing, are not different for boys and girls and so do not explain this gender variation. The value placed on children, especially sons, in smaller middle-class families, can result in indulgent feeding because food treats are a cultural index of parental caring. Parents also value child fatness as a sign of health. These obese Mexican children have no greater social problems (peer rejection or stigma) or psychological problems (anxiety, depression, or low self esteem) than their non-obese peers. More study specifically focused on feeding practices in the home environment is required to explain very high rates of child obesity. The differences in obesity risk related to specific aspects of children's developmental microniche emphasize the importance of including a focus on gender as a socio-ecological construct in human biological studies of child growth, development, and nutrition. Am. J. Hum. Biol. 15:446–460, 2003. © 2003 Wiley-Liss, Inc.