Studies have documented substantial increases in obesity throughout most of the industrialized world in recent decades. The majority of explanations for these increases have centred around environmental factors such as the increasing availability of high-fat, high-carbohydrate foods and sedentary lifestyles. This study sought to determine if genetic factors might be contributing to the increases in the proportions of North Americans who are obese and overweight. The body mass index (BMI) for a large sample of two generations of United States and Canadian subjects was correlated with family fertility indicators. Small but highly significant positive correlations were found between the BMIs of family members and their reproduction rates, especially in the case of women. For instance, mothers in the sample (most of whom were born in the 1940s and 50s) who were in the normal or below normal range had an average of 4·3 siblings and 3·2 children, compared with 4·8 siblings and 3·5 children for mothers who were overweight or obese. When combined with evidence from twin and adoption studies indicating that genes make substantial contributions to obesity, this study suggests that recent increases in obesity are partially the result of overweight and obese women having more children than is true for average and underweight women. It is speculated that improvements in medical treatments for conditions associated with obesity – particularly diabetes and heart disease – are making it possible for overweight women to live longer and to be more fertile than was true historically.