Girls are born lighter than boys. The consistency of this observation across different populations is striking, suggesting that it may have fundamental significance for those conditions linked with lower birth weight, such as diabetes. Previous hypotheses relating low birth weight to subsequent diabetes have addressed differences in insulin resistance within the sexes, not between them. Here, we propose that gender-specific genes affecting insulin sensitivity are responsible for the gender difference in birth weight – the genetically more insulin resistant female fetus is less responsive to the trophic effects of insulin and is therefore smaller. These genes also render female subjects more susceptible to diabetes, explaining why reports of type 2 diabetes (T2D) in younger populations show a female preponderance. Consistent with our proposal, concentrations of insulin and/or its propeptides are higher at birth in female populations and they are intrinsically more insulin resistant throughout life, with attendant impact on their metabolism, and the regressions describing the relationship between insulin resistance and adiposity in female and male subjects have similar gradients, but different constants. These gender-specific genes have a demonstrable impact on fetal growth and insulin resistance. Diabetes and cardiovascular disease are thought to be driven by insulin resistance, and the observations reported here may help to focus the search for genes that control it.