Having two X chromosomes provides women with a backup if a genetic mutation occurs, men meanwhile have a single X chromosome to express all their genes. The female’s genetic reserve might allow a greater repertoire of effective neural, endocrine, and immune responses to potentially damaging environmental demands. Also, female hormones, particularly estrogen, and the resiliency of the female body to accommodate pregnancy and breast-feeding are also thought to promote longevity. Estrogen has a beneficial effect on lipids in the blood and seems to protect against premature heart disease.