There is no shortage of theories as to why women live longer than men - an apparently simple question, but one balanced on such a mountain of complex data and partial knowledge that it cannot be definitively answered at the present time. Here is another theory for the stack: "Mitochondria are inherited only from mothers, never from fathers, so there is no way to weed out mutations that damage a male's prospects. ... [Researchers] analysed the mitochondria of 13 different groups of male and female fruit flies. Mitochondria, which exist in almost all animal cells, convert food into the energy that powers the body. ... the results point to numerous mutations within mitochondrial DNA that affect how long males live, and the speed at which they age. ... Intriguingly, these same mutations have no effects on patterns of ageing in females. All animals possess mitochondria, and the tendency for females to outlive males is common to many different species. Our results therefore suggest that the mitochondrial mutations we have uncovered will generally cause faster male ageing across the animal kingdom. ... They suggest this is because there is no evolutionary reason for the faults that affect males to be picked up - because mitochondria are passed down by females. ... If a mitochondrial mutation occurs that harms fathers, but has no effect on mothers, this mutation will slip through the gaze of natural selection, unnoticed. Over thousands of generations, many such mutations have accumulated that harm only males, while leaving females unscathed."