Researcher Thomas Kirkwood revisits the well known difference in life expectancy between the genders: "It turns out that the females of most species live longer than the males. This phenomenon suggests that the explanation for the difference within humans might lie deep in our biology. ... Many scientists believe that the aging process is caused by the gradual buildup of a huge number of individually tiny faults - some damage to a DNA strand here, a deranged protein molecule there, and so on. ... We might well ask why our bodies do not repair themselves better. Actually we probably could fix damage better than we do already. In theory at least, we might even do it well enough to live forever. The reason we do not, I believe, is because it would have cost more energy than it was worth when our aging process evolved long ago, when our hunter-gatherer ancestors faced a constant struggle against hunger. ... If you can avoid the hazards of the environment for a bit longer by flying away from danger or being cleverer or bigger, then the body is correspondingly a bit less disposable, and it pays to spend more energy on repair. Could it be that women live longer because they are less disposable than men? This notion, in fact, makes excellent biological sense. In humans, as in most animal species, the state of the female body is very important for the success of reproduction. The fetus needs to grow inside the mother's womb, and the infant needs to suckle at her breast. So if the female animal's body is too much weakened by damage, there is a real threat to her chances of making healthy offspring. The man's reproductive role, on the other hand, is less directly dependent on his continued good health."