It's well known that women outlive men by about five to six years, says Thomas Kirkwood in Scientific American. But the reasons why have long been murky. Hypotheses built around lifestyle choices — that women are better at avoiding stress or unhealthy habits like smoking — fall short. To truly answer the question, we must take "a wider biological perspective": Since women carry and nurse babies, their long-term health is more important from an evolutionary context than males'. Here, an excerpt:
In fact, high levels of testosterone, which boost male fertility, are quite bad for long-term survival. ... As many dog and cat owners can attest, neutered male animals often live longer than their intact counterparts. Indeed, the evidence supports the notion that male castration might be the ticket to a longer life.
Might the same be true of humans?... The historical record is not good enough to determine if eunuchs tend to outlive normal healthy men, but some sad records suggest that they do. A number of years ago castration of men in institutions for the mentally disturbed was surprisingly commonplace. In one study of several hundred men at an unnamed institution in Kansas, the castrated men were found to live on average 14 years longer than their uncastrated counterparts. Nevertheless, I doubt that many men —myself included — would choose such a drastic remedy to buy a few extra years.