Dear Cecil: According to the Humane Society, you can extend your dog's life a couple of years by getting him neutered. Are testes really lethal? Does neutering your dog really extend his life? Would the same thing work for men? Dave Greenaway

Cecil replies:

You’re not going to want to hear this, Dave. But according to one much-cited study, castrated men live nearly 14 years longer than their intact brethren, which if true means there’s some elective surgery you may want to think about. But first let’s talk about dogs.

It’s not hard to find statements in the veterinary literature that neutering — here meaning gonadectomy in either sex — prolongs the life of both male and female pets. To cite an obvious advantage, a neutered male dog is unlikely to get testicular cancer, while spaying female dogs virtually eliminates uterine disease and mammary tumors. Animal welfare groups promoting neutering have been happy to spread the word about these benefits.

Researchers into human longevity have also been interested, although for a different reason. Life expectancy in the U.S. has increased sharply over the past century, but more so in women than men. In 1900, a newborn boy could expect to live 46.3 years and a newborn girl 48.3 years, a difference of two years. By 1970, in part due to fewer maternal deaths during childbirth, baby girls could expect to live 74.7 years, baby boys 67.1 years, a difference of 7.6 years. U.S. males have caught up some since then; in 2007 a baby boy could expect to live 75.4 years, a baby girl 80.4. But the question remains: why, despite ongoing advances in health care, do women still live considerably longer than men?

A landmark 1969 study seemed to provide an answer. James Hamilton and Gordon Mestler compared the lifespans of 297 castrated inmates at a Kansas institution for the mentally retarded with those of 735 intact males at the same facility. The castrated males had gone under the knife at ages from 8 to 59 years old, with the average age ranging from 12 (!) in 1898 to 30 in 1923. They didn’t vary markedly from intact inmates in terms of IQ, type of mental disability, and so on, suggesting there had been no firm criteria for the operation other than possibly your getting on the hospital staff’s nerves — too bad if you were an inmate but lucky for science, since except for castration the two groups were indistinguishable.

Result: the castrated inmates on average lived 13.6 years longer than the intact ones (55.7 vs 69.3 years). What’s more, the earlier you were castrated, the longer you lived. Conclusion: testosterone kills.

OK, Hamilton and Mestler didn’t put it that dramatically. But they did believe their research applied to all males, not just the mentally retarded, in part because castrated animals in general were thought to live longer than those left intact. Their view has largely carried the day as the explanation for why women outlive men. My assistant Una found their paper had been cited at least 130 times by later researchers.

You’re thinking: come on. What toxic effects could male hormones possibly have that would account for a 14-year difference in lifespan? It wasn’t male predilections for smoking or violence, or male-only conditions like testicular cancer. Rather, according to Hamilton and Mestler, it was infections.

I know, makes no sense to me either. One explanation I’ve seen is that castration was used to pacify the rambunctious. Troublemakers who didn’t get orchidectomized instead were bound to chairs or beds, making them more vulnerable to chronic urinary infections and such. In other words, it wasn’t so much castrated inmates living long lives, but rather intact ones dying young.

Which gets us back to dogs. Remember, Hamilton and Mestler believed their conclusions applied to everyone, not just the mentally retarded, because castrated animals in general lived longer. But it turns out the evidence for that is thin and contradictory.

Research on Rottweiler longevity is instructive on this score. A 2003 study found that of 21 dogs who lived exceptionally long lives by Rottweiler standards — more than 13 years — two-thirds were female and 90 percent had been neutered, supporting the conventional wisdom. On looking closer, however, we see that whereas five of seven male dogs had been neutered, all 14 of the females had been. Implication: while neutering helps male dogs live longer, it helps females even more.

So sex hormones of any kind mean an early grave? Not so fast. Matters may be confused by the failure to consider when in an animal’s life neutering is performed. More recent Rottweiler research indicates the longer a female dog has ovaries, the longer she lives.

The supposedly lethal impact of testosterone may also be exaggerated. Browsing through the databases, we find a 1982 analysis of 2,000 canine post-mortems showing no significant difference between the lifespans of intact and neutered animals of either sex.

Inquiry into this murky business continues. For now about all we can say is that having sex organs doesn’t necessarily shorten your life. Not to celebrate prematurely, but I say: whew.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.