Something about marriage -- in the sense of cohabiting with a woman and raising children, seems to lower a man's testosterone.
Peter Gray, a University of Nevada researcher, has been measuring testosterone fluctuations in men for several years. He and colleagues test the levels in men's saliva several times over the course of several days. They've found that
Gray is unusual among researchers in that he's extended his studies outside of the U.S. and doesn't confine them to student populations. (Many researchers use students because they're very easy to obtain for studies; sometimes, they're required to participate in studies as part of their coursework. However, this can really skew results, especially when it comes to relationship studies. It's not at all clear that a 19-year-old student's relationships mirror those of a 40-year-old's.) Gray has done studies in China, North Africa and Jamaica.
He's published two new studies. One looked at fathers and non-fathers in Jamaica; the other, in North Africa.
Ariaal men, members of a tribal population that tends to marry late and focus relationships more on other men than on the family, still had lower levels of testosterone than single men. And, while you might expect men with several wives to be high-testosterone (in order to be able to handle all those women), the same thing held true for the polygamous men.
In the other study, in Jamaica, married men had lower testosterone and higher prolactin levels than single men.
Because testosterone mutes the effects of oxytocin, it's possible that this reduction in testosterone is part of a natural process that allows men to be more uxorious and keeps them more available to the family. Prolactin is released in men after orgasm; perhaps -- and this is only my own speculation -- increased sexual activity in marriage is responsible for the higher prolactin and lower testosterone.
Here's an abstract of the Kenyan study: Testosterone and Marriage Among Ariaal Men of Northern Kenya.
Here's a press release about the studies: Marriage and Fatherhood Linked to Lower Testosterone Levels.