Here’s another unsettling new trend: men’s testosterone levels are dropping.

Nov. 3, 2006 – Over the last two decades, American men have made a number of major lifestyle changes—taking on a greater share of the housework, consuming an ever-widening array of skin-care products and even leaving jobs to stay home and raise the kids while their well-paid wives earn the dough. Now, a new study published online in the Journal of Clinical Endocrinology and Metabolism suggests that today’s men are also changing on the inside: sporting significantly lower testosterone levels than their counterparts 10 or 20 years ago.

Using blood samples collected from more than 1,500 healthy men age 45 to 79 in the Boston area over a period of 17 years (originally gathered as part of the Massachusetts Male Aging Study), scientists at the New England Research Institutes (NERI) measured both total testosterone and “bio-available� testosterone, the portion of the hormone readily available to cells. Then they compared men of the same age in different decades. “We see about a 1 percent decrease per calendar year across men of the same age born at different times,� says NERI biostatistician Thomas G. Travison, lead author of the study. “In other words, a typical 50-year-old today would have about 20 percent lower [testosterone] than his father would have had 20 years ago. What was most surprising about the result was that we were able to explain so little of it.� [full text]

There are likely multiple factors contributing to decreasing testosterone in men, which the article only alludes to, but which commenters on the article flesh out in more detail. Possible contributing factors include: increased obesity, pesticides and fungicides which have an estrogen-like effect, estrogen in the water supply as a result of women taking birth control and hormone replacement therapy drugs, and the massive increase in the use of plastics which also have an estrogen-like effect.