Hormonal birth control and its consequences have been a disaster for the human race. Typical testosterone levels (perhaps not just humans either but in other mammals as well) have been dropping significantly since approximately the time hormonal birth control became widely available. The drugs pro Before you snicker and make jokes, reduced testosterone in men has a strong correlation with depression, which leads to i…
Smoking increases testosterone levels[0].
Like any drug, nicotine has a variety of side effects, and one of them seems to be increasing testosterone levels (the exact measures here are complicated, read the article for more info).
So, trying to untie the decease in smoking to a total decrease in male testosterone seems to be an active area of research and one that the literature is still working on. Maybe there is a falling amount that is not due to less smoking, maybe there isn't.