But I'm not more aggressive—a behavior change often tied to testosterone. That's not surprising to Robert Sapolsky, ., a neuroendocrinologist at Stanford University and a leading researcher on stress and behavior. "It's really not the case that testosterone 'causes' aggressive behavior," he says. "Instead, it makes the brain more sensitive to social cues that trigger aggression. And in support of that, a guy's testosterone level isn't a very good predictor of how likely he is to be aggressive."
Males start an approximate 1% yearly testosterone decline anywhere from their twenties to thirties, and this will also continue on through life. By the middle-aged years, a man can be suffering from a change of 20 to 30% or more . This can have a dramatic adverse effect on his sex drive and performance. The production of testosterone and sperm also are interrelated, and some men dealing with Low T find it difficult to father a child. Treatment with testosterone therapy has brought about remarkable benefits in the area of sexual performance for men.