There are several explanations for why testosterone levels seem to be dropping so measuredly between generations, but scientists have struggled to determine which variable is most to blame. Some have cited increased rates of obesity; others have pointed to reduced rates of smoking in men (which ironically can lead to lower testosterone levels). Still others have alluded to environmental factors, including various chemicals that may act as endocrine disruptors, resulting in below-average testosterone levels.

Other behavioral factors may be at play. Anthropologists at Harvard University found the testosterone level of a man decreases significantly when he holds an infant and that “married men, whether fathers or not, have markedly lower testosterone levels than single males.” Experts have reasoned that such a trend, for the purposes of a biological timeline, makes sense. Decreasing testosterone levels—and thus, a decrease in competitive mating behaviors—is better suited to the stability of home life. Yet fewer young men today are married and parenting their own children than in previous generations, meaning this can’t be driving the overall trend. But it could be related to men mimicking more feminine behaviors.

Conversely, several studies have shown that engaging in “traditionally masculine” behavior, such as competitive or aggressive activities, can increase testosterone levels.