Mice or humans lacking the aromatase enzyme show reduced aggression despite high testosterone levels, and blocking aromatase prevents testosterone-induced aggression.

“Now of course it is the case that if testosterone is low, that a person or an animal will exhibit less aggressive behavior, but that's not because of reduced testosterone per se. It's because of the subsequent reduction in testosterone, meaning if there's no testosterone to aromatize into estrogen, estrogen will also be lower.”

“What's going on is that testosterone can be converted into estrogen through a process called aromatization. There's an enzyme called aromatase. ... So the aromatase enzyme converts testosterone into estrogen, and it is actually testosterone, aromatized, converted into estrogen and then binding to these estrogen containing neurons in the ventromedial hypothalamus that triggers aggression.”

“Now this effect of estrogen causing aggression in the brain is very robust, so much so that if you take a mouse that lacks the aromatase enzyme or a human that lacks the aromatase enzyme, and they do exist, then there is a reduction in overall aggression, despite high levels of testosterone.”

“and as you say, it's because many of the effects of testosterone, although not all, many of them are mediated by its conversion to estrogen, by a process called aromatization, it's carried out by an enzyme called aromatase.”

“So it testosterone if you block aromatase you get essentially female uh female rodent brain. So >> does that mean that you need aromatase to get testosterone in the brain or does it need mean that you need the testosterone to become estrogen to go into the brain? >> No, the testosterone only the testosterone gets into the brain. Estrogen is actually prevented the um peripheral testosterone is prevented by a protein called alpha feta protein in rodents and it's so maternal estrogen is bound >> uh so that females are not masculineized. So tea enters the brain and is aromatized produced the te the testosterone from the male testicles >> is high that gets into the brain once it gets passes the blood well it's yes gets in there >> once it gets past because it doesn't have the alpha protein that's a pretty elegant solution >> it is an elegant solution so it is uh clear that it's um estrogen acting via estrogen receptors that are masculineizing sexual and aggressive behavior which is just very clear in rodents because you have lordosis in females and mounting in males and you have higher rates of male aggression etc. So um that's via”