The names have been seared into our memories. Eric Harris and Dylan Klebold. Seung-Hui Cho. Jared Lee Loughner. James Holmes. Adam Lanza. And now, allegedly, Dylann Roof. Young men driven to unspeakable acts of violence, often motivated by hatred.

In many ways, it's not surprising that perpetrators of these crimes are overwhelmingly young, male and mostly white. Nationwide, arrests for homicide peak in the late teens and early 20s, declining gradually afterward, according to the Bureau of Justice Statistics.

It's hard to compare acts of violence like murder -- whether it involves the killing of one person or many people -- and it's hard to generalize. But some experts say it's important to recognize the fact that young men are the main perpetrators of both. "In all my research, every multiple death act of violence has been perpetrated by one to two male adolescents," psychology professor Abigail A. Baird told the Huffington Post in 2013. "I'm not comfortable saying that's a coincidence."

Digging deeper into the numbers, homicide arrests peak at age 19. Dylann Roof, the alleged Charleston shooter, is 21.

The relationship between violence and late male adolescence has been previously documented -- the "age-crime" curve, it's called. "The age-crime curve probably reflects decreasing parental controls, a peaking of peer influence in the teenage years, and then increasing family and community controls with age," criminologist David P. Farrington wrote in the 1980s. In the intervening years, researchers have identified biological components, too: young men experience elevated testosterone levels that, in some circumstances, can make them aggressive and violent. And late adolescence is a time when a host of other problems typically emerge, like drug dependence and mental illness.

When violent acts like this one happen, we tend to cast about for explanations. But we may not be paying close enough attention to the simple biological facts of age (most violent criminals are young) and gender (among homicide arrests, men outnumber women 9 to 1.)

To put it another way: males aged 15 to 30 make up 10 percent of the population, but commit 63 percent of the homicides in the United States.