Tamerlan Tsarnaev, 26, left, and Dzhokhar Tsarnaev, 19.

Tamerlan Tsarnaev, 26, left, and Dzhokhar Tsarnaev, 19.

Why did they do it? It's a familiar question we're asking in the aftermath of the Boston bombings; the only variation is that with Tamerlan and Dzhokhar Tsarnaev, we're talking about murderous brothers instead of a homicidal lone wolf.

Was it radical Islamism, or Chechen nationalism, or a growing disaffection with American materialism that drove these young men to kill and maim so many last week? While pundits debate these possibilities, they're missing the real answer, writes Lisa Miller in New York Magazine.  That answer, she claims, is the testosterone that courses through young men's bodies, driving all to distraction and some – like the Tsarnaevs – to unspeakable violence.

“Evil may not have a single face, but it can be reliably found within one kind of body: that of an angry man in his late teens or 20s,” Miller writes, referencing US killers such as Adam Lanza, Timothy McVeigh, Jared Lee Loughner, James Eagan Holmes and Seung-Hui Cho.

An updated FBI wanted poster of Dzhokhar Tsarnaev.

An updated FBI wanted poster of Dzhokhar Tsarnaev. Photo: AP/FBI

In exploring why virtually all mass murderers are young men, Miller suggests we think of testosterone, “the aggression drug”, as “perhaps the only place to start” looking for an answer: “[The] male proclivity to assert power through violence has been true for males, and not for females, for millions of years, which is why when you give your four-year-old daughter a toy sword to play with, she may just turn it into a fairy wand and go on with her day.”

Like so many who rely nearly exclusively on biological explanations for human behaviour, Miller sees sex differences as a dichotomy, rather than a spectrum. In reality boys are as different from each other as they are from girls – and girls are capable of remarkably enthusiastic violence.

(I don't know if Miller actually has a four-year-old daughter, but I'm blessed with just such a creature. I guarantee you that if I were to hand Heloise a toy sword, she would not, as Miller writes, 'just turn it into a fairy wand and go on with her day'. My four-year-old would soon smite her baby brother a mighty blow. She has tea parties with her dollies, and when she's done, she not infrequently smashes them to bits. She lacks impulse control not because she's “boyish” but because she's four.)

Those who insist on rigidly gendered explanations for tragedies like Boston don't just posit young men as helpless in the face of a testosterone tempest. They also refuse to recognise that girls, despite much lower levels of that “aggression drug”, also have a very real capacity for anger, irrationality, and lust.

Miller claims that “men are likelier than women to act out vengeance, partly because their brains do not propel them to seek help, to pick up the phone or see a shrink, when enraged”. This is the classic error of mistaking cultural conditioning for biological predisposition.

From Chechnya to Cambridge, Massachusetts, boys are raised to see “seeking help” as something feminine, and therefore to be avoided. When boys are beaten and mocked for showing weakness, they hide their vulnerability.

When they're praised for aggression and taking foolish risks, they learn that recklessness and violence are key to establishing their masculine credentials. Our young men are not consumed with anger because they're at the mercy of their hormones, but because they've been denied access to any emotion other than rage.

You don't have to believe that “nature” has no impact on human behaviour in order to argue that “nurture” (how we raise our children) offers an equally important influence. Hormones are part of our human hardwiring, but socialisation is what teaches boys when and how to direct the aggressive, protective, sexual urges that testosterone creates.

Impulses may be rooted in biology, but how those impulses manifest has everything to do with culture. Create a culture in which boys have options other than violence and they will become less violent; create a culture in which women can pursue intellectual, sexual, and sporting ambitions, and they will become far more frank about what it is they want and how badly they want it.

In his best-selling The Better Angels of Our Nature, Steven Pinker makes a compelling, well-documented case that violence has been on the decline for centuries.

While conceding that in both past and present it is young men who commit the overwhelming majority of killings, Pinker argues that the “civilising influences” of urbanisation, marriage and even women's empowerment have radically reduced the incidences of male violence around the world. Young men still have testosterone coursing through their veins, but culture has the demonstrated power to determine what kind of behaviours that “aggression drug” will actually cause.

While we don't yet know all we need to know in order to understand what motivated the Tsarnaev brothers, we do know that what causes terrorism isn't testosterone. The source of such appalling violence isn't what flows inside young men's bodies. The source of the violence is the cultural straitjacket that suppresses and shames any sign of weakness and any plea for help.