Every year, nearly 40,000 Americans kill themselves. The majority are men, and most of them use guns. In fact, more than half of all gun deaths in the United States are suicides.

Experts and laymen have long assumed that people who died by suicide will ultimately do it even if temporarily deterred. “People think if you’re really intent on dying, you’ll find a way,” said Cathy Barber, the director of the Means Matters campaign at Harvard Injury Control Research Center.

Prevention, it follows, depends largely on identifying those likely to harm themselves and getting them into treatment. But a growing body of evidence challenges this view.

Suicide can be a very impulsive act, especially among the young, and therefore difficult to predict. Its deadliness depends more upon the means than the determination of the suicide victim.