With 12 dead and 58 injured, the July 20 massacre at the midnight premiere of The Dark Knight Rises in Aurora, Colo., is one of the largest mass shootings in U.S. history. Aurora is only 20 miles from Columbine High School, where seniors Eric Harris and Dylan Klebold killed 13 and injured 24 in 1999. We may think we know what makes the perpetrators of mass shootings — mostly boys and men — tick. Though psychology doesn’t always lend itself to hard statistics, there are some patterns that might surprise you.

1. Shooters are insane.

The 2002 Safe School Initiative report, by the Secret Service and the Department of Education, looked at 41 attackers across 37 incidents from 1974 to 2000. It concluded that only 17 percent “had been diagnosed with mental health or behavior disorder prior to the attack.” Most had never had a mental-health evaluation. However, 78 percent “exhibited a history of suicide attempts or suicidal thoughts.”

Adult and teen shooters do not suddenly snap. Their anger and planning often develop over time. And the more they plan, the more an argument can be made, in the eyes of the law, that they are sane.

Indeed, mental illness is not automatically an excuse, legally speaking. Charges against James Holmes, the Colorado theater shooting suspect, were filed Monday. Whether he could invoked an insanity defense hinged on whether it can be proved that he had a mental condition that rendered him unable to distinguish right from wrong.

By contrast, most experts (after his death) have declared Columbine shooter Eric Harris a psychopath. With that label, he could not have pleaded insanity because, while he would be seen as coldblooded, he would also be considered rational, calculating and aware of his actions.

Mass murderers may have various diagnoses (the other Columbine shooter has been called a depressive), but they are usually seen as being fueled by anger and vengeance.

2. Cutting down on illegal gun sales would help.

Not necessarily. The Safe School Initiative found that 68 percent of the attackers obtained guns from their homes or those of relatives. Case in point: Jonesboro, Ark., middle-school shooters Mitchell Johnson, 13, and Andrew Golden, 11, took guns from the homes of Golden’s parents and grandparents.

A friend of the Columbine shooters purchased at a gun show three of the four weapons they used and passed them on to the killers without breaking any laws. Holmes legally obtained four firearms from local gun stores, according to various news accounts. Many shooters do not have the type of criminal record, such as a felony conviction, that would prohibit them from legally obtaining guns.

3. A shooter’s loved ones should have seen it coming.

Teenagers, including those who commit mass shootings, often hide things from their parents. In the case of Columbine, the vast writings and videos left behind by the 17- and 18-year-old killers do not indicate that anyone knew of their plans, and there was no criminal prosecution to that effect. Civil suits accusing the killers’ parents of not stopping the shootings were settled, although their depositions have been sealed for years to come.

While young shooters may not leak information to a parent, they may drop hints to others. The Safe School Initiative found that in 81 percent of the cases studied, “at least one person had information that the attacker was thinking about or planning the school attack.”Almost always — 93 percent of the time — that person was “a peer, a friend, schoolmate or sibling.”