But to prevent a crisis from happening in the first place, experts say the best bet is to pay attention to behavior. As to the specific behavior, that's harder to say. It's something that "makes you stop and think and worry about the person," Randazzo says. That might include an obsessive interest in violence or a school essay riddled with disturbing content or even telling someone about a dangerous scheme. Behavior can't predict a certain outcome – "the same person would do something different in different situations" – but it can provide details for a composite picture that a professional can assemble and assess, she says.
If you experience a visceral reaction to someone a la the hair standing up on the back of your neck, that's a good indicator of possible danger – but often, people misread each other, says Mary Ellen O'Toole, a retired FBI agent profiler and author of "Dangerous Instincts: How Gut Feelings Betray Us." Why? Because you need a baseline of behavior to recognize whether someone strays from his or her norm. Otherwise, it's too easy to confuse your biases with your gut, she explains. "We believe that we're born with this inner barometer that tells us if somebody is safe or nonthreatening," she says. But too often, "we look at the trappings of normalcy," like where someone went to college, "instead of the behavior that they are exhibiting right in front of us."
Erratic behavior, people speaking in ways that are hard to understanding or expressing delusional thinking – all these things should raise your antenna, Appelbaum says. But most often, these patterns are detected by friends and relatives, who must grapple with whether to report a loved one to authorities. O'Toole acknowledges the pain in this decision but advises those who notice aberrant behavior to report it by contacting someone who can help such as the person's primary doctor, psychologist or psychiatrist, local psychiatric hospital or 911.
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Spotting such signs early makes punishment much less likely, Randazzo says. As a situation worsens, your options are likely to become fewer and more severe such as arrest or an involuntary psychiatric examination.
According to Appelbaum, the attempt to identify a potentially violent criminal is nearly impossible. So, he argues for vigilance around erratic behavior and "better screening for and access to mental health services on a general basis." "Those warning signs of mental illness ought to be tended to – not because those are people who are likely to commit violence, because only a tiny number of them will," but to safeguard them from their own self- destruction.
Start by talking with the person about getting treatment and consider asking a family member or supervisor to intervene if need be, he says. To learn more about how to get help, Appelbaum suggests visiting the websites of the National Institutes of Mental Health, National Alliance on Mental Illness and the American Psychiatric Association.
Corrected on 10/11/2013: A previous version of this story misspelled Paul Appelbaum's name.