Last week the cold case of the 1979 abduction (and presumed murder) of 6-year-old Manhattan boy Etan Patz was revived when a man named Pedro Hernandez confessed to abducting the boy on his way to school.

Police never found Patz’s body, nor caught his killer. As they do after most high-profile disappearances, psychics came out of the woodwork to offer the Patz family and police information about the boy’s abduction — but all of it was wrong. Police had no leads, and the case remained open while the family prayed that one day the case would be solved.

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But not everyone was completely surprised by the confession. In fact Hernandez had apparently given it several times before, including to a prayer group at a Catholic church a few years after Patz’s disappearance.

According to The New York Times, Hernandez “told the prayer group he had strangled a boy and left the body in a Dumpster, according to Norma Hernandez, his sister, and Tomas Rivera, a leader of the group who said he was present. Mr. Rivera, speaking Sunday at his home in Blackwood, N.J., said it was not his place to call the police ‘because he did not confess to me’ one on one. ‘He confessed to the group,’ Mr. Rivera said.”

Why wouldn’t a group of as many as 50 people — much less a Catholic prayer group — who’d just heard one of their members confess to murdering a child report the crime to police?

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Chalk it up to human nature and a phenomenon psychologists and sociologists call “diffusion of responsibility.” It’s when the responsibility for a task or action is spread out among many different people, so that no one person feels that they have a personal obligation to carry it out.

Everyone hears the confession, so everyone has equal responsibility to report it — so no one reports it.

In fact Hernandez’s confession (whether true or not — some people do falsely confess to crimes they did not commit) was an open secret among his friends and family. It now seems likely that dozens of people heard him talk about killing the boy over the past decades. It’s possible that they thought he was joking about it, or reporting a psychotic episode or delusion he’d had. It’s also possible that the confession wasn’t specific enough to alert anyone if he didn’t mention the name or age of his victim (though one would think that would hardly be an excuse).

There are some real-life lessons to take from this; for example if you or someone else is having a medical emergency such as a heart attack on a busy street, do not simply call out to the crowd, “Someone call an ambulance.” Instead, single out one specific person nearby and point to him or her directly and say, “You, call an ambulance!” or “You, help me stop the bleeding.”

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Research shows that this is far more effective and is much more likely to result in immediate and decisive action. It’s not that people are indifferent, cruel or lazy; it’s that without a specific, clear, immediate request, people are much more likely to assume someone else will take care of it. Parents can also use this phenomenon to their advantage, by for example assigning specific responsibility to one child instead of telling a group of kids, “I want this mess cleaned up by the time I get back.”

Interestingly, Hernandez’s sister Norma claims that she did in fact tell New Jersey police that her brother told her he killed a child 25 years ago, but nothing was ever done. Her claim has not been confirmed, but assuming it’s true, what would make his own sister turn him in when dozens of strangers didn’t? For one thing, he told her in person and in private.

Image: On April 19, 2012, New York City police officers watch over a crime scene where investigators searched for evidence of Etan Patz, who has been missing for 33 years in New York City. Credit: Justin Sullivan/Getty Images.