Cohen says that some police investigators just rely on one piece of evidence. "The problem is that they don't look anywhere else," he says. "They shoot an arrow and draw a circle where it lands."
During these cases, especially the JonBenet Ramsey saga, the media has been criticized for its relentless questioning and breathless coverage. But it seems to play a dual role, sometimes helping and other times hindering the investigation.
"I know the media has done a lot of good in these cases," says Ed Smart, explaining that the press plays a crucial role in reeling in potential witnesses. "For someone missing a child, you can't do it without the media."
Gerry McCann's sister, Phylomena McCann, had mixed feelings about the media's involvement in the case. Some journalists, she told BBC Radio 4, "overstepped their mark." Yet, "If it hadn't been for the help of the media, she says, "Madeleine's case might have disappeared."
Studies show that the media only play a useful role some of the time. "Thirty percent of the time, the media was helpful in advancing the investigation," says Keppel, who conducted a study on the effects of media coverage. "They publicize what went on, and a viewer who saw something calls in. But that means that 70 percent of the time, the coverage was not necessarily helpful."
Clay Calvert, a professor of journalism and law at Pennsylvania State University, says that these types of stories are made for the world of 24/7 journalism on the Internet and cable TV. And part of that appeal, he explains, comes down to the audience playing detective and questioning the motives of parents like the McCanns.
"We know that initial suspects always tend to be those who are closest," says Calvert. "It makes for perfect tabloid fodder. We all love to second-guess the responses and to find the most sinister intentions lurking in people."
Calvert attributes part of this reaction to the counterintuitiveness of these cases. "Here we have another beautiful blond girl who's missing," he says. "And the parents are clean-cut-looking people. How can anybody do anything like that to such a beautiful girl?"
In the end, he believes, the media don't usually help the investigation. "It's interesting to speculate, but we're not really solving the crime. Maybe more energy should be devoted to that and less to running stories about every aspect of the characters in the middle of this tragedy."
ABC's Maeva Bambuck contributed to this report.