Do you believe that serial killers have common traits that can appear as early as childhood? Many people do - thanks to the Macdonald Triad. The triad states that three common behaviors during childhood can point to murder-prone children: bed wetting past the age of five, animal cruelty and fire-setting. But is the stereotype true?

The Macdonald Triad comes from a 1963 paper called "The Threat to Kill", written by forensic psychiatrist J.M. Macdonald. In the paper, Macdonald observed that these behaviors were frequently seen in his most aggressive and sadistic patients. He interviewed 48 psychotic patients and compared them with 52 non-psychotic patients who had threatened to kill someone. Over half of the participants were men, and the participants ranged in age between 11 and 83. Macdonald's paper relied on clinical observation, and he did not believe that his small and unrepresentative study had any predictive value. Nonetheless, other researchers latched onto the study to test it again.

A few years after Macdonald's paper was published, a team of researchers tried to recreate its findings. It divided 84 incarcerated offenders into two groups: the 53 nonaggressive offenders and 31 aggressively violent offenders. Three-quarters of the violent offenders had evidence of one or two of the behaviors of the Macdonald Triad, and 45 percent had all three. The study was small and poorly designed but subsequent trials that were better controlled and tested larger groups were also able to recreate its findings.

The Macdonald Triad really caught fire though when the FBI's Behavioral Analysis Unit offered evidence of the Triad's veracity during the 1980s and 1990s. Their studies, too, were flawed. They gathered information from just 36 murderers, with only 25 serial killers. All had volunteered for the interviews, but the samples were too small to draw any definitive conclusions.

Nevertheless, several agents used the data to develop theories and publish articles. They found that, within their samples, half came from single-parent households; three-quarters had negligent or indifferent parents; three-quarters had paraphilias, or atypical and extreme sexual behavior; and three-quarters had been abused. A majority of those interviewed had a psychiatric history, while the average IQ was on the intelligent side of normal.

Some violent offenders do indeed have backgrounds that include fire setting, animal cruelty and bed wetting past the age of five years old. But rarely do these offenders have all three behaviors in their past, while, according to Psychology Today, other ones - like callous disregard for others - show up much more frequently. In addition, some of the data about the Macdonald Triad and its link to serial killers has seeped into public consciousness from novels and websites, written by authors who believe that the link has been indisputably proven.

Only recently have researchers begun to challenge the Macdonald Triad. In 2010, Kori Ryan published a thesis that performed the most extensive review of the triad to date and found that it was utterly inconclusive.

The triad can be an indication of a stressed child unable to cope well, or a child with developmental disabilities. But until better empirical data can be found to support the hypothesis, saying that these children are murder-prone should probably be avoided.

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