Can you really spot the signs of a serial killer early on? Scientists once thought so, but now they’re not so sure.

Animal cruelty, fire-setting and persistent bed-wetting, also known as the Macdonald triad, or the homicidal triad, are often cited as tell-tale childhood behaviors engaged in by budding murderers. The triad is often referenced, though not always by name, in pop culture and news articles about serial killers. They are the signs that something has gone wrong -- a child that sets fires and tortures animals is just practicing for later horrors involving people, the thinking goes.

But now researchers say this triad is more of an urban legend than a reliable predictor of future violence.

Psychiatrist John Macdonald first described the triad in a 1963 paper in the American Journal of Psychiatry, titled “The Threat to Kill.” In a study of 100 patients who had threatened to kill someone, Macdonald found that more aggressive and psychotic subjects were more likely to have a history of fire-setting, cruelty to animals and prolonged bed-wetting than subjects who were not aggressive or psychotic. Over the next few decades, the idea of the portentous triad caught on in some criminology circles and in the popular imagination.

In 1966, two psychiatrists, Daniel Hellman and Nathan Blackman, published their own paper purporting to support the triad theory. They looked at 84 prisoners and found that three-quarters of the 31 charged with the most violent crimes exhibited all three behaviors in the Macdonald triad.

In their paper, Hellman and Blackman stressed “the importance of early detection of the triad and serious attention toward resolving the tensions that precipitated it.”

But now, most researchers question whether these relatively small studies are really evidence of a reliable method for predicting homicidal tendencies. Macdonald himself began to have doubts -- in his 1968 book, “Homicidal Threats,” he said he could find no statistically significant link between the triad and murderers.

Bed-wetting, also called enuresis, in particular, has almost been completely discredited as a marker of sociopathy and violent tendencies.

Anthropologist Gwen Dewar, the proprietor of website Parenting Science, notes that bedwetting is fairly common in children, with up to one-fifth of five-year-olds committing it once in a while.

“Research indicates that bedwetting is usually caused by relatively benign medical conditions, like a tendency to sleep deeply or overproduce urine at night,” Dwar wrote.

In a 2009 article in the Journal of Urology, Australian researchers published the results of a survey of more than 8,000 school children in Sydney. They found that children's emotional stress was associated with moderate bed-wetting but not with severe enuresis.

For her master’s thesis, California State University Fresno researcher Kori Ryan reviewed the literature associated with the Macdonald triad and found “little empirical support” for it.

The myth of the triad may cause troubled children who set a fire or injure an animal to be falsely labeled as potentially dangerous, according to Ryan.

Forensic psychologist Katherine Ramsland, writing at Psychology Today, says that though some violent offenders exhibit one of the three behaviors in the Macdonald triad in childhood, they rarely exhibit all three. Meanwhile, other traits, like callous disregard for others, are much more consistently found in violent offenders.

There needs to be more empirical studies conducted before researchers and the media latch onto certain behaviors as predictive of future crimes, according to Ramsland.

“Together or alone, the triad behaviors can indicate a stressed child with poor coping mechanisms or a developmental disability,” Ramsland wrote. “Such a child needs guidance and attention” -- not the label of future serial killer.