Genetics researchers have also made inroads. They have found that certain factors play a role in violent behavior.

A major player appears to be monoamine oxidase A gene (MAOA), which "has earned the nickname 'warrior gene' because it has been linked to aggression in observational and survey-based studies," researchers led by Rose McDermott reported in a paper published in the Procedures of the National Academy of Sciences. A defect in the x-chromosome seems to be behind it.


In 2015, a Finnish study reported in Molecular Psychiatry showed that genes coding for the protein CDH13 — which has been shown to impact autism, schizophrenia and ADHD — were also "plausible" factors behind violent behavior, according to Popular Science.

In addition, there is evidence that implicates genes that code for a protein known as atechol-O-methyltransferase (COMT). A German report in 2011 found a correlation between murderous behavior and COMT.

That said, scientists — and ethicists — warn that genetics does not alone predict actual behavior, and stress that what happens in your environment very much impacts the role genetic makeup plays.

After all, having the genes for heart disease or cancer does not mean you inevitably end up with either. The same, warn scientists is true for "killer" genes.

“If you inherit small glitches, little pieces of noise, this sets you on a path," Lieber Institute for Brain Development director Daniel Weinberger told the magazine. "But it doesn’t determine you will end up with mental illness. These glitches aren’t fate. They are for risk. Environmental factors are at play too.”

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