As headlines scream of new mass murders on a regular basis, the grief-stricken and the fearful frequently can be heard asking, "what can be done to stop these killers before they act?"

For years people have sought to predict future behavior, sometimes using frightening and uncomfortable approaches that raise the hoary specter of eugenics, phrenology and the like.

But in recent decades, advances in brain scanning and genetics have helped researchers make significant progress in the hunt for brain-structure and genetic clues to hyper-aggressive, murderous behavior.

One such investigator is New Mexico-based neuroscientist Kent Kiehl, who has been scanning the brains of prison inmates for the past 25 years.

"We’ve found that psychopaths have 5 to 10 percent reduced gray-matter density in and around the limbic regions," he told Wired in a 2014 interview. "We’ve also found — and a group in Germany has published a similar finding — that the tissue that connects the limbic system to the frontal lobes is disrupted. There have also been lots of studies published showing reduced responsivity in those circuits during emotional processing and moral decision-making."

Kiehl, who recounted his findings in his book, "Psychopath Whisperer", has now collected data on over 4,000 imprisoned criminals, using fMR to record their reactions to exposure to statements about things like stealing, or pictures of violence and mayhem.

Contrary to Hollywood stereotypes, in both behavior and frequency in the population, "psychopaths are way more common than people believe," he stated, estimating that about one in every 150 people meet the strict clinical criteria, although most don't become violent criminals. The condition, he noted, is "actually far more common and banal." Psychopaths tend to live lives that are "disorganized and nomadic" and "tend to eventually end up in some sort of trouble."

And they are free from conscience and different, even from each other. He described one to the magazine.

"One of my favorites I call Shock Richie because he did some things that most of us would not even think were possible — the types of crimes he committed, the things he would do to the bodies afterwards. He would do things purely for shock value. When he walked into the maximum security prison on his first day he took off all his clothes and walked around completely naked out in the rain. All the other inmates were wondering what the hell was going on. I interviewed him later that day and he told me he does stuff like that to make sure people understand he’s capable of anything."

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.”