At first, it seems ridiculous — serial killers “retiring.” After all, murdering isn’t a career. And don’t serial killers have an innate urge to kill, a need that is overwhelming, complete and doesn’t falter throughout their lives?

The truth is that the urge to kill can come and go in a serial killer’s life due to many factors. A number of murderers have taken long breaks from killing or even have stopped altogether.

The Golden State Killer, Joseph James DeAngelo, for instance, appears to have stopped cold in 1986 after committing at least 13 murders, 51 rapes and 120 burglaries between 1974-1986. He wasn’t arrested until 30 years later and does not appear to have killed anyone during that timeframe.

Why Do Serial Killers Stop Killing?

A 2007 FBI report highlights many reasons why serial killers will murder for years, but then take breaks or stop killing. According to the report, serial killers may cease to kill as a result of their life circumstances changing, either making murdering more difficult or less appealing to the killer — for example, “increased participation in family activities, sexual substitution and other diversions.”

According to Mark Safarik, a former member of the Federal Bureau of Investigation’s Behavioral Analysis Unit and a lecturer at the University of Pennsylvania, these circumstances could be as simple as them changing jobs and working the night shift so that they’re not available to “predate” or stalk those who they might murder later. They could also become involved in a relationship, says Safarik, that would cause them to look suspicious if they left the house at night. 

Read More:Serial Killers Have Rapidly Declined Since The 1980s

Substitution and Life Circumstances

This appears to have been true in the case of Jeffrey Gorton, who raped and murdered a college professor at the University of Michigan-Flint in 1986 and didn’t kill again until 1991 when he raped and murdered a flight attendant before he was caught in 2002. According to the FBI report, Gorton was married and said he “engaged in cross-dressing and masturbatory activities, as well as consensual sex with his wife in the interim,” which served as a substitute for murdering.

Others may make decisions not to kill because they’re nervous that police are coming close to catching them. “Jeffrey Dahmer became afraid that police were going to identify him on a number of occasions, so he would stop for a period of time,” says Safarik.

Read More: Jeffrey Dahmer And Other Serial Killers Have These 5 Traits In Common

Do Serial Killers Age Out of Killing?

Additionally, research has shown that most serial murderers reach a certain age and stop killing; in some ways, they age out of it. A 2017 study that Safarik co-authored in the Journal of Forensic Sciences found that older sexual homicide offenders slow down once they reach age 50.

The older a killer gets, the less likely they are to participate in heinous rapes and murders because it becomes physically more difficult. “It’s part physical, it’s part hormonal, there just isn’t that drive anymore,” he says. It’s rare to have these types of offenders ever being over the age of 55.

Dennis Rader, also known as the BTK Killer (a nickname he gave himself, short for “bind, torture, kill”), murdered at least 10 people between 1984 and 1991. While it’s thought that he committed all of his murders during this time period, he wasn’t arrested until nearly 15 years later; he doesn’t appear to have murdered anyone in the intervening years.

“It seemed like as I got older I started making… well, physically, I was just wasn’t up to it,” Rader said in Dateline NBC interview after he was caught. “I knew if I’d have to fight with somebody it would have to be an older person because I’d be just winded or wouldn’t be able to fight physically.”

According to Safarik, the ability to stop or take a break from murdering highlights that serial killing is something a killer can control; it’s a myth to think that their choices are not conscious or they don’t have control over their actions. Most have some degree of psychopathy, meaning they don’t have a lot if any, empathy. But with regard to the law, most are not insane, and they know the difference between right and wrong. “They know it’s wrong, they just choose to ignore it,” says Safarik.

Read More: Female Serial Killers Do Exist And Are Known As ‘Quiet’ Killers