Other killers might have changed behavior after moving away from the original epicenter of activity. Ted Bundy mutilated and murdered perhaps more than 30 young women in the 1970s. Yet there were stretches along his peripatetic travels when he was not associated with murders in those areas.
In some cases, jobs and families might have stabilized and exacerbating sources of stress might have faded, some experts said.
Dr. Michael H. Stone, a professor of forensic psychiatry at Columbia University who has extensively studied serial killers, noted that Gary Ridgeway, the Green River Killer, murdered prostitutes during his first two difficult marriages. He married a third time, more happily, and the killings dwindled.
“Some of these men have little oases of compassion, within the vast desert of their contempt and hatred of women,” Dr. Stone said.
So far, the authorities have not offered any public explanation as to why the Golden State Killer spree, for which Mr. DeAngelo was arrested this week, began and continued, much less stopped. But some experts point to the most banal explanation: In 1986, when he was 40, Mr. DeAngelo may have aged out.
“The testosterone levels are down,” Dr. Stone said. “His capacity to perform is weakened,” he added, noting that he was merely speculating. The prey drive is lessened.
Mark Safarik, a retired F.B.I. criminal profiler and consultant to crime shows like “Bones” and “The Blacklist,” recently worked on a study with academic researchers about older sexual homicide offenders. “They are really rare over age 50,” he said. “We just don’t see them. Pedophiles over 50, yes. But not rape-murderers.”
There is little research on why spree killers desist for reasons other than getting caught. No one knows what has happened to a serial killer of young women on Long Island.
“There has never been a survey of serial killers asking them why they stopped,” said Eric Witzig, a retired homicide detective and chairman of the Murder Accountability Project, a database of unsolved murders. “All we have are anecdotal hunches,” he said.
Perhaps a victim struggled and spooked the attacker, he said. The killer “might think then, ‘Maybe I don’t want to do this anymore because I might get caught.’ Or, ‘I want to stop and reflect on the carnage I wreaked in the past.’”
Or, said Dr. Bruce E. Harry, a retired forensic psychiatrist with the University of Missouri medical school: “Maybe they get tired or bored and just don’t want to do it anymore.”
Mr. Safarik, the retired F.B.I. crime profiler, was a beat cop in the late 1970s in Davis, Calif. and remembers being on stakeouts for the Golden State Killer, watching with night surveillance scopes for a man scrambling over rooftops or escaping into nearby fields.
Then as now, police believed that the suspect had military or law enforcement training, which helped him evade detection. One reason the rapist-murderer may have stopped in the late 80s, speculated Mr. Safarik, is that he was becoming aware of the ability to collect DNA evidence, left by the most meager material. The public was becoming familiar with it, he said, particularly through television dramas.
Several experts, underlining the notion that so little is yet known about Mr. DeAngelo — including most significantly whether he committed the crimes — wondered whether the criminal behavior did stop entirely in 1986.
“I would want to look at other rapes and murders in the areas where he lived over time,” Dr. Meloy said. “I would be skeptical that there was a complete shutdown at age 40.”
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