What data on 3,000 murderers and 10,000 victims tells us about serial killers

Back in the early 1990s, Dr. Mike Aamodt, then a forensic psychology professor at Radford University, started to collect and code data on serial killers. At the time, “there wasn’t much out there,” he says. “I wanted to change that.”

Over a 25-year period, Aamodt and his students went through public records of serial murderer cases around the globe, collecting granular data on nearly 3,000 US serial killers and 10,000 victims. The cases, ranging in data from 1900 to present, each contain hundreds of variables, shedding eerie light on serial killers’ motives and methodologies and giving us a specific idea of whom they most frequently target.

First, we’ll explore why and how serial killers kill (according to public records); then we’ll take a look at who the victims have historically been and where they tend to live. Ultimately, though, as we’ll show, there isn’t much to fear: Serial killers have declined by 85 percent over the past three decades.

Why do serial killers kill?

When we think of the serial killer in popular culture, figures like Ed Gein or John Wayne Gacy come to mind — troubled, often psychotic individuals who go on massive killing sprees dressed as clowns or eat their victims’ flesh.

But the Serial Killer Database uses the FBI’s official definition of serial killing (which is also the most common): “the unlawful killing of two or more victims by the same offender, in separate events.” This includes not just typical folkloric serial killers like Charles Manson, but also gang members and organized criminals — people who frequently commit repeat murders without much publicity.

Aamodt and his researchers compiled and analyzed publicly available information — news clips, court reports, books — for thousands of these serial killers. Whenever a motive was specifically defined (through interrogation, investigation, or admission), they recorded it.

What they found is that the majority of serial killers simply kill for enjoyment.

Gang activity only accounts for 6.3 percent of all serial murders — though gang members often don’t reveal their affiliation in interrogations or post-trial psychological evaluations. It is possible that many of them provided the more general response that they did it for money.

How do serial killers kill their victims?

Serial killers are often reputed to have repulsive killing techniques. For instance, Jeffrey Dahmer, who killed 17 young men in the 1970s and ’80s, would drill holes in his victims’ heads and inject them with acid before strangling them.

But historically, most serial killers have preferred “easier” methods. A review of 9,915 US serial killer victims reveals that nearly half were shot to death. Following this, 21.7 percent were strangled, 14.8 percent were stabbed, and 9.2 percent were bludgeoned with blunt objects. Less commonly, 145 victims were axed to death, 94 were drowned by force, and 63 were burned alive.

Interestingly, there is a correlation between a serial killer’s murder method and what he or she scores on an IQ test. Murderers frequently use an insanity plea in court to avoid the death sentence, and are, at times, given IQ tests to gauge their mental capacity. Using IQs from these records, we can see a differentiation by murder method.

An IQ of between 85 and 100 is considered average in America. In general, criminals score half a standard deviation below that. But serial killers who concoct bombs average 140 — well into the realm of what is considered “superior.”

Generally, the messier and more direct the murder method, the lower the IQ score.

Who are the victims?

Looking deeper into the Serial Killer Database data, we’re also able to get a better idea of who these 9,915 victims are.

Gender-wise, victims are essentially split down the middle, with a slight lean toward women. (Note: There are naturally more women than men in the general US population, and this is only slightly more than proportional.)

Across race lines, two-thirds of serial killer victims are white. But when gauged with US Census figures of the general population, black people are by far the most overrepresented: They account for 13.3 percent of the US population but a whopping 24 percent of all victims.

Serial killers also prefer younger victims: About 18 percent of all victims fall under the age of 18, while just over 10 percent are over the age of 60.

After peaking at age 29, the chances of being murdered by a serial killer dramatically decrease in one’s 30s, 40s, and 50s.

Of course, where you live also impacts your chances of being a serial killer victim: Certain states have much higher rates of repeat killings than others. The map below isn’t perfect — the serial killer data spans from 1900 to present, and we used population data from the 2015 census — but it still gives us an idea as to where killers have most frequently struck historically.

Washington, DC, has, by far, the highest rate of victims, at 25 per 100,000. Distantly trailing — but still shockingly high — are Alaska (6.9) and Louisiana (6.4).

California has the most total victims (1,628), but it also has the largest population of any US state. South Dakota has the least, with just seven deaths in 116 years.

While the outer reaches of Alaska seem ripe for serial killers, America’s other offshore state, Hawaii, is the safest place in the country with a per-100,000 figure of 0.7. New Hampshire is equally killer-free.

Good news: Serial killers are on the decline

There is a silver lining to all of this, which is that serial killers are not as potent as they once were.

Charting out the data over time, by decade, we see that instances of serial killers — that is, people who kill two or more people on two or more separate occasions — have been on the decline since the 1980s.

When looking at that massive rise from 1900 to 1960 above, keep in mind that the data is likely skewed, as serial murders were less frequently reported in the olden days, and older case files are harder to find. What’s more shocking here is the massive drop in serial murders over the past four decades. Dr. Aamodt, who spearheaded the collection of this data, offers a few theories for this.

“One of the keys here is a change in parole laws — longer sentences, things like the three strikes rule,” he says. “If you take a look at serial killers in our database prior to 1980, 22 percent had killed someone, gone to prison, been released, then killed again when they got out. You get a big reduction in that after 1980.”

Aamodt also cites improved forensic science (“We’re getting better at catching criminals after one murder instead of three or four,” he says”), as well as a culture that generally engages in less risk-taking. It used to be acceptable to hitchhike or let a child ride her bicycle alone in a park; today, it is more difficult for serial killers to find vulnerable victims like this.

Breaking down serial killers by their number of total kills, we also see a tremendous decrease in “prolific” killers — or those who kill five or more victims.

Just 40 years ago, nearly one-third of all serial killers in the US got away with five or more murders before getting caught. Today, that figure is down to 13 percent — and nearly half get caught after two killings.

As Aamodt says, “That’s good news any way you slice it.”