Based on the wild popularity of shows like Dexter and CSI, it would seem our fascination with the ultra-creepy criminals called serial killers knows no bounds. But why do these unusual murderers exist? And how can insights into their behavior be used against them? Here's what you need to know about the mind of a serial killer.

Serial killers have existed throughout history (like Elizabeth Báthory and Jack the Ripper), but it wasn't until the 1970s and the seminal work of FBI investigator Robert Ressler that the phenomenon finally received formal recognition. Today, much of the work done by the FBI and other investigative units are extensions of Ressler's early work (including criminal profiling, which we'll get to in just a bit).


What makes a serial killer?

But even after years of research, neuroscientists, psychologists, and behavioral scientists have still not come to a consensus on what makes someone a serial killer. According to experts Ronald Holmes and Stephen Holmes, there simply hasn't been enough serial killers to study. And it's because of this, they say, that no meaningful scientific statement can be made about the exact role that biology plays in creating these modern day monsters.


Now that said, psychologists are not running blind — they have pinpointed some likely factors. Some serial killers were abused as children — either emotionally, physically, or sexually. Many are compelled by sexual fantasies that have gotten out of control, sometimes driven by overlapping psychological disorders like schizophrenia and psychopathy.

And indeed, many serial killers are in fact psychopaths — that much we know. Murderers who rank high on the Hare psychopathy scale are often motivated by an overwhelming desire to control things and exert their will on others by whatever means possible. For some, this leads to periodic violent behavior. But not all serial killers are psychopaths, and vice versa (by an absolutely massive margin).

As work on serial killers continues, therefore, we may very well discover that there is not just one type or one cause that's responsible for it all.


What defines a serial murder?

Now, as Resler's work with the FBI attests, the first step in catching a serial killer is in recognizing that one exists — something that's easier said than done; it's not always obvious that a serial killer is involved. In some situations, like the Robert Pickton case in Canada, it can take years for the police to realize that they have a serial killer on their hands — an oversight or mistake that, like in the Pickton case, can cost dozens of lives.


Many investigators define a serial killer as someone who has killed three or more people in separate incidents over a period of more than a month — an interval that they refer to as a "cooling off period."

But late last decade the FBI adopted a more liberal definition, stating that a serial murder is more properly defined as the "unlawful killing of two or more victims by the same offender(s), in separate events." They also noted that a serial murder can involve one or more offenders, and that a requisite time period between murders needs to exist to distinguish them from mass murderers (who tend to go on a single killing spree, such as the horrors perpetrated by Anders Breivik and James Holmes).

But the moment that two or more cases can be associated, the investigators can then start to change gears and adapt their techniques accordingly.


How are serial killers profiled?

Trying to find a serial killer can be like looking for a needle in a haystack. Thankfully, investigators have some tools that can give them a broad sense of where in that daunting haystack that needle might be located.

Given that many investigations have virtually nothing to go by aside from what the forensic scientists have uncovered, police have turned to a technique called ‘offender profiling' in which they create a rough sketch or prediction of the person they're looking for. These profiles are compiled by referring to years of accumulated data, along with the insights gleaned from psychologists and neuroscientists. Serial killers, it should come as no surprise, have certain behavioral patterns, traits and characteristics that are fairly predictable.


Surprisingly, police have learned that a serial killer's motivation is one of the more unhelpful elements in an investigation. Serial murders are typically committed for psychological gratification, and not for any material or tactical gain. Their motivations tend to be driven by hedonism, sexual satisfaction, the thrill, lust, and a sense of dominance — aspects that make for weak clues.

What's more insightful for investigators are the commonalities that can help them outline their perpetrator. For example, serial killers tend to be males in their 20s and 30s (spiking around the age of 28) who tend to kill within their own racial group. While these are admittedly gross generalizations, they create a probabilistic profile that eliminates a huge swath of the general population, allowing the investigators to narrow their search.

But there are other patterns as well. Serial killers tend to use the same method of killing every time, and their victims are often of a certain type (such as prostitutes, or a certain age group). These killers encompass all races in fairly similar proportions (the notion that whites have a higher propensity for serial killing is a myth) who target strangers that live near their home or places of work.


Interestingly, female serial killers are exceptionally rare, and those who are often fall outside of most profiling schemas. Women tend to know their victims (who are almost always male), they murder them for material gain, and are often part of a serial killing team (typically with a man). Notorious examples include Paul Bernardo and Karla Homolka, and more recently, Terri-Lynne McClintic and Michael Rafferty (who were mercifully caught before they had a chance to kill more than once).

It's worth noting that Aileen Wuornos, who was featured in the film Monster, is a very rare exception to these rules; her killing behavior was very much like that of a man's.


Organized or disorganized?

Perhaps the broadest and most telling of the profile categories is in determining whether or not the killer is "organized" or "disorganized."

Organized killers typically have above average intelligence and are very discreet and careful about their crimes. They are are also known to be sociable, they have friends, a stable job, and even families — the kind of person no one would suspect.


This runs in stark contrast to the disorganized killer. Their crimes are more impulsive and often executed with whatever weapon is available at the time. Their crime scenes tend to be a mess and they're not typically careful about how they dispose of the body. Some are necrophiliacs and have a history of mental illness. For disorganized killers, their motivations tend to be about exerting extreme physical and/or sexual violence and fulfilling some kind of fantasy.

But just to make life difficult for investigators, there is also the ‘mixed' killer — a perpetrator who exhibits both organized and disorganized characteristics. The classic example of this is Jeffrey Dahmer, a methodical and careful killer who was searching for a "perfect lover." But at the same time, he dismembered his victims, kept body parts in his freezer, and tried to bring corpses back to life by drilling into their skulls. He also experimented with cannibalism to "ensure his victims would always be a part of him."


What are the limits of profiling?

Indeed, the Jeffrey Dahmer example provides a potent and upsetting example of how limited and even misleading criminal profiling can be. Not only did he have mixed characteristics, he also killed outside of his racial group. And because he killed men (Dahmer was gay), this confused the investigators.

And during the Beltway sniper attacks, for example, the police profile described a white male in his 30s from the DC area who was acting alone. In reality, the crimes were committed by two black males, one of whom was 41 and the other 17 years old — and both from the west coast.


These cases, plus an array of other factors, have even led some observers to declare that criminal profiling is both misleading and unscientific. Back in 2007, Malcolm Gladwell railed against the practice in the New Yorker by complaining that, "It doesn't do any good to get a specific detail right if you get general details wrong." He writes:

A profile isn't a test, where you pass if you get most of the answers right. It's a portrait, and all the details have to cohere in some way if the image is to be helpful. In the mid-nineties, the British Home Office analyzed a hundred and eighty-four crimes, to see how many times profiles led to the arrest of a criminal. The profile worked in five of those cases. That's just 2.7 per cent, which makes sense if you consider the position of the detective on the receiving end of a profiler's list of conjectures. Do you believe the stuttering part? Or do you believe the thirty-year-old part? Or do you throw up your hands in frustration?

To be fair, profiling is, at best, a tool that can help investigators steer the direction of the investigation. And indeed, analyses of successful investigations often show that the profiling was quite accurate. A good example comes from psychologist and criminologist David Canter who, in a 1986 case, created a profile for John Duffy in which 13 of his 17 predictions were absolutely spot on.


But Gladwell is right when he says that profiling very rarely leads to an arrest or a solid lead — but it can help to short-list potential offenders and confer confidence to the officers during the the arrest process.

To catch a killer

More often than not, serial killers are typically caught as a result of their own carelessness. Some get nabbed during their first crime as a result of their inexperience, while others get sloppy and over-confident after a few years and make a mistake.


Also, it's not uncommon for an escaped witness to identify the killer (which is how Dahmer got caught), or for a murderer to get caught during the act itself. Some are caught with a body in the trunk of their car (Ted Bundy was arrested during a routine traffic stop). And a prior history of misconduct will often put a killer under suspicion, which often leads to surveillance and an eventual arrest. Or, when working as a team, one will turn the other in. And David Berkowitz, the "Son of Sam," was initially picked up for loitering — the police thought he was a witness to the crimes instead of the killer.

There's always something to be said for serendipity.

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