When Dennis L. Rader was arrested as the B.T.K. (bind, torture, kill) serial killer suspect—and eventually charged with 10 killings—his Wichita, Kan. community reacted with shock and contradiction. After all, he was a leader in his church, a husband and father of two, living among the very neighbors whom he secretly terrorized.
It left people to wonder what would drive a man who appeared so normal to his neighbors to commit murder. According to new research, the answers may lie in evolutionary psychology.
Dr. David Buss, evolutionary psychologist, studies motives for murder.
Dr. David Buss, an evolutionary psychologist at The University of Texas at Austin and author of the just-published “The Murderer Next Door: Why the Mind is Designed to Kill” has conducted an unprecedented set of studies investigating the underlying motives and circumstances of murders, from those of serial killers to the perfectly friendly next-door neighbor who one day commits murder. He examined FBI files of more than 400,000 murders, conducted a highly detailed study of nearly 400 murders, and led, with his collaborator Joshua Duntley, the largest homicidal fantasy study ever conducted.
“Killing is fundamentally in our nature because over the eons of human evolution murder was so surprisingly beneficial in the intense game of reproductive competition,” Buss said. “Our minds have developed adaptations to kill, which is contrary to previous theories that murder is something outside of human nature—a pathology imposed from the distorting influences of culture, media images, poverty or child abuse.
“Though we may like to think that murderers are either pathological misfits or hardened criminals,” he added, “the vast majority of murders are committed by people who, until the day they kill, seem perfectly normal.”
To determine what would drive people over the edge and cause them to kill, participants in one of the studies were presented with more than a hundred different scenarios in which they recorded the probability they would kill.
“Nearly all people express a willingness to kill in some circumstances—to prevent being killed or to defend their children from killers,” Buss said. “The study reveals the specific circumstances in which normal people said that they would kill, with some surprising findings. For example, men indicate an increased willingness to kill as their mating prospects become dire; women do not.”
In Buss’s sample of 429,729 homicide FBI files, 13,670 were cases in which a husband killed his wife. A husband discovering his wife having an extramarital affair is one of the leading causes of women being murdered, particularly when the woman is dramatically younger than her husband. This leads to a disturbing theory—the more good-looking, healthy and fertile the woman, the more motivated the man will be to kill her upon discovering a sexual infidelity.
Separation is also a powerful trigger for murder. According to a study of homicides in Chicago, 50 percent of wife killings took place within the first two months of the separation, and an astonishing 85 percent of these women were killed within the first year. In contrast, among the women who contemplated killing their mates, getting dumped accounted for only 13 percent.
“Among women killed by a partner they have separated from, 88 percent had been stalked prior to being killed,” Buss said. “Although most stalkers do not kill their victims, most mate-killing men do stalk their victims. Stalking is one danger sign that women should not ignore.
“Just when women feel as though they have successfully escaped a bad marriage is precisely the time when their lives are most in danger,” he added. “It is likely that the key danger is not the length of time per se but, rather, when the man realizes she will never return to him.”
Based on existing research, Buss concluded for the few mate killings that occur a year or more after estrangement, it seems the couple actually had sexual contact during the year even though the woman had moved out. The hope that she might return, as indicated by sex, offers a protective buffer, lowering the odds the man will try to kill her. But then when the sex stops, and he realizes she will never come back, the woman’s life is in danger.
Previously, Buss led one of the most massive cross-cultural studies ever undertaken, documenting mating desires among 10,047 people in 37 cultures, residing on six continents and five islands. Taking these findings into consideration, when and why men or women commit murder can be explained by the differences in evolutionary pressures and the differences of what they value in a mate. The most desired characteristics in a romantic partner, valued by both men and women, included a mate who is kind, understanding, dependable and intelligent. However, in comparison, men had a much stronger desire for beauty, youth and fidelity. Women had special desires for economic success and high status men.
Buss led the largest homicidal fantasy study ever conducted, using 5,000 people, 375 who were actual murderers. The study looked into why people have homicidal fantasies and the specific circumstances in which they contemplate killing. The research team discovered how homicidal fantasies are used to build and work through scenarios of killing, how they help channel murderous intentions into other means of seeking redress, how they can also be used to simulate and rehearse carrying out murder and how particular passions come into play in evaluating whether or not to turn fantasy into reality.
Buss’s homicidal fantasy research revealed that 91 percent of men and 84 percent of women have had at least one vivid fantasy—often intense and astonishingly detailed—of committing murder.
“As with most instances of homicidal fantasies, few thoughts are translated into deeds,” Buss said. “Most people work through cost-benefit calculations, figure out alternative means of solving the problem and decide that the costs of killing are too high.”
One indication of how infuriated both sexes feel when romantically rejected comes from an analysis of whether torture was part of the fantasy. The response to getting rejected and discovering a partner’s sexual infidelity proved to be about equal for women and men, with 57 percent of the victims of each experiencing torture fantasies.
“The key differences between the sexes is not so much in having homicidal fantasies about the mates who have left them but, rather, in the likelihood of acting on them,” Buss said. “Among the men who entertained thoughts of murdering their mates, 54 percent were triggered by the woman ending the relationship.
“Whereas men kill mates who have dumped them,” he said, “women kill mates who sequester, abuse and threaten them so heavily that they see killing as the only way out.
“Many abused and battered women choose a time to kill when their partner is more vulnerable, such as when he is drunk or asleep,” he said. “Because the laws typically state that a person’s life has to be in imminent danger to invoke self-defense, defense lawyers often have a difficult time convincing juries that a battered woman who waited until her husband fell asleep was actually acting in self-defense.”
Interestingly, women who live close to their kin experienced dramatically less violence at the hands of husbands than women whose families live hundreds or thousands of miles away.
Repeated physical, sexual and psychological abuse are by far the most common triggers of women’s homicidal fantasies, and also the leading predictors of when women kill their mates. A major trigger of homicidal fantasies for women, though much less frequently translated into actual murders, is having been raped. Although many women express strong fear about being murdered by a rapist, in fact only one in 10,000 rapists kill their victims. Sexual predators exploit fear of being killed as a strategy designed to successfully carry out a rape.
“Most participants feared getting killed by strangers more than acquaintances,” Buss said, “despite the fact that most murderers know their victims, often intimately.”
The method for killing in the fantasies also differed between the sexes. Because men are larger and stronger than women on average, women must use different means of killing, even in their fantasies—where one would presume they could do whatever they wanted. Women are far more likely than men to use poison. In fact, only one man in the sample of more than 5,000 participants mentioned poison in his homicidal fantasies.
“When we asked people to estimate the probability that they would carry out their homicidal fantasies if they could get away with them undiscovered, most men thought that the likelihood would quadruple,” Buss said. “The most frequently cited reason for not carrying through on the homicidal fantasies was the fear of getting caught and spending their life behind bars. Many of us owe our lives to the fact that murder is so costly to commit in the modern world.”
Public humiliation tends to lead to especially violent fantasies about killing the tormentor. The vividness and elaborated details of such homicidal fantasies highlight the magnitude of the social cost and psychological agony experienced by those whose reputations are damaged. For men there are even higher stakes, given that their status and reputation are important in attracting a mate. Instances of being embarrassed or ridiculed in public may lead to a violent reaction. Men are far more likely than women when losing their jobs, to take revenge by shooting a boss or rival co-worker whom they blame for their demise. Another scenario is that a man may suffer more ridicule when his wife cheats and his reputation may suffer catastrophic damage.
Serial killers account for only one to two percent of all murders, and according to Buss’s theory, may murder because they seek vengeance for status denied, while mass murderers kill to get to the top in a status hierarchy and stay there.
“I would argue that the underlying motivations that drive them to kill are the same as those behind the everyday killings over status and reputation,” Buss said.
For example, Ted Bundy, one of the world’s most prolific serial killers with a death toll of at least 36 female victims, began his spree after a beautiful woman of higher social standing spurned his offer of marriage. Bundy explained his motivation for killing as “stealing the most valuable possessions of the established classes, their beautiful and talented young women.”
“Serial killers and mass murderers, at least the ‘successful’ ones, almost invariably achieve a certain sort of status,” Buss said. “Many, in fact, married and had children after they were caught and convicted. The children of Charles Manson and Ted Bundy are among us today.”
Amazingly, even today convicted murderers remain highly appealing to some women. Scott Peterson, recently convicted of murdering his wife and unborn child, has been bombarded with hundreds of love letters and marriage proposals.
For centuries, power and status has affected a man’s reproductive success. Oxford geneticist Dr. Chris Tyler-Smith and his colleagues discovered roughly half a percent of the world’s population, 16 million people, are likely modern descendants of fierce Mongol emperor Genghis Khan.
“People might mistakenly assume that the theory of adaptations for murder implies approval or acceptance of killing,” Buss said. “It doesn’t. I would suggest instead that those who create myths of a peaceful human past, who blame killing on the contemporary ills of modern culture, and who cling to single-variable theories that have long outlived their scientific warrant tread on dangerous moral ground. The problem of murder cannot be solved by wishing away those aspects of human nature that we desire not to exist.
“As an evolutionary psychologist,” he added, “I’ve become accustomed to critics who confuse what is with what ought to be. We can prevent murder, in principle, through a deep understanding of its underlying psychological circuits and designing environments that prevent their activation.”
Office of Public Affairs/College of Liberal Arts