Those who seek to harm learn the most direct routes to vulnerability.

In England, Stephen Port was recently convicted of four murders committed over 15 months, and of assaulting seven other men. Once he met the men, he invited them to his apartment, drugged them, and raped them; he overdosed those who died. Port found his prey via online dating apps, using various names and photos on sites such as Sleepyboy, Grindr, Slaveboys, Gaydar, Daddyhunt, Hornet, and Couchsurfing. Predators like Port think first of themselves and their own gain. To get what they want, they realize they must be stealthy. They try to present themselves as safe, with ordinary pursuits, while trying out the most effective bait. They learn this by studying the key areas of human vulnerability, and then they find or create ways to gain access. They look for people who are motivated by desire or desperation to engage with someone they don't know well (or at all).

Katherine Ramsland Katherine Ramsland is a professor of forensic psychology at DeSales University in Pennsylvania, where she also teaches criminal justice. She holds a master's in forensic psychology from the John Jay College of Criminal Justice, a master's in clinical psychology from Duquesne University, a master's in criminal justice from DeSales University, and a Ph.D. in philosophy from Rutgers. She has been a therapist and a consultant. Dr. Ramsland has published over 1,000 articles and 54 books, some of which include: Snap! Seizing Your Aha Moments, Blood and Ghosts: Paranormal Forensics, The Mind of Murder a Murderer: Privileged Access to the Demons that Drive Extreme Violence, Inside the Minds of Serial Killers, The Forensic Psychology of Criminal Minds, The Forensic Science of CSI, The Criminal Mind: A Writer's Guide to Forensic Psychology, True Stories of CSI, Beating the Devil's Game: A History of Forensic Science and Criminal Investigation, Inside the Minds of Healthcare Serial Killers, Inside the Minds of Mass Murderers, The Human Predator: A Historical Chronology of Serial Murder and Forensic Investigation, Psychopath, The Vampire Trap, The Ivy-League Killer, Piercing the Darkness: Undercover with Vampires in America Today

Editor: Muhammad Talha

During World War I in France, Henri Landru repeatedly placed the same ad in the newspaper France Matin to lure women: “Widower with two children, aged 43, with comfortable income, serious and moving in good society, desires to meet widow with view to matrimony.” He knew what frightened or lonely war widows were seeking—a solid, safe, reliable man. He would meet them in a public place, the Luxembourg Gardens in Paris, establish that they had money, impress them with his manners, and eventually persuade them to travel to his country home. In four years, he lured and murdered at least 10 women, burning their bodies in his stove while he emptied their bank accounts.

Landru demonstrates this method: Figure out what the target group most desires and become that person. Dissolve their hesitations by using “trust cues” like confidence, helpfulness, reassurance, and an ordinary demeanor. Keep affirming that you offer what they want—you’re perfect for them—and that it’s safe to advance further. Use several steps that gradually engage them, each one proving your sincerity until they commit. Predators know how to work on low self-esteem, loneliness, alienation, a need for love, and even on narcissism (“You are the kind of person who deserves only the best.").

Using ads is a common way to lure prey, because people actively look in such places and willingly meet with strangers to acquire goods, services, and partners. Classified ads have been used in a number of cases where people have not only been fleeced but also raped and/or killed. Predators only need to know how to bait the hook.

Norwegian-American Belle Gunness, a devious predator, also employed “matrimonial” ads. After killing her first husband and two children for insurance money, she purchased a pig farm in Indiana. There, she married and quickly killed again. Collecting that money, she placed ads in several Norwegian papers to lure men with money, generally seeking immigrants from their common “home country.” She presented herself as a woman of means who needed a man to take over, appealing to their sense of protection and desire to own property. Once she hooked someone, she penned flowery expressions of eternal devotion, asking her new beau to bring all of his money and tell no one where he was going. She died or disappeared in 1908, whereupon police searching for a missing man dug up the remains of more than a dozen people from her property. There was suspicion that she had fed some to the pigs.

Generally, predators prey on the lonely. One team was even dubbed the “Lonely Hearts Killers.” A male con artist, Raymond Fernandez, used matrimonial ads during the 1940s to attract and fleece vulnerable women. Martha Beck posed as his sister. Fernandez corresponded with Delphone Downing, a widow in Michigan with a toddler. She invited Fernandez to visit, which ended her life and her daughter's. This team was suspected in many more such murders.

Today we have online marketplaces for goods and services, like Craigslist, which predators exploit as well. Sometimes predators place an ad, and sometimes they respond to ads their victims place. It appears that several New York-area prostitutes met their killer through Craigslist, and he dumped their bodies on an isolated stretch of Long Island. A massage appointment trapped Julissa Brisman with her killer, Phillip Markoff.

Of great concern are predators who prey on children. With the rising popularity of social networking sites, it has become easy for pedophiles to “befriend” future victims. Children spend hours on social media talking with strangers until those strangers feel more like friends. They often miss the signals of deception as they seek a friend, romantic partner, or innocent adventure. Predators can easily learn enough facts about kids to make them feel “known” and “understood.” Once they’re “friends,” an offline encounter feels to the child like a natural next step.

Among the creepiest of predatory ads was the one placed by Armand Meiwes requesting volunteers who wanted to be eaten. Surprisingly, he got several respondents, although most eventually backed out. But Bernd Brandes committed. They set a date, Brandes arrived, took some pills, and Meiwes killed him slowly as they consumed pieces of his body together. At Meiwes' trial, his defense was that he was guilty only of “killing on demand,” or assisted suicide. It didn’t go as well as he’d hoped.

So how does one stay safe? Should you stop trusting everyone? That's not necessary. The majority of ads and respondents are legitimate. It's important to be aware of your vulnerabilities—how much information is available to others about you, and how someone might use your expressed desires to bait you. (Men responding to Gunness, for example, should have been suspicious that she wanted them to keep where they were going a secret.)

Take precautions about inviting strangers to your home or meeting them alone, especially if they insist or make it a condition for providing their service, product, or shared interest. Be alert to how an ad you place could attract the wrong attention, and develop a list of safety rules that can serve as reminders should someone tempt you to do something potentially dangerous. Don’t lead with need; lead with caution.