The crimes of the convicted serial killer Lucy Letby have rattled the UK and the world. One can think of nothing worse: babies who were grievously ill, killed or injured by a woman who was meant to care for them. Why was Letby able to kill so many young victims? Why wasn’t she recognised as a murderer sooner? Some are saying that Letby’s case is a “one-off”, and is thus difficult to process or learn from. While serial murder is rare, to those of us in the field of serial homicide research, the crimes and victims of Letby are less surprising. In many ways, Letby fits the profile for the “typical” female serial killer (FSK) that my team and I compiled for The Journal of Forensic Psychiatry & Psychology in 2015. By analysing cases in the US, we found that nearly 40% of female serial killers are nurses, nurses’ aides or other healthcare workers. Our analysis showed that a FSK is likely to be white, probably Christian, average looking or attractive, and in her 20s or 30s when the crimes start. She has an elevated probability of being a healthcare worker, often in charge of caring for those who are helpless. Those familiar to her are at risk, especially vulnerable people such as infants and the sick. She may murder for money or power. She may be arrogant or at times withdrawn, and may have experienced a recent relationship issue. Experts such as Eric Hickey and Patricia Pearson have also compiled information about FSK backgrounds, crimes, motives and victims. The caveat is that a forensic profile is an imperfect predictor of future crimes, and the description above probably applies to many nurses you know. Moreover, almost every nurse or medical professional who has ever lived would never harm anyone. We created a composite FSK profile based on frequencies and trends in previous cases that we hoped would serve as a starting point – a statistical tool for investigating suspected female-perpetrated serial homicide offences. In fairness, too, Letby did not check some of the boxes in our profile. Most FSKs have been married; Letby never was. Most FSKs do not keep souvenirs of their crimes; Letby kept bags of patient records and a diary with victim initials and their death dates. In our analysis we found a documented history of mental illness in about 40% of FSK cases. I have seen no reports yet that Letby has such a history. Many FSKs had parental issues. It is my understanding that Letby’s parents were supportive to the point of attending her trial every day. Yet the profile is a starting point for whom and what to look for when serial murder is suspected. So how can we make sure that law enforcement and the public have access to what we know about FSKs as an investigative tool? My 2015 article about FSKs is reported to be the most-read article in the journal’s history. But the point is moot if the information isn’t disseminated to people who can use it. We can try to do better. It does seem bewildering that administrators at Countess of Chester hospital failed to recognise Letby’s crimes after doctors raised concerns . Some outlets have put the hospital’s reaction down to a “a culture of arrogance”. As a research psychologist, however, I can offer a different perspective, and one that I offer to law enforcement when given the opportunity to engage with them. We all have a “schema” – a cognitive framework – of how people, places and things typically operate to help us process the world around us. We create each schema from the slew of information we have encountered in our previous experiences. Evidence repeatedly shows that we find it very difficult to process or believe information that does not fit our schemas. We have a schema for “nurse”. A nurse is a skilled and compassionate caretaker. A nurse monitors, nourishes and soothes. “Killer” does not fit into our “nurse” schema. More broadly, we also have a gender schema. Fairly or not, people have a preconceived notion of what we feel a “woman” and a “man” ought to be, based on our previous encounters and feedback. Evidence shows that, throughout history, women are typically perceived as nurturing, caring and kind. Most people are not ready to believe that a woman can kill. Now think of “female nurse”, and how “murderer” is probably brushed off as being impossible. The initial failures of the Countess of Chester hospital echoes what we have seen in previous FSK cases. Other hospital-based serial killers such as Kristen Gilbert, Reta Mays and Genene Jones killed multiple helpless victims before hospital authorities realised that the deaths were unnatural. When I give training talks to law enforcement (while acknowledging that their crime-solving skills are exponentially more developed than mine), I challenge them to drop their schemas. If you think a woman will not kill or a nurse will not kill, then think again. On the heels of the Letby case, what can we do to stop this type of homicide, or at least to limit the number of future victims? As mentioned previously, we can become aware of the science out there, and perhaps we in the scientific community can do a better job of disseminating information. In addition, we can learn to look beyond our preconceived notions of who a murderer might be. One must be ready and willing to recognise that a crime is happening, even if the suspect seems unlikely based on our preconceived ideas. Of Letby, Detective Ch Insp Nicola Evans said: “There isn’t anything outstanding or outrageous about her as a person.” This is reminiscent of the Mays case in West Virginia in the US, when District Judge Thomas Kleeh, sentencing Mays to life in prison for murdering seven elderly veterans under her hospital care, told her: “You are the monster no one sees coming.” We must be prepared to recognise that, sometimes, the monster is a vanilla nurse who took dance lessons, fancied a staff doctor, and had teddy bears, fairy lights and a polka-dot dressing gown in her bedroom; an otherwise ordinary woman who took salsa lessons and a holiday with friends in Ibiza, and yet destroyed lives in a most extraordinary way. Dr Marissa Harrison is a professor of psychology at Penn State Harrisburg. She is the author of several scientific articles and a book on female serial killers, Just as Deadly: The Psychology of Female Serial Killers. She is a member of the Atypical Homicide Research Group network