Text: Marjolein de Jong | Photo: David Meulenbeld | 3 June 2026
I once came across the intriguing term ‘crime profiler’ next to your name. What exactly do you do?
'When you say you are a criminologist, people tend to assume your work is very exciting. In reality, I mainly conduct research at the university. However, I also work closely with the police and contribute to active investigations. At the moment, for example, we are looking at approaches to explosion-related incidents, and in the past one of our studies contributed to the investigation that led to the identification of a serial killer targeting sex workers, who later became known as the ‘Rotterdam Ripper’.
What we see is that, in practice, a great deal is done, but many aspects of police work are still only weakly supported by scientific evidence.'
Can you give an example of police work that could benefit from stronger scientific foundations?
'We see that in serious crimes it is not yet standard practice to investigate whether a case might be part of a series, even though this is often the case. There are relatively few offenders, but they tend to have long criminal careers and therefore commit many offences. As a result, we know that patterns exist within their crimes.
Law enforcement agencies are often highly focused on individual incidents. They are dealing with a case right now, and they need to act on it right now. But that focus can mean missing the opportunity to recognise a series of offences and address the underlying problem. The chronic understaffing within the police does not help either, yet focusing on potential series can actually make investigations more efficient.'
How do you identify a serial offender?
'Because of all the crime dramas on television, people immediately think of DNA. That would certainly make things easier, but in reality DNA is not always available, and even when it is, it does not always produce a match.
In sexual offence cases, for example, we might look at the way an offender approaches a victim, how they overpower them, or what they say during the offence. It is usually a combination of behaviours that can be highly specific to an individual offender.'
'What I found particularly interesting was that offenders often have a kind of moral compass when it comes to their own criminal behaviour'
You also study how offenders make decisions. What has surprised you most?
'For my PhD research, I interviewed burglars and robbers. What struck me was how differently offenders arrive at the decision to target a particular location. Some are extremely cautious, while others are remarkably casual about it. Many different factors play a role, such as how busy a location is, how familiar an area feels, or where someone believes they can make a quick escape.
What I found particularly interesting was that offenders often have a kind of moral compass when it comes to their own criminal behaviour. Burglars, for instance, would say that burglary was acceptable because people were not directly harmed by it. Robbery, on the other hand, was considered terrible because it involves threatening someone. Robbers tended to turn that logic around and argue that robbing a shop was acceptable because it was insured, whereas breaking into someone’s home was crossing a line. Everyone finds a way to justify their own behaviour.'
An important part of your work is the project Reasonable Doubt. What does that involve?
'Together with a team, we examine criminal cases in which there may have been a wrongful conviction. We do this by analysing both the evidence itself and the way in which that evidence has been assessed. We do not only analyse evidence that points towards guilt; we also take alternative explanations seriously. To what extent does the evidence fit a scenario in which the person is innocent?
It sounds straightforward, but it actually represents a very different way of thinking. In practice, evidence is often interpreted from the perspective that it confirms what investigators already believe: namely, that someone is guilty. In psychology, this is known as confirmation bias; in policing, it is often referred to as tunnel vision. But that is only half the story. If you genuinely want to establish the truth, you must also test alternative explanations and determine whether they can, or cannot, be ruled out.'
'What did we find? The blood did not wash out in that manner and remained detectable using standard forensic techniques'
Can you give an example of a case where you felt the evidence was not strong enough?
'We worked on a case in which a victim had been struck several times on the head, causing significant bleeding. Clothing was found in the suspect’s washing machine that had allegedly been worn on the evening of the incident, according to witness statements. Naturally, that looked highly suspicious. The clothing was only removed from the washing machine much later due to a mistake by the police. No traces of blood were found on the garments. The conclusion was then that this made sense because the blood had been washed out. But we wondered: is that actually possible?
We recreated the situation to test whether the blood could indeed be removed in that way. We obtained fresh sheep’s blood, used comparable clothing and washing machine, and even used the same detergent to test the assumption. What did we find? The blood did not wash out in that manner and remained detectable using standard forensic techniques. The key point is that assumptions about evidence must themselves be subjected to critical scrutiny.'
'That does not necessarily mean the person was innocent; they may well have committed the offence. But there was insufficient evidence to justify a conviction'
Is there a case that has particularly stayed with you?
'One case involved a man with significant social difficulties who was convicted of murder. When I read the statements, I was shocked. I saw someone who spoke very little, seemed unsure of how to handle the situation, and was likely on the autistic spectrum. When you then examine the evidence, you find problems at every turn. It makes you wonder: how did this ever result in a conviction? That affects me deeply. Here was someone who struggled to stand up for himself, who may not have received the right legal defence, and whose case was not subjected to enough critical questioning. Yet that person spent years in prison.
Over the past 25 years, we have seen many cases in which we believe there may have been a wrongful conviction because the evidence supporting guilt was so weak. That does not necessarily mean the person was innocent; they may well have committed the offence. But there was insufficient evidence to justify a conviction.'
But suppose the case concerns a very serious crime, such as a sexual offence or the murder of a child, and your analysis could lead to someone being released, even though instinctively everyone believes they are guilty.
'I understand what you mean. But my position has consistently been that if we genuinely conclude there is insufficient evidence, then that is simply the reality of the situation. I firmly believe that we should not tamper with the boundaries of evidence. Once you start doing that, every boundary quickly disappears, and decisions become based on intuition rather than proof.
At the same time, it is incredibly difficult. That is especially true in highly emotional cases where the stakes are enormous. But that is precisely why we must remain rigorous in determining what can and cannot be proven. If we abandon those boundaries, we lose something fundamental as a justice system.'
