Looking back on 25 eventful years as a forensic expert, I reflect with a half-smile on the many challenging cases that have kept me up day and night. However, as of January 1, 2025, with a few exceptions, I will no longer accept new expert reports from the German judiciary. This decision has not been easy, but it finally opens up the space I desperately need for my research, my family, and the completion of my specialist book on person identification.
Cognitive bias in forensic science is not an abstract concept, but a creeping virus that, in the worst cases, can infect entire justice systems. To make the workings of this insidious phenomenon more tangible, let’s look at some examples that clearly illustrate the potential impact of cognitive distortions in practice.
The investigation of perpetrators through DNA analysis has established itself as an indispensable method in modern criminology. A sensational case from the early 2000s exemplifies the significance of this procedure impressively: a serial killer in South Louisiana could only be apprehended through the analysis of his DNA. Until that point, the FBI’s search focused on a white man who was supposedly driving a white pickup truck, at least according to eyewitness reports. However, the DNA analysis brought about a groundbreaking turn: it not only enabled the identification of genetic matches or familial connections but also determined the ethnic origin of the perpetrator. This realization led to the conclusion that the perpetrator could be an African American or Afro-Caribbean, rendering all previous leads obsolete. It became clear why previous DNA analyses of the suspected phenotype had not been successful and why the killer could continue to murder.