Eyewitnesses play a crucial role in establishing the truth and solving crimes. Traditionally, they are interviewed individually to create the most objective reconstruction possible of what they observed. Until now, however, the practical effects of interviewing two witnesses simultaneously had not been studied. Would it lead to new or additional information? Van Rosmalen and Vredeveldt conducted a field study in which witnesses were interviewed jointly. Their findings are promising: in all joint interviews, witnesses provided new, crucial details that were previously unknown to the police.
The researchers gained access to interviews conducted by the Dutch National Criminal Investigation Department and analyzed five serious cases, including a potential murder case and police-involved shooting incidents. In total, they coded fifteen interviews, where witnesses were first questioned individually and then together. The results were striking: during the joint interviews, an average of 131 new details emerged.
“Witnesses actively helped each other by repeating, elaborating on, or confirming one another’s statements,” Van Rosmalen explains. These processes—known as ‘retrieval strategies’ that focus on the content of the statement—stimulate the joint reconstruction of events and proved to be highly effective. In one specific example from the study, one witness described the victim as “stiff,” after which the other added that the body was cold—both new and crucial details for the investigation.
Should all witnesses be interviewed together from now on? Not necessarily, Van Rosmalen emphasizes. It remains important to first interview witnesses individually. “But when an investigation is stalling, this method can help uncover additional details and correct faulty memories. It’s a powerful and valuable tool for criminal investigations.”
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The research was recently published in Applied Police Briefings, a digital journal that informs police professionals about relevant academic findings and their practical applications. The field study had already appeared earlier this year in the scientific journal Journal of Criminal Psychology, in a special issue on contemporary developments in obtaining and evaluating witness statements.