By Carey Marr

Novel, uncontrollable, and unpredictable – the key elements of a recipe for stress. It’s therefore perhaps unsurprising that many people experience stress while witnessing a crime. Though it may be an unfamiliar situation to you, being an eyewitness is not actually as infrequent as you may think. In fact, one past survey showed that nearly a third of adults reported witnessing a crime within the past year. Reports provided by eyewitnesses can be critical for police investigations, particularly when other evidence is limited. But how might the stress that people feel during the crime affect their memories?

Investigating the effects of acute encoding stress (i.e., stress experienced during an event) on memory performance is not a new question. Researchers have been interested in this idea for decades, though experimental results paint a complex picture of mixed findings. Experts in this research area also show a lack of consensus on the answer. In a recent survey of eyewitness memory experts and fundamental memory experts, the majority of fundamental experts agreed that experiences encoding stress enhances memory performance. However, most of the eyewitness memory experts did not agree with this statement.

This lack of consensus may stem from the fact that many factors are at play when considering how encoding stress affects memory. For example, how severe is the stress? In more specific statements, most experts in this same survey expressed the belief that eyewitnesses who experience moderate encoding stress will display better memory than those who experience low levels of stress. At the same time, many experts agreed that severe stress—but not moderate levels of stress—may harm eyewitness memory. Empirically examining mild to moderate levels of stress is possible in the laboratory using validated stressors such as the Maastricht Acute Stress Test, but researchers cannot ethically induce severe stress. Thus, investigating severe stress can only take place in unique settings, such as field studies observing military trainees or firefighters.

Other important factors to consider stem from our biology, including the complex timeline of our physiological stress response. Our brains help us deal with acute stress in several stages, including an initial and immediate release of adrenaline a more delayed release of cortisol, our main stress hormone. Differences in timing of stress can therefore mean different impacts of acute stress on our memory. Other biological differences—such as differences in sex hormones or female menstruation cycles—can also affect the way that stress affects our memory.

And what kind of memory do we mean? Researchers often use simple stimuli such as words. However, in real life, police are interested in different kinds of memories – such as what the perpetrator looked like. The type of detail might matter, with research also suggesting that though memory for central details may be enhanced by stressed, memory for peripheral details may be harmed.

There are many other factors that could also play a role in this complex relationship. Though many conflicting findings have been found in past research, newer research experiments combining the methodological strengths of the eyewitness and fundamental memory fields show a lack of a reliable effect of encoding stress on memory performance, particularly for person recognition. Thus, at the moment, more research is needed before we can generalize any results and confidently state how encoding stress affects eyewitness memory performance.

However, stress experienced during a crime is not the only stressful part of this process: the police interview can also be a stressful experience for many eyewitnesses. For many people, this unfamiliar experience holds many of the same elements that evoke a stress response: novel, uncontrollable, and unpredictable. Research findings and expert consensus is more consistent in this area, with the majority of studies suggesting that retrieval stress impairs memory performance.

Important to note, however, is that past research has been conducted in the fundamental memory field. As such, there is limited applied work investigating how retrieval stress affects eyewitness memory. One recent study examining recognition and recall memory for details of a complex scene found no effect of retrieval stress on memory performance, suggesting that negative effects of retrieval stress on memory do not always emerge.

Regardless of how retrieval stress affects memory performance, police and other legal practitioners should do what they can to reduce an eyewitness’ stress level. Beyond potentially helping their memory, reducing stress level where possible is important for improving eyewitness wellbeing, particularly for those who have already been through the stressful experience of witnessing a crime. This can potentially be done by reducing the feelings of uncontrollability and unpredictability of the police interview. For example, eyewitnesses could be provided with an overview of how the interview process will work and could be given control of the conversation that they have with the police.

Much future research is still needed in this area and a recent review highlights some suggestions for the way forward. Combining the applied strengths of the eyewitness research field and the methodological strengths of the fundamental research fields and using open science practices will yield a more reliable base of research that can be used to better inform police and legal practitioners. Further researching how to mitigate any negative effects of acute stress on memory will also be valuable, ideally boosting both memory performance and eyewitness wellbeing.