What Affects Our Memories

When a crime is investigated, witness statements are often an important part of the evidence. Witnesses are usually asked to carry out two types of tasks: recounting a sequence of events and identifying a person. Describing what happened is the most common form of testimony.

Even though witness statements can be very valuable, human memory is not an exact recording of reality. Memory is affected by a range of different factors, both within the individual and in the situation in which the event occurs. Because of this, two people who witnessed the same event can give different accounts without either of them deliberately providing false information.

Witness-specific factors

Several individual factors affect how well a witness perceives, stores, and later recalls an event. Examples of such factors are:

• age
• occupation
• previous experiences
• expectations and prejudices
• attitudes
• sensory limitations, for example impaired vision or hearing

All people interpret their surroundings based on their previous experiences. We do not only register what happens around us; we also interpret the information. Our memories are therefore always subjective to some degree.

Prejudices and attitudes shape experiences and memories

The psychologists Gordon Allport and Leo Postman carried out a classic experiment showing how preconceived notions can affect people's memories.

Participants heard a story about an event on the London Underground involving a Black man and a white man. During the story, it was mentioned that a knife was drawn, but it was never made clear who was holding the knife.

When participants later retold the story, 12 of the 16 participants did not mention who was holding the knife. Four people, equivalent to 25%, instead retold the story as if it was the Black man who had drawn the knife, even though this had never been stated.

The experiment shows how people's prejudices and prior assumptions can affect how an event is stored in memory and later recalled.

Another classic experiment was carried out by the psychologists Albert Hastorf and Hadley Cantril. Students from Princeton University and Dartmouth College watched a film of an American football game between the two universities. The game included many rule violations, and a player was injured during play.

After the film, the students were asked to estimate how many rule violations each team had committed. Even though all participants had seen exactly the same film, the results differed clearly. Students from Princeton judged that Dartmouth had committed an average of 9.8 rule violations, while Dartmouth students estimated that their own team had committed only 4.3 rule violations.

The experiment shows that people's loyalty, group membership, and expectations can affect how the same event is perceived and interpreted.

Situational factors

In addition to individual factors, eyewitness memory is also affected by the circumstances that existed when the event took place.

Examples of such situational factors are:

• the offender being masked
• the event being very brief
• poor lighting conditions
• fog or heavy rain
• a large distance between the witness and the event
• high stress or chaos at the crime scene

The poorer the conditions for observing an event, the greater the risk that eyewitness memory will be incomplete or contain inaccuracies.