Retention Interval and Eyewitness Memory

Retention interval refers to the amount of time that elapses between the end of a witness's encounter with a perpetrator and any subsequent testing of the witness's memory for that encounter. Testing of a witness's memory for a perpetrator's identity is obviously important whenever the prosecution seeks to prove that the [Page 694]perpetrator and the defendant are indeed the same person. When eyewitness testimony is provided, the trier of fact must decide whether the testimony is accurate. Unless the trier of fact believes that human memory operates with the fidelity of a video camera, he or she will need to estimate the strength of the witness's memory at the time of his or her memory being tested. To increase the precision of the estimate, the trier of fact needs three pieces of information: An estimate of the original strength of the witness's memory representation of the perpetrator's face, the length of the retention interval, and the nature of the forgetting function. The forgetting function is the curve that describes the strength of the memory trace over the course of the retention interval.

Inasmuch as the trier of fact ordinarily has access to a relatively precise measure of the length of the retention interval, with both the time of the incident in question and the time of the memory test being well established, the problematic pieces of information are an estimate of the original strength of the witness's representation of the perpetrator and knowledge of the course of the forgetting function during the retention interval. Let us first consider what is known about the nature of the forgetting function. Researchers interested in how memory for the human face is affected by the retention interval have conducted several dozen published studies wherein they have assessed memory accuracy after two or more different retention intervals. Assessments of the average effect size for the retention interval (measured in standard score units) taken across all these published studies have revealed that, statistically speaking, one can safely conclude that memory traces of human faces encountered but once previously will be weaker at longer retention intervals than at briefer ones. However, simply knowing that memory for unfamiliar faces is less accurate at longer retention intervals does not specify the time course of the forgetting function. The trier of fact would like to know just how rapidly memory strength declines for an unfamiliar face.

Published surveys of the opinions of psychologists who qualify as experts in the science underlying the psychology of testimony have shown that more than 80% of them believe that the nature of the forgetting function for the human face follows the same form as that of the forgetting function first described by the early experimental psychologist, Hermann Ebbinghaus, and reproduced in introductory psychology texts. That is, the experts believe that the forgetting curve declines rapidly right after viewing of a perpetrator's face and then levels off over time. It turns out that when theoretical forgetting functions are fit to retention interval data from studies wherein three or more retention intervals were tested, theoretical functions that fit the data very well describe a forgetting function that is mathematically quite similar to that of Ebbinghaus.

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