perpetrator and the defendant are indeed the same per-
son. 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 mem-
ory 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 representa-
tion of the perpetrator’s face, the length of the reten-
tion 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 inter-
ested 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 inter-
vals. 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 know-
ing 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 andthen levels off over time. It turns out that when theoret-
ical 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 mathemati-
cally quite similar to that of Ebbinghaus.
Given that there are theoretical forgetting functions
that make relatively accurate predictions regarding the
memory accuracy of the typical witness in a laboratory
or field experiment, memory accuracy at any particular
retention interval, it should not be surprising that one
can “work backward” from the earliest tested retention
interval to make a prediction as to what the original
strength of the witness’s memory representation was.
When this estimate is translated into a proportion cor-
rect measure of accuracy, one then has a reasonable
estimate as to the maximum level of accuracy expected
for the typical witness under the conditions prevailing
in the experiment—or in more realistic situations, to the
extent that conditions are the same as in the experiment
in question. The expectation is that memory accuracy
will only decline from this level at forensically typical
retention intervals. Interestingly, the retention interval
most frequently encountered by the British police has
been reported as 1 month. One theoretical forgetting
function that fits empirical data well makes the predic-
tion that the strength of the memory trace for an unfa-
miliar face at a 1-month retention interval, depending
on a number of factors, would likely be in the range of
40% to 60% of its original memory strength.
Researchers have identified a number of factors
that affect initial memory strength and, therefore, the
amount of strength remaining after any retention
interval. Longer exposures to an unfamiliar face, bet-
ter lighting, and greater facial distinctiveness (as com-
pared with the typical face) have all been shown to
increase initial memory strength. Estimates of witness
memory accuracy when tested with a lineup or photo
spread have also been shown to be a function of how
distinctive (or similar) the unfamiliar target face is rel-
ative to the other faces presented. A high degree of
similarity will produce a lower estimate. Events
occurring during the retention interval can also seri-
ously affect witness memory accuracy. For instance,
exposure to mug shots before the ultimate memory
test, followed by a memory test that includes one of
the faces from the mug shots, increases the probabil-
ity of erroneously selecting the face seen in the mug
shots rather than at the crime scene.Kenneth Allan Deffenbacher694 ———Retention Interval and Eyewitness MemoryR-Cutler (Encyc)-45463.qxd 11/18/2007 12:43 PM Page 694