Here we can see a traditional photographic line-up. However, there are a few key aspects
of the set up here that psychological research has identified as being problematic. Before we talk
about those issues, I’m going to ask you to imagine that you’re the eyewitness to
the crime that we’ve shown in our drama. Imagine you’ve been called to the line-up,
and we’ll see if you can pick out the perpetrator of the crime. So which of these individuals
committed the crime? Maybe pause the video at this point so you can decide on your answer.
Start the video again when you’ve decided. So, we’ll come back to whether you got the
correct answer later.
Let’s look at how this line up was carried out and what psychological research has identified
as being problematic with this way of doing things. The first issue is that the administrator
knew, in advance, who the suspect was. This is known as a single-blind procedure, where
only the eyewitness doesn’t know in advance who the suspect is.
The problem with this is that, even with the best intentions in the world, it is possible
that the administrator might accidentally provide cues to the eyewitness as to which
person is the suspect. That is, the eyewitness might end up choosing the suspect not because
they saw them committing the crime, where of course the suspect might be innocent, but
because they’ve been subtly “tipped off” by the administrator as to who to pick.
The administrator isn’t necessarily doing this deliberately – the cues they’re providing
may be involuntary.
Sarah Greathouse and Margaret Kovera of the
City University of New York found that situations, in which the administrator knew who the suspect
was, did indeed lead to increased false identifications of the suspect as being the criminal. So – what’s
the solution?
The solution is to employ what’s known as a double-blind procedure – where you ensure
that the administrator doesn’t know who the suspect is. That is, you essentially treat
the line-up as if you were running a behavioural experiment. Sarah Greathouse and Margaret
Kovera found that what they called the “diagnosticity” of the eyewitness, essentially a measure of
how accurate their overall judgement was, doubled when the administrator didn’t know
who the suspect was – when the double blind procedure was used. That is, we’ve got evidence
that the double-blind procedure leads to better outcomes.
The next issue that I’m going to talk about is related to the first. Greathouse and Kovera
found that the administrators who knew who the suspect was, also performed a higher proportion
of what they called biasing behaviours. Biasing behaviours refer to anything the administrator
does that might prejudice the eyewitness towards identifying the suspect – remembering that
the suspect may, of course, be innocent. However, even when the administrator doesn’t know
who the suspect is, bias can be introduced through the instructions that are given to
the eyewitness. For example, it’s likely that the eyewitness will assume, given the
police have bothered with a line-up in the first place, that there’s a good chance
that the perpetrator is present. That is, their judgement will be coloured by the expectation
that the criminal is somewhere in the line-up and their job is to choose the person in the
line-up who most closely resembles the criminal.
In fact, Lorraine Hope of the University of Portsmouth and her colleagues report evidence that 90% of eyewitnesses
in a simulated line-up situation expected the suspect to be in the line-up. So – how
should we deal with this? Fortunately this is one problem for which there appears to
be an easy solution. It turns out that simply instructing the witness that the person they
saw committing the crime may or may not be present in the line-up is sufficient to undermine
this expectation and substantially reduce the chance of a false identification.
In a classic study by Roy Malpass and Patricia Devine of the State University of New York,
they compared the effect of biased and unbiased instructions to eyewitnesses on the percentage
of judgement errors in a line-up situation. So, the unbiased instructions included reminding
people that the person they saw committing the crime may or may not be in the line-up.
They found that when the suspect was in the line-up, it didn’t seem to matter whether
biased or unbiased instructions were given. Eyewitnesses were pretty similar in their
chance of correctly ID’ing the criminal. However, when the criminal was not in the
line-up, an innocent person was identified as the criminal nearly 80% of the time. This
compares with only 33% of eyewitnesses who had received the unbiased instructions.
This finding has been confirmed by plenty of other studies since then, although I should
point out a point of contention here was raised by Steven Clark of the University of California,
Riverside. He argued that, based on a meta-analysis, there was evidence that biased instructions
might increase correct ID’s when a perpetrator is in the line-up.
One way of conceptualising what Clark is arguing is by showing the outcomes in a 2x2 contingency
table. Here we can see the four possible outcomes of a line-up. Two are good. There’s a correct
ID. So the criminal is in the line-up and the eyewitness correctly identifies them as
the criminal. There’s a correct lack of ID, where the suspect in the line-up is innocent
and the eyewitness correctly does not indicate that they are the criminal.
The other two options are bad. There’s a false ID, where the suspect in the line-up
is innocent but the eyewitness incorrectly identifies them as the criminal. Finally,
there’s an incorrect failure to ID, where the criminal is in the line-up but the eyewitness
fails to identify them.
So, biased instructions, where the eyewitness believes the perpetrator is in the line-up
have the effect of increasing false ID’s. Meaning that we increase the chance of an
innocent person being identified as the criminal. That is, we’re moving judgements from this
box to this box.
However, Clark argues that, in principle, biased instructions would also have the effect
of increasing the chance of a correct ID if the criminal is actually in the line-up.
We’re essentially moving judgements from this box to this box. To put it another way,
the instructions are affecting what we call the response bias of the eyewitness, making
them more likely to identify the suspect as the criminal. That is, all judgements are
being moved from the bottom row to the top row.
This is good if the suspect really is the criminal but bad if the suspect is actually
innocent. Steven Clark argued that his analyses, in contrast to previous analyses, confirmed
this theory. That is, biased instructions can be both good and bad.
So, does this mean it’s okay to use biased instructions under the right circumstances?
Well – no.
The problem, of course, is that in real life you could never be sure that the perpetrator
was actually in the line-up. So by using biased instructions you’re always risking sending
an innocent person to prison.
So, even Steven Clark’s conclusion is that police should still always use unbiased instructions
despite the understandable temptation to do otherwise if the officers involved are convinced
they have the right person. So what we have here is some disagreement
between researchers as to the underlying mechanisms, which is valuable in understanding what is
actually going on, but the end result is that they still agree on what best practice ought
to be.
In the next presentation we’ll turn our attention now to how the members of the line-up
are presented to the eyewitness.