So, eyewitness reports are incomplete, unreliable, partially constructed, and can be manipulated.
Can we improve them? Let’s start with something controversial: forensic hypnosis.
Hypnosis is a process in which people are guided to place themselves in an altered state
of consciousness. It’s argued that, in this altered state, individuals have heightened
focus and concentration, allowing then to potentially concentrate better on a specific
thought or memory. In this state, people also appear to be particularly open to suggestion.
The FBI, New York police, Texan police, and the Israeli police amongst others have all
used hypnosis to improve eyewitness recall. Forensic hypnosis has been reported to be
useful, especially when the witness experiences trauma from the crime they observed. To illustrate
this, let’s look at a couple of crimes where forensic hypnosis was key in catching the
suspect, as reported in an article by Marilyn Smith of the University of Toronto.
The first example is known as the Chowchilla kidnapping case. In July of 1976, twenty-six
Californian school children and their bus driver were abducted at gunpoint, herded into
vans, and driven to a quarry where they were put into a cave and the entrance sealed. Eventually,
they managed to dig their way out and escape. However, when the bus driver was questioned
by police, his recall of the events was sketchy. He had seen and tried to memorize two of the
van license plate numbers but had been too frightened to concentrate under the constant
surveillance of the kidnappers.
Under hypnosis, it was suggested to the driver that he imagine himself sitting in his favourite
chair watching the events unfold as if he was watching a documentary on TV. In this
state, he called out two license plate numbers. and one of these numbers, with the exception
of a single digit, turned out to be the licence plate of one of the vans driven by the kidnappers.
This led to their arrest after one of the biggest manhunts in Californian history. That
is, the use of forensic hypnosis was credited with capturing the criminals when all other
leads had failed.
In a second example, two girls, aged seven and fifteen, were kidnapped by a man from
the San Francisco area. He told them that the doors of his car were wired with explosives,
which he would trigger if stopped by the police. He drove them all the way to Mexico and raped
the older girl in a hotel near the border. He released them a few days later, telling
them that if they disclosed any details of events, their parents would be killed. Under
normal questioning by police, neither girl could remember much of use. The older girl
did identify the motel room and the false name the man had given at reception - but
these clues didn’t turn out to be useful. Then the police tried hypnosis on the older
girl. Under hypnosis, she was able to recall unique rust spots on the car as well as details
about things inside the car and parts of the car. For example, she remembered that the
gear lever was held in place by tissue paper and the passenger window made a noise when
it was rolled up the last few inches.
She was also able to remember a transaction at a San Diego petrol station on a hilltop
where the man had the car repaired. She remembered conversations between the man and the mechanic,
and that he had paid for the repairs using a credit card. The FBI agents managed to locate
the garage and the mechanic and they managed to identify the exact credit card transaction.
This gave them the man’s name and address and this led to his arrest at his house in
Northern California.
So, again, the use of forensic hypnosis proved absolutely crucial in capturing the suspect
in a case where all other leads had come to nothing. So, it’s not hard to see why, on
the basis of anecdotal evidence like this, some police forces have been enthusiastic
in the use of hypnosis. In the US, some detectives are even trained
to carry out the hypnosis themselves. This all sounds great, but unfortunately this is
one example of a situation in which anecdotal evidence and scientific research don’t tell
us the same story. With a few exceptions, most laboratory studies of hypnosis have not
been successful in demonstrating the effectiveness of hypnosis in improving memory recall. Some
have argued that this is because the laboratory conditions used in most scientific studies
were not realistic enough. For example, they’re weren’t realistically stressful. But even
studies that have used very realistic situations, for example where participants believed the
crime they saw was genuine, these studies found no advantage of hypnosis.
But if this is the case, why do we get all of the compelling anecdotal reports that forensic
hypnosis works? Marylin Smith raised one possibility, which is that while under hypnosis people’s
suggestibility is increased. This means people’s threshold for what they consider to a genuine
memory changes. This may indeed increase the number of correct memories recalled, but it
will also increase the number of false memories recalled. We can represent this using a contingency
table that shows the four possible outcomes in a situation when an eyewitness attempts
to recall some detail about the crime.
So, the eyewitness can correctly remember a detail about the crime – a correct hit.
They can also correctly reject that detail about the crime as being incorrect – so
that is they are sufficiently unsure about the detail to decide that it probably isn’t
correct. Both of these are good outcomes. However, things can go wrong in two ways.
First, we can have a false rejection and this is where the detail is correct but the eyewitness
rejects it as being a real memory. Then, we have false positives which is where the eyewitness
incorrectly recalls the detail as being correct when in fact it’s incorrect.
So changing the threshold for what people consider to be a correct memory shifts things
upwards from the bottom row, where the detail is rejected as being untrue, to the top row
where the detail is accepted as being true. So the argument is that hypnosis is shifting
things up regardless of whether they are true or not. So this is great if the detail happens
to be correct, but a problem if the detail happens not to be correct.
That is, overall accuracy may not be improved, even though it might appear to be if we ignore
the cases when forensic hypnosis has resulted in people reporting false memories. This is
essentially what we are doing when we refer to the anecdotal evidence of it working, where
the incidents are far more likely to become anecdotes if the hypnosis results in a positive
outcome than if it results in a false positive – which frankly don’t make such good stories.
Research by Elizabeth Loftus’s team found that you can get a similar effect to hypnosis
by just asking people to guess when they are uncertain about a memory judgment. That is,
you are effectively asking them to lower their threshold of certainty for what they report
as being a correct memory. If they’re doubtful about a memory then they should err on the
side of assuming it’s true. What they found was that people who were asked to guess were
more confident of the truth of their false guesses in a later test of their memory – even
when they were initially unsure. That is, the act of guessing had the effect
of cementing that false memory as being true.
However, this doesn’t mean we want to write off hypnosis, just that we need to be careful
in how it’s used. That is, it may be a good idea as a way of generating leads in investigations
when no other leads are forthcoming, which is exactly what we saw in those two anecdotes.
That is, we want to know about anything the eyewitness can tell us – even if they’re
uncertain about it – and we acknowledge that there’s a high chance of it being inaccurate
information. However – in terms of being evidence in its own right, we need to be suspicious
of memories generated under hypnosis, as there’s a good chance that information generated this
way won’t be true.
So, it’s still not quite what we want in terms of improving eyewitness memory. However,
another way of looking at this is to see if there’s any element of forensic hypnosis
that we can leverage to improve memory without risking lots of false information? That is,
does hypnosis give us clues that we can extract and use to create more effective memory enhancement
interventions?
For a start, is it really the case that all there is to the alleged improved memory generated
by forensic hypnosis is that all just down to threshold changes for memory acceptance,
or might there be more to it? Well, one other suggestion for how hypnosis might potentially
be aiding memory in some cases is that it is reinstating the context of the crime. In
a great experiment by Carol Krafka and Steven Penrod of the University of Wisconsin-Madison
examined whether context reinstatement without hypnosis improved eyewitness memory. An experimenter
posing as a customer turned up at a convenience store. He bought a can of drink using a ten
dollar travellers’ cheque, producing two forms of ID. He then had a brief chat with
the shop assistant then left. The first researcher gave a description of the shop assistant to
a second researcher.
Either two or twenty-four hours later, the second experimenter went and found the shop
assistant. He told him he was a law intern and said
“I am trying to identify a man who may have been in your store in the last twenty-four
hours. If so, he probably bought something small and paid for it with a ten dollar travellers’
cheque”. The researcher then produced six photos and said that the man might be one
of them. Half the time, he was not one of the six photos. Half the time he was.
They repeated this procedure for 98 shop assistants, and ended up with 85 people they were able
to use in the study. Half the time, the second experimenter tried to get participants to
reinstate the context. He asked the shop assistants to try and remember what had happened during
the transaction. He also produced one of the non-photo forms of ID used. What they found
was that the act of reinstating the context of the shop assistant’s interaction with
the customer dramatically improved the identification accuracy of the customer without increasing
the number of false identifications too much. So, finally we’ve got something – an intervention
– that we could potentially leverage to improve eyewitness memory.
Of course, what’s needed now is to find a way of turning this and other findings from
psychological research into practical techniques that can be used during police investigations.
And that’s what I’ll be talking about in the next presentation.