Interrogations-and-Confessions-Handbook

382 A Psychology of Interrogations and Confessions
more limited intellectual resources to assist them to cope with an unfamiliar
task, such as interrogation.
There appears to be a significantly negative relationship between interrogative
suggestibility and intellectual functioning, which has been demonstrated in
a number of studies with different groups of subjects. However, there is strong
evidence that the relationship between suggestibility and intelligence is significantly
affected by range effects. That is, it is only studies utilizing subjects
of average intelligence or below, or where a large range of IQ scores are used,
that significant results emerge. An IQ range of average or above appears to
have no significant correlation with suggestibility. That is, subjects with IQs
above average are no less susceptible to suggestive influences than subjects of
average IQ, but subjects with IQs well below average, such as those who are
borderline or mentally handicapped, tend to be markedly more suggestible.
Two early studies suggested that there was a relationship between intelligence
and the ability to give accurate recall. Howells (1938) found a small, but
a positive correlation (r = 0.27) between accuracy during an eyewitness experiment
and intelligence. In other words, there was a slight tendency for the more
intelligent subjects to give more accurate accounts of events.
The second study is that by Burtt (1948). He found a correlation of −0.55
between intelligence and suggestibility, which indicated that subjects of lower
intelligence tended to be more suggestible than those of higher intelligence.
In the first ever study on the GSS 1, I found that IQ, measured by the WAIS,
correlated negatively with both Yield 1 and Shift (Gudjonsson, 1983). The correlation
with Full Scale IQ was −0.55. Similar correlations were found for Verbal
(r =−0.47) and Performance (r =−0.50) IQs.
Tully and Cahill (1984) found a correlation of −0.69 between intelligence,
measured by Raven’s Coloured Matrices and the Crighton Vocabulary Test,
and suggestibility, which was measured by the GSS 1. The correlation is exceptionally
high because the authors, unwisely in my view, pooled together for
their analysis the scores of 15 normal control subjects and 30 learning disabled
subjects. Some of the subjects in the learning disabled group had IQs
of 50 or below, which is likely to have seriously skewed the distribution of
IQ scores.
In one study I analysed the relationship between suggestibility and IQ among
60 normal subjects and 100 forensic patients (Gudjonsson, 1988b). The correlations
with Full Scale IQ were −0.52 and −0.58 for the normal and forensic
patients respectively. However, in spite of the highly significant negative correlations
between IQ and suggestibility, the relationship between the two variables
was dependant upon the range of IQ scores. That is, IQs above 100 in the
two groups did not correlate significantly with suggestibility, whereas IQs below
100, as well as the entire IQ range, correlated significantly with suggestibility.
These findings have important implications for studies that have relied on
subjects whose IQs fall in the average range or above, such as college students.
In one study I looked at the types of intellectual skill that most highly correlated
with suggestibility among 60 forensic referrals (Gudjonsson, 1990b).
The subjects had all completed the WAIS-R (Wechsler, 1981). A negative correlation
of −0.44 was found between Full Scale IQ and total suggestibility on