CEDRIC HART AND PHII WILMOT
PPG measures have been convincingly demonstrated to be most sensitive indicators
of male sexual arousal. Research with convicted sex offenders has demonstrated
that unlike normal males, many of those who have offended against children show
a sexual preference for children when assessed by PPG. In fact Murphy, Haynes,
and Worley (1991) in their review could find only one study which failed to
differentiate paederasts from non-paederasts using measures of penile
tumescence.
Given this it might be hoped that PPG would have clinical value and validity with offenders. In particular it might be hoped that PPG would contribute in a clinical setting to:
 | cutting through cognitive distortions; |
| obtaining information about sexual interests and activity; |
| establishing treatment needs; |
| establishing a baseline for the individual; |
| establishing a baseline against which the programme might be evaluated and |
| setting the treatment scene - establishing openness and honesty. |
How well these goals are achieved will depend on how well the PPG protocols used succeed in uncovering sexual preferences which are related to offending. This study looked at the ability of different PPG methods to identify aspects of sexual preference which are related to offending.
121 Sex offenders assessed at two prisons prior to their participation in the SOTP Core Programme.
| Some 39% were serving their sentence for offences against adults, |
| 29% for offences against girls under 16, |
| 26% for offences against boys under 16, and |
| 6 % for offences against both boys and girls. |
Two sets of stimuli were used. One for detecting a sexual preference for
children and the other for detecting a sexual preference for rape or violence.
The age preference material consisted of still images of unclothed figures in
non-sexual poses. The images included figures of both genders and covered about
the age 5 through to adulthood. The rape-preference material consisted of short
films depicting rape, assault, or consenting sex between a man and a woman.
The age preference material was either organised as individual images, each
exposed for 30 seconds and then followed by a detumescence period, or as bursts
of 4 images from the same age/gender category, each burst being composed of 4 x
20 second images and then being followed by a detumescence period. These two
variations were referred to as BURST-FIRE
or SINGLE SHOT stimuli.
The other variation was in the order of the two sets of stimuli: for some
subjects the rape preference stimuli were used first whereas for others it was
the age preference stimuli which were presented first.
These two variations gave four possible combinations of PPG protocol and
successive subjects were randomly allocated between the four conditions.
Responses to the age preference material were transformed to Z-scores to
remove differences in the overall level of responsiveness. The largest Z-score
for a deviant (child) category was then compared to the largest Z-score for a
non-deviant (adult) category to produce an age-preference
index.
Table 1 shows the effect of the four conditions on the magnitude of the age-preference index.
Note that an age-preference index of greater than 0 indicates a stronger response to images of children than to images of adults.
|
|
One-shot |
Burst-fire |
Rape material first |
0.09 | 0.39 |
Age material first |
0.11 | 0.50 |
Significantly stronger preferences for children were apparent using the
Burst-Fire Material.
Table 2 then shows the age-preference indices according to whether men had been
convicted for more than one offence against a child. Results are shown
separately for One-shot and Burst-Fire stimuli.
|
|
One-shot |
Bust-fire |
Offences against one or no children |
0.15 | 0.29 |
Offences against 2 or more children |
0.11 | 1.02 |
There is a statistically significant difference between the ability of the two types of stimuli to distinguish the men who had offended against more than one child. Essentially the One-Shot stimulus protocol showed little ability to distinguish these groups of men whereas the Burst-Fire protocol powerfully distinguished them.
The value of PPG is likely to depend greatly on the kinds of stimulus protocol used. The Burst- Fire protocol appears to provide a much more effective method of uncovering the sexual preferences of recidivistic paederasts.
Murphy, W.D., Haynes M.R., & Worley. P.J. (1991) Assessment of adult
sexual interest. In C.R. Hollin & K. Howells (Eds) 'Clinical approaches to
sexual offenders and their victims.' 77-92 West Sussex, Wiley.