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Can Phallometry Differentiate Offender Groups?

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Phallometry's usefulness resides partly in its potential to identify the sexual orientations of individuals. The less well it is shown to do this, the less we can trust it in clinical decision making. 

Hall, Proctor and Nelson (1988) took a group of otherwise mentally and intellectually normal sex offender males who were in the state hospital system for assessment for their offending. On the basis of both their sexual offence history and additional interviews, the men were classified as rapists of adult women, sex offenders against children up to 12 years old, and sex offenders against young people between 13 and 16 years. About a third of the original sample was lost because they only offended against underage males or their offences came under two or more categories. On average, the men had been arrested for only about one offence. Among the offenders against children, just over a quarter had used force.

The stimulus materials included various audio tapes depicting consenting intercourse with an adult female, rape of a female child, consenting intercourse with a female child and non-sexual assault against a female child. Penis circumference was measured (using a mercury-in-rubber strain gauge) while listening to these tapes. From most arousing to least, the erotic materials were consenting child, consenting adult, child sexual assault, and child non-sexual assault.

While one might expect sex offenders against children to be aroused most by the child sexual activity, in fact the three types of offenders did not differ! Similarly, it might be assumed that the violent abusers would be more turned on by the sexually violent material. Against expectations, there were no differences between these violent and non-violent offenders in terms of how aroused they were by violent sexual materials. Interestingly, the immaturity of the victim did not seem to inhibit sexual arousal but the use of force did. There was a reasonable correlation between the paedohiles' arousal to both deviant and non-deviant sexual materials; those who showed the greatest erection to depictions of sex with children owed the highest degree of erection to adult female depictions. 

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Another study compared physiological and verbal responses to a variety of slides including boys, girls, adult males, adult females and couples. The offender groups were male and female orientated paedophiles, rapists and non-sex offenders (Wormith, 1986). Using a statistical technique that produces optimum classifications of cases on the basis of the data collected (discriminant function analysis), it is possible to compare their known offender category with that predicted on the basis of their sexual arousal. The overall accuracy of classification based on penile circumference measures of arousal was 64%. Specifically, only 50% of paedophiles were  
classified correctly whereas 42% were classified as normal. About a third of non-sexual offenders were classified as rapists! 

Self-reported arousal to the slides was also effective at separating the paedophiles from the rest, largely on the basis of arousal to slides of male children. Nearly 60% of paedophiles were correctly classified but 17% were misclassified as normals! 

Finally, a content analysis of the verbal descriptions of the slides made by the offenders proved to be the most effective at discriminating. Paedophiles made more non-evaluative comments about the slides and avoided descriptions of the sexual features of the slides. Using this sort of comment, 83% of the paedophiles were correctly assigned to the paedophile category, although a quarter of the non-sex offenders were classified as paedophiles and a third of the rapists.

While Wormith does not discuss this, it would appear that non-physiological measures of arousal were at least as effective in discriminating paedophiles from the rest. The fact that measures of penile circumference were used may explain the somewhat unsuccessful findings. 

What about the value of penis volume for diagnosing paedophilia? 

In a preliminary assessment as part of a study of this, Freund and Watson (1991) compared offenders against adult women with community volunteers (believed not to be sexual offenders) in terms of their arousal to explicit slides (sometimes accompanied by an erotic commentary) of adults and children. Few of the offenders against women were actually diagnosed as having a preference for minors on the basis of the laboratory tests (only 3.1% of cases). In contrast, nearly one in five (19.4%) of the normal men were misclassified as having an erotic preference for minors. 

This has important implications for estimates of the prevalence of paedophiliac tendencies in the general population. For Freund and Watson, these findings Imply that offenders against adult women 

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may be a better control group for studies of paedophiles than so-called "normals"! The explanation of the findings, they argue, might lie in offenders having a vested interest in giving no signs of sexual interest in children! 

In the study proper, sex offenders against female children (not adolescents) were compared with those against male minors (adolescents and younger children combined), because these were the only sizeable paedophile groups available for research. They excluded admitters of paedophiliac attraction using the two questions given on p. 107 (Freund and Blanchard, 1989), leaving just deniers in the final sample. Incestuous fathers and homosexuals were also excluded.

The presentation of the data is complex. Essentially, the authors calculated the number of men with an erotic preference for underage children. A proportion of their sample was discounted on the basis that a percentage of offenders are not paedophiles at all and would not show how the typical paedophile response to children. 

In other words, adjustment was made so that these non-paedophile molesters do not dilute estimates of the power of phallometry to identify paedophiles. Furthermore, apparently, those who were not sufficiently responsive on the phallometric apparatus and those judged to be faking were also excluded. The actual percentages based on the original (unadjusted) sample sizes who were correctly identified by phallometry as having a sexual interest in children are given in Table 4.2. Notice that the test was far better at identifying the preferences of those who admitted a sexual orientation towards children.

Table 4.2. Phallometrically correctly identified 

Offence deniers


Offence admitters


Multi-offences against girls


Against girls


Multi-offences against boys


Against boys


Single offence against girls




Single offence against boys




Multi-offences against both




Phallometry's lack of success in differentiating boy-oriented paedophiles from girl-orientated ones is confirmed by another study (Freund et al., 1991). Differences in arousal to male and female stimuli were examined in men in the following categories: 

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(1) Paedophiles preferring boys; 

(2) Paedophiles preferring girls; 

(3) Offenders against adult females; 

(4) Volunteers who preferred women; and 

(5) Volunteers who preferred men. 

Men who sexually prefer adults showed marked differences in sexual arousal to men and women, whereas paedophiles showed relatively little gender differentiation; they were aroused more equally by both sexes.

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