"...the subjects [of previous studies] are usually incarcerated and [their] information is strongly influenced by possible negative consequences from the penal or judicial systems..."
"when information is obtained from nonincarcerated offenders, the negative social climate regarding sexual assaults, as well as the confidentiality of the data, results in the offenders' reluctance to reveal the true extent of their deviant interests and behaviour."
In other words, whether incarcerated or not, the researchers assume that sex offenders will always tend to underreport their offending. But that is no more than an assumption. I suggest that, in view of the kudos attached to sexual success ("scoring") among some classes of men, and conversely the shame associated with a lack of sexual "conquests", it may well be that some sex offenders will, given the chance, exaggerate, perhaps vastly, the extent of their undetected offending.
My hypothesis receives some circumstantial backing from Abel et al.'s reported figures, for in each of their categories the median number of self-reported offences per offender is much lower than the mean, indicating that a very few individuals are responsible for a very large proportion of the self-reported offences. Here is an extract from their tables:
paraphilia no. of subjects per subject
paraphilic acts mean median
pedophilia (nonincest)
female target 224 23.2 1.4
male target 153 281.7 10.1
pedophilia (incest)
female target 159 81.3 4.4
male target 44 62.3 5.2
rape 126 7.2 0.9
On the basis of the mean figures the authors suggest that thousands upon thousands of sex crimes go undetected, when perhaps all that has happened is that a few of their subjects have engaged in some rather unsavoury bragging.