What goes on inside the mind of a paedophile? What causes a person to become sexually attracted to children, and what then possesses him or her to act on those feelings?
These questions constitute a hole in psychologists' understanding of paedophilia, a sexual preference for children and that has limited their ability to assess and manage the disorder.
Now, a team of Canadian researchers is launching a study to mine for answers, but their controversial methodology has rankled with victims' rights advocates.
The study, to be conducted by the University of Toronto and University of Lethbridge, plans to pay convicted child molesters and porn offenders C$60 (HK$424) each to look at images of fully clothed children and discuss their responses.
Academics involved say the study will elicit invaluable information about what they view is a serious mental health problem, while critics argue it rewards criminals for thinking and potentially behaving in the very manner that got them into trouble with the law in the first place.
Although there is general consensus that paedophilia cannot be cured, the debate over the new study has also drawn lines between those who believe offenders can be rehabilitated and those who think they should be incarcerated for life.
Dennis Fitzpatrick, vice-president of research at the University of Lethbridge, said investigators in the study were being trained to interview about 250 offenders who have been released from prison. The study is expected to take two years.