The study, published in the journal Archives of General Psychiatry, found that paedophiles' brain responses to photos of nude adults and children are different than normal people.
Analysing this, the researchers said, could not only help identify whether an individual is paedophile, but also whether his urges are hetero or homosexual -- a distinction that may call for different approaches to treatment, the Los Angeles Times reported.
In the study, the researchers used functional Magnetic Resonance Imagery (fMRI) to map distinct patterns of brain activation in 24 acknowledged paedophiles and 32 healthy male volunteers of similar age and intelligence levels.
Among the acknowledged paedophiles were 11 who were attracted to prepubescent girls and 13 who were homosexually paedophilia. The healthy volunteer group was made up of 18 men attracted to adult women and 14 men attracted to adult men.
While lying in the MRI scanner -- which detects increased blood flow to specific regions of the brain in response to stimuli -- each man was shown the same random sequence of 490 pictures.
Scattered among neutral photographs were 280 nude photos of children and adults, both male and female, shown for just a second. The patterns of activation in several regions of the brain where sexual impulses and arousal are processed were quite distinct across the four groups.
Only one group -- heterosexual paedophiles -- failed to show a significantly distinct pattern of arousal when shown pictures of adult women and female children.
But their brains responded to pictures of female children in ways that made them easily distinguishable from the heterosexual control group.
The researchers -- from Kiel and Berlin in Germany and Hvidovre, Denmark -- said this is a first-ever effort to use "neurofunctional pattern classification" to assess paedophilia.
Compared to the existing test of phallometry, the measurement of penile changes in response to various pictures, the use of fMRI seems to be slightly less sensitive -- meaning it may not detect all paedophiles.
But it appears to be much more specific than phallometry, meaning it is less likely than phallometry to identify people as paedophilic who are not.
An accurate gauge of paedophilia could be an important tool in detecting whether court-ordered treatment is working, and whether a convicted paedophile can be safely released into society.
It might also identify those who need the most intensive treatment: Among single-victim offenders, the phallometry test suggests that only about half have the sustained sexual attraction to children that classifies them as paedophilic.
