The sudden and uncontrollable paedophilia exhibited by a 40-year-old man was caused by an egg-sized brain tumour, his doctors have told a scientific conference. And once the tumour had been removed, his sex-obsession disappeared.

The cancer was located in the right lobe of the orbifrontal cortex, which is known to be tied to judgment, impulse control and social behaviour. But neurologists Russell Swerdlow and Jeffrey Burns, of the University of Virginia at Charlottesville, believe it is the first reported case linking damage to the region with paedophilia.

“We’re dealing with the neurology of morality here,” says Swerdlow. Since the area does not affect physical health, “it’s one of those areas where you could have a lot of damage and a doctor would never suspect something’s wrong,” he says.

“He wasn’t faking,” says Burns. “But if someone argues that every paedophile needs a MRI, the difference in this case was that the patient had a normal history before he acquired the problem. Most paedophiles develop problems early on in life.”