"Brian's brain has very low levels of density in a system we call the para-limbic system," he explains.
The para-limbic system is a "behaviour circuit" of the brain, including brain regions known as the amygdala and pre-frontal cortex.
Scientists have long known that these areas are associated with the processing of emotions.
Over the past century or so, people with brain damage to these areas have been studied because their behaviour suddenly changed and became anti-social.
"Those systems, we think, didn't develop normally in Brian," says Dr Kiehl. Psychopathy seems to involve a lack of development in these regions - which may be genetically determined.