Studies of the neurobiology of individuals with narcissistic personality disorder have one of the same issues as genetic studies. The suggestion to an individual with this condition that their personality is abnormal is a threat to an already fragile self-esteem. This fact limits the pool of volunteers, limiting study sizes. However, there is some evidence patients with narcissistic personality disorder have deficits in the grey matter in prefrontal areas, which may lead to lack of control in emotional reactions. Functional MRI scans on individuals show less activity in the anterior insular cortex, the area of the brain where empathy arises, when asked to deal with the emotions of others. This indicates these individuals are predisposed to have a limited ability to feel, much less express, empathy.