Ulrik Fredrik Malt, a psychiatry professor at the University of Oslo, said the 33-year-old Breivik was suffering from Asperger's syndrome, Tourette's syndrome and narcissistic personality disorder, but was likely not psychotic.
The question of Breivik's sanity is key to his ongoing trial. Though judges are certain to find him guilty, they must decide if he was criminally sane or not.
Their decision would affect whether he gets mental treatment in a secure psychiatric facility.
Asperger's is a developmental disorder on the autistic spectrum that often is characterized by a lack of empathy. Tourette's is a neurological disorder marked by tics and verbal outbursts.
Malt said Tourette's could explain why Breivik has frequently smiled inappropriately throughout the trial.
The psychiatrist left open the possibility that Breivik was suffering from paranoid psychosis but said the chances of such a condition were less than 25 percent.
Breivik, who admitted killing 77 people in a July 22nd bomb attack and shooting rampage, wants to prove his sanity because he thinks more people would give credence to his extremist ideology -- described as a crusade against multiculturalism and a pending "Muslim invasion" of Norway and Europe.
Malt's opinion goes against two evaluations carried out by two other psychiatric teams.
The first found Breivik to be psychotic and suffering from paranoid schizophrenia, whereas a second opinion found no sign of psychosis but did declare Breivik to be asocial and narcissistic.
Malt's opinion is based on his observations of Breivik during his trial, which started on April 16th, but he has not interviewed the defendant.
Although Malt's opinion would appear to have been along the lines of what Breivik is trying to prove in court, the defendant lashed out against the diagnosis.
"I want to congratulate Malt for a very good demolition of my personality," he said.
If he is found sane, he faces a 21-year jail term which could be extended indefinitely if he is still considered a threat to society. If he is found insane he could receive closed psychiatric care, possibly for life.