A psychiatrist who claims to have witnessed numerous cases of demonic possession throughout his career says that dabbling in evil or the occult is the most common reason why people suffer from demonic attacks.

Dr Richard Gallagher, a psychiatry professor from New York, witnessed exorcisms when some exorcists asked him to conduct the discernment process, which is the first step in finding out whether a patient is in need of exorcism.

In an interview with the Daily Mail, the psychiatrist detailed some of his encounters with victims of supposed demonic attacks.

He said that many of the victims spoke in different languages, including Chinese, Latin and ancient Greek while being possessed by ancient demonic creatures.

Giving his opinion of what the demons are, he told the Daily Mail: 'They're fallen angels. This is what I literally believe. They're extremely bright; they're much brighter than humans. They've been around for millenia – so they speak all languages,' he added.

Gallagher, a practicing Catholic, said the most common reason why people become targets of demonic attacks is because of their participation in occult activities.

'It's not random at all; there's almost always a discernible cause. The most common cause ... is someone has turned to evil or the occult. And paradoxically, it's often when they try to get away from that that the demonic world feels they have a hold on that person,' he told the Daily Mail.

The psychiatrist said the first case he encountered was a victim of oppression, which involves an evil spirit attacking an individual without taking control of their personality.

Gallagher said the victim was a Hispanic housewife who was a devout Catholic.

'She and her husband both swore that she would be lying in bed, and all of a sudden, she would have the feeling of being assaulted by evil spirits, and bruises would appear on her body – so I needed to do a medical workup,' the psychiatrist said, as reported by Daily Mail.

He conducted a psychiatric assessment and performed standard medical checks to establish whether she had a physical condition that caused clotting on her body.

Gallagher concluded that she did not have any psychiatric illness and came to believe her story, even though he had not seen many cases of oppression at the time.  He believes that the woman was attacked by evil spirits because of her holiness and her charitable works.

Gallagher says he witnesses around four cases of demonic attacks each year but he is not worried that evil spirits will eventually come after him because he says he is sustained by his own faith and people who pray for him.

Gallagher profiles some of the possession cases that have been referred to him by exorcists in his forthcoming book, 'Demonic Foes, A Psychiatrist Investigates Demonic Possession in the Modern United States.'