Demon possession is confirmed by Jesus. He never said "Oh, that's not demon possession that's a psychiatric disorder!" Jesus did know about lunacy and disease:
MATTHEW 4 . KJV . look up
23 And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people.
24 And his fame went throughout all Syria: and they brought unto him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatick, and those that had the palsy; and he healed them.
Many behaviours resulting from possession by devils are described throughout history. They are still the same today. If there were no demon possession, how could Jesus cast out the demons?