Lost & Found
In this room is a seven-year-old boy. He is crying softly in the corner. As you enter the room, he looks at you, stands up, and runs to you. He embraces you and says how glad he is that you found him. He will refer to himself as your son.
If you keep him for long enough, you will believe him. If you keep him even longer, your family and friends will believe him, too. Eventually, records such as school transcripts and birth certificate and others will confirm this state of affairs even though you will continue to have no memory of how he was born.
This mass delusion can only be reversed by putting the boy back in the room in which you found him.