Is this true? Since you are talking of something that needs a study or research, do you have a source for this, where it is explained that there is indeed a psychological aspect to over or under betting? Or do you at least have data that back this observation?

In my experience it is dependent on the specific game. If the game is between two teams that are high scorers, I will probably be betting on the over. But if the game is between low scoring teams I might be betting on the under.