Are you aware of the psychological aspect of how bettors choose between over or under? For starters, over/under bets is a type of sports betting where bettors try to predict whether a combined number of score of both teams is over or under a predetermined value. It is observed that beginners tend to bet on the over side. Why? Because over means excitement.
I'm not really sure what you men by the psychological aspect on the over and under. Maybe if you are a beginner on the O/U then you might be overwhelm by it. However, if you are a experience one, then you have to look and analyze the numbers itself.
Over means the bettor is anticipating a lot of action and this is how bookmakers get to them. If you have been watching and betting for a long time however, you may be able to objective and exercise restraint and not be overly excited over a match and only bet on the under side. Anyway, under is not always the correct answer as it really depends on the match, the teams, the gameplay and many other factors but most beginners end up getting too excited and betting mostly on over without proper analysation.
And that's why as I have said, there are data to look at and see where the game might go. You have to look at the teams and how they score on the past. And not just beat base on how excited you are on the game. And there are times that it's really hard to see how the game will turn out. And even if the odds might be looking good in the beginning, you still need to really analyzed the game before betting.