Sometimes psychology findings do come across as being obvious, or common sense. People read about a recent study and say to themselves, "I already knew that!" The problem is that many things that seem intuitive and obvious aren't actually true, and many things that are not actually obvious at all can seem obvious in retrospect (we call this the I-knew-it-all-along fallacy).

Just to demonstrate, is it obvious that "birds of a feather flock together"? But what about the saying that opposites attract? Does absence really make the heart grow fonder, or are people "out of sight, out of mind"? Contradicting conclusions can be supported by common sense.

The consequence of this is that when scientists run studies to conclusively demonstrate a particular psychological phenomenon - say, that we prefer to date others who are quite different from ourselves - people may claim that it's an obvious, common sense finding. But often, the fact is that these same people would believe the opposite argument equally easily. Indeed, I used this example because the reverse is actually true here - people generally prefer to date others who are quite similar to themselves.

Intuition can be faulty, which is why we need data to parse out what's true and what's not.