Psychology is the science of human thought, emotion, and learning, including interpersonal relationships. Bad psychology is what happens when a fanbrat attempts to use psychological concepts in fan fiction... and fails. Badly. It is common in bad hurt/comfort fics, in stories involving rape, incest, abuse, and angst. It almost always involves pulling the canons OOC, because bad psychology inevitably involves unrealistic human behavior. Bad biology is a similar offense.