Most people won’t admit it, but we size up other people's characters all the time. In fact, research suggests that it takes just 30 seconds to make up our minds about someone’s intelligence and personality (we make other assessments even faster) and that these evaluations are surprisingly accurate.
In one study, researchers showed participants short videos of different couplesinteracting, and participants were able to detect which individuals had cheated on their partners. Likewise, observers watching videos of randomly selected speed daters were able to infer participants' level of romantic interest. Even when the people being evaluated are children, observers can infer their character with a similar degree of accuracy than the children’s parents do.
So the idea that you can’t judge a book by its cover is inconsistent with the evidence: People, it seems, are fairly transparent and we can see through them pretty easily and accurately
Via The Learning Factor, Mark E. Deschaine, PhD