Political correctness in American society takes it's toll on cognitive ability. It appears that white Americans, when placed in a situation where they are interacting with blacks, spend so much of their cognitive capacity ensuring that they show no racial bias that it impedes their ability to do other cognitive tasks.

Understandably the more bias you actually have the more effort it takes to manage the interaction with a black individual. A lengthy period of interaction leaves these people mentally depleted and not much good for anything else, thinking wise, for quite some time.

The research was done by Assistant Professor of Psychological and Brain Sciences, Jennifer Richeson, at Dartmount College. You can read more about the research here.