To the litany of arguments against prejudice, scientists are now adding a new one: Racism can make you stupid.
That is the message of an unusual and striking new series of experiments conducted at Dartmouth College, with the help of brain-imaging equipment and a crew of undergraduate volunteers.
According to the findings, the more biased people are, the more their brain power is taxed by contact with someone of another race, as they struggle not to say or do anything offensive. The effect is so strong, the team found, that even a five-minute conversation with a black person left some of the white subjects unable to perform well on a test of cognitive ability.
"Just having a prejudice makes you stupider," said John Gabrieli, a professor of psychology at Stanford University who was not involved in the research. "It is really interesting."
Researchers cannot yet predict how racial bias as measured in the lab will translate into overt racist attitudes or actions. But the new brain-imaging work, reported in today's edition of the journal Nature Neuroscience, represents the most detailed look yet at the way racial biases function in the brain.
The work also paints a dispiriting portrait of the state of the nation's race relations, the lead researcher said, even among the well-educated, well-meaning Dartmouth undergraduates whom the scientists studied.
"I think people are getting caught in this trap where they are trying not to do the wrong thing, rather than trying to act natural," said Jennifer A. Richeson, an assistant professor of psychological and brain sciences at Dartmouth College. "Somehow we have to get past this awkward phase."
Richeson and her colleagues began by recruiting a group of white Dartmouth undergraduates and asked them to perform an "Implicit Association Test," a test that is widely used to measure unconscious racial bias. The subject is given a screen and two buttons. First, the subject is asked to push the button on the left if the word that appears on the screen is a positive word, like beauty, or a common first name for a white person, such as Nancy. Otherwise, they are instructed to push the button on the right.
After a session, the test is changed slightly, and the names given are those more common for a black person, such as Tyrone. The greater the difference between the reaction times in the two sessions, the more the person has trouble associating black names with positive concepts.
Next the team had each of the students speak briefly with a black experimenter and then perform a test of cognitive ability called the Stroop test. They showed that the higher a bias score the student had in the IAT test, the worse they did on the Stroop test after speaking with the black experimenter.
To uncover what was behind this effect, the team used a functional magnetic resonance imager, which is able to peer inside the brain and measure the level of activity in different areas.
Each student was then shown a series of photographs, some of white males and some of black males. The more biased a student was, the more the team saw a certain area of their brain activate, an area associated with "executive control," conscious efforts to direct thinking. This, Richeson said, is a sign the brain is struggling not to think inappropriate thoughts.
Based on the findings, the team suggested that when a biased person interacts with someone of another race, even briefly, it exhausts the part of the brain in charge of executive control, leaving it temporarily unable to perform as well on the Stroop test and, presumably, other tasks.
The report is the first time that researchers have shown a connection between racial bias and the parts of the brain responsible for higher functions, according to several neuroscientists who were not involved in the research.
It is part of a nascent movement to study the neurological basis of social phenomena, in particular racism. One study, by Elizabeth A. Phelps at New York University, found that biased people are more likely to have greater activity in their amygdala, a portion of the brain associated with negative emotions like fear, when shown the picture of a black person they don't know.
Another, conducted by Stanford's Gabrieli and other scientists, showed that the brains of white people process white and black faces differently from the moment they see them.
Gareth Cook can be reached at email@example.com.