Black Americans have been found to score lower on tests of verbal aptitude when tested by white proctors than when tested by Black proctors. This best illustrates the impact of

Black Americans have been found to score lower on tests of verbal aptitude when tested by white proctors than when tested by Black proctors. This best illustrates the impact of stereotype threat.

Explanation:

Studies have shown that black students tend to perform poorly when they feel at risk of confirming negative stereotypes related to their ethnic, racial, gender, or cultural group. In other words, being afraid of performing poorly is precisely what makes them perform poorly. That is called is stereotype threat, and it is observed in cases such as the one described in the question. While being tested by white people, students are subjected to stereotype threat. They do not feel the same fear, however, when being tested by black people.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.