Final answer:
The concept that best explains why prejudices can be observed non-verbally even among those trying to suppress them is confirmation bias, which highlights the tendency to focus on confirming evidence for pre-existing beliefs.
Step-by-step explanation:
The reason that a person's prejudices tend to leak through the non-verbal chain of communication in interracial interactions is most consistent with the concept of confirmation bias. This phenomenon occurs when individuals pay more attention to information that confirms their pre-existing stereotypes and beliefs, and disregard information that contradicts them. When interacting with someone from a racial out-group, individuals might unconsciously seek out behaviors that confirm their prejudicial beliefs, while ignoring behaviors that do not. This effect can be potent even among individuals who consciously attempt to control their expressions of prejudice, due to deep-seated implicit attitudes that can be difficult to override in the moment.