Behavioral Confirmation happens when your expectations about how someone will act actually cause them to act that way. Sociologists have performed a bunch of clever experiments to prove that this is a real and significant effect. Here's how the author describes one experimental setup:
"Fifty-one male and fifty-one female undergraduates at the University of Minnesota participated, for extra course credit, in what had been described as a study of the "processes by which people become acquainted with each other." These individuals interacted in male-female dyads in a getting-acquainted situation in which they could hear but not see each other( a telephone conversation). Before initiating the conversation, the male member of each dyad received a Polaroid snapshot of his female interaction partner. These photographs, which had been prepared in advance and assigned at random to dyads, identified the target as either physically attractive (attractive-target condition) or physically unattractive (unattractive-target condition). Each dyad engaged in a ten-minute unstructured telephone conversation that was tape-recorded. Each participant's voice was recorded on a separate channel of the tape. In order to assess the extent to which the actions of the female targets provided behavioral confirmation of the male perceivers' stereotypes, twelve observer-judges listened to the tape recordings of the getting-acquainted conversations. The observer-judges were unaware of the experimental hypotheses and knew nothing of the actual or perceived physical attractiveness of the individual whom they heard on the tapes. They heard only those tape tracks containing the female participants' voices. Nine other observer-judges listened to and rated only the male perceivers' voices."
Not only did the judges find that the men treated the women differently based on the mens' biases, but the women unconsciously conformed to those biases!
Analysis of the observer-judges' ratings of the tape recordings of the conversations indicated that female targets who (unbeknown to them) were perceived to be physically attractive (as a consequence of random assignment to the attractive-target experimental condition) actually came to behave in a friendly, likeable, and sociable manner. This behavioral confirmation was discernible even by outside observer-judges who knew nothing of the actual or perceived physical attractiveness of the target individuals.
In this demonstration of behavioral confirmation in social interaction, the "beautiful" people became "good" people, not because they necessarily possessed the socially valued dispositions that had been attribuvted to them, but because the actions of the perceivers, which were based on their stereotyped beliefs, had erroneously confirmed and validated these attributions.
As the author, Mark Snyder, puts it: "They treated their targets as sociable or unsociable persons, and, indeed, these targets came to behave in a sociable or unsociable fashion."