Final answer:
W. I. Thomas is the theorist who proposed that the interpretation of a situation by people has real consequences, a concept known as the Thomas theorem.
Step-by-step explanation:
The theorist who argued that if people define a situation as real, it is real in its consequences, was W. I. Thomas. This concept is known as the Thomas theorem, which suggests that people's behavior is influenced by their subjective perception of reality rather than by objective reality itself. For instance, the labels we attach to people can influence their behavior and self-identity, causing them to embody the characteristics associated with those labels. For example, a teenager who is repeatedly given a label-overachiever, player, bum—might live up to the term even though it initially wasn't a part of his character.