Sociologists examine financial collapses, such as those of the 1930s, in which rumors of insolvency, when believed by enough depositors, resulted in real bank failures. what sociological concept describes this phenomenon?

The Thomas theorem describes this phenomenon. William Isaac Thomas developed this theorem in 1928. According the theorem people make decisions based on their interpretation of the situation, whether that interpretation is correct or not. In this case the depositors believed in the rumors and that resulted in real bank failures.