Historians are usually quite reluctant to come out with such grand propositions.
That is one reason why the assertion may at first sight come as a surprise. The second reason for surprise is of a more sociological nature. Even though no extended period of world history is without some important changes, the second half of the twentieth century could be considered one of unusual institutional stability. In global terms, the most significant institutional transformation was certainly de-colonization. But if the focus is on Western societies, all that seemed to happen was gradual change without major ruptures or unpredicted events.