Reconstructionist Judaism is a movement within Judaism that started in the 1920s. People like Mordecai Kaplan (1881-1983) believed that Judaism is like a civilization that constantly develops. The Jewish society therefore develops much like any other society. As a result, the Jewish religion needs to adapt to these changes. There are about 13 million Jews in the world, about half of them live in the United States. Of these, about three percent, or 180,000 people, see themselves as Reconstructionist.