premise: Rastafari, sometimes termed Rastafarianism, is an Abrahamic religion. Classified as a new religious movement, it developed in Jamaica during the 1930s. It lacks any centralised authority and there is much heterogeneity among practitioners, who are known as Rastafari, Rastafarians, or Rastas.
hypothesis: People have been following Rastafarianism for around eight decades. 
reason: The statement People have been following Rastafarianism for around eight decade"is definitely correct because it developed in Jamaica during the 1930s and it's now the 2010's, meaning approximately eight decades have passed. 