Complex sentence: A corollary of the Pygmalion effect is the golem effect, in which low expectations lead to a decrease in performance; both effects are forms of self-fulfilling prophecy.
Split into simpler sentences: A corollary of the Pygmalion effect is the golem effect, in which low expectations lead to a decrease in performance. The Pygmalion effect and the golem effect are forms of self-fulfilling prophecy.