Keller created the Personalized System of Instruction (PSI) in the late 1960s in order to help students in Brazil be able to learn course material without an instructor standing by their side. Soon after, he brought his Personalized System of Instruction program back to the United States. Due to its heavy reliance on behavioral principles, it was quickly adopted by many psychology professors and by individuals outside of psychology. Keller (1968) outlined five basic components that he deemed to be essential for a PSI class: (1) mastery of course material, (2) the use of proctors, (3) self-pacing, (4) stress upon the written word, and (5) use of lectures and demonstrations primarily for motivational purposes.