Conversation so far:
What did Michael Shermer earn his PhD in?
Michael Shermer lost interest in psychology and switched to studying the history of science.
Where did Michael Shermer earn his PhD?
Michael Shermer lost interest in psychology and switched to studying the history of science, earning his PhD at Claremont Graduate University.
In what year did Michael Shermer earn his PhD?
Michael Shermer lost interest in psychology and switched to studying the history of science, earning his PhD at Claremont Graduate University in 1991.
Where did Michael Shermer begin teaching?
Michael Shermer then became an adjunct professor at Occidental College, California.

Question: What did he teach?
Self-contained rewrite: What did Michael Shermer teach?