The Art of Manliness has a good 3-part series on the topic of sheep, sheepdogs and wolves. A quote from part one:

Last year I took a handgun class at the U.S. Shooting Academy here in Tulsa. During one of our breaks, our burly, mustached instructor shared an insight from retired Army Lt. Col. and author Dave Grossman that’s given me a lot of food for thought this past year.

According to Grossman, the human population can be divided into three groups: sheep, wolves, and sheepdogs.