For an extensive and fascinating discussion of the use and pitfalls of rewards, see Edward Deci, Why We Do What We Do: Understanding Self-Motivation (New York: Penguin, 1996); Alfie Kohn, Punished by Rewards: The Trouble with Gold Stars, Incentive Plans, A's, Praise, and Other Bribes (New York: Houghton Mifflin, 1999); Daniel Pink, Drive: The Surprising Truth About What Motivates Us (New York: Riverhead, 2009).