In his fascinating book on moral intuition, The Righteous Mind, psychologist Jonathan Haidt tells the fictional story of what he calls a "harmless taboo violation." A sister and brother, Julie and Mark, are traveling together. While staying at a beach cabin in France, they decide it would be a fun new experience to have sex with one another. They take strict precautions to avoid pregnancy. They make love and enjoy it. It makes them feel closer. They then decide never to do it again. They keep the incident a secret between them.
