Imagine you’re in a relationship with someone who is usually brilliant, funny and loving. However, this same partner is sometimes the exact opposite, engaging in screaming rages that leave you crushed and bewildered. Do you stay in the relationship, or do you leave? Psychology professor Roy Baumeister was in this position when he was young. Unsure what to do, he kept track of the days spent with his partner, recording which were good, bad and neutral. A few months of data showed they enjoyed...