Gretchen Rubin's The Four Tendencies begins with a question she started asking everyone she knew: "How do you respond to expectations?" The answer, it turned out, split reliably into four types — and those four types explained, with surprising clarity, why some people keep their New Year's resolutions and others abandon them by February, why some employees thrive with external deadlines and others need to understand the "why" before they'll comply, and why the same motivational approach that ene...