If your goal is to be happier in the year ahead, you might focus on your body rather than your mind. You can start right now by sitting up a little straighter. Then give a brief smile—even a fake one.

Predicting whether the outcome of a decision will make you happy is often impossible. Predicting a decision’s impact on you is not.

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Kaplan is the author of many popular books including the New York Times bestseller The Gratitude Diaries and her new book What Your Body Knows About Happiness. She is the former editor-in-chief of ...