Nancy Etcoff in Science & Spirit:
Some people are happier than others, some people seem never to be happy, and others seem glad to be unhappy. In his memoir, Surprised by Joy, C.S. Lewis contrasts his father’s sentimental, passionate family with his mother’s cheerful clan, which “had the talent for happiness in a high degree—went straight for it as experienced travelers go for the best seat in a train.” In American psychologist William James’ words, “There are men who seem to have started in life with a bottle or two of champagne inscribed to their credit, whilst others seem to have been born close to the pain-threshold, which the slightest irritants fatally send them over.” The question is: What accounts for such enduring individual differences?
One answer comes from behavioral geneticists trying to determine how much of the variance we see in happiness levels may be due to genetic differences.