Experts who study "positive psychology" generally agree that we inherit a genetic "set range" for positive emotions: Both lottery winners and paraplegics tend to revert to their usual level of cheeriness or dissatisfaction within a year of their life-changing events. The good news: By practicing better habits of mind—and action—you can learn to occupy the top of your personal range.

"You do your body, mind, and spirit a favor when you practice thinking 'up,'" says M.J. Ryan, author ofThis Year I Will (Broadway, 2007). Thinking in ways that are positive—cultivating optimism, gratitude, generosity, hope—activates the brain to flood the body with endorphins, the "feel-good" hormones. On the other hand, thinking negatively—with fear, anxiety, hate, jealousy—fills the body with stress hormones that depress the immune system and trigger the fight-or-flight response.

Not surprisingly, research shows that happier people tend to live longer. During a ten-year study of 1,306 older men, those identified as optimists had half the risk of coronary heart disease than more pessimistic men (Psychosomatic Medicine, 2001, vol. 63, no. 6).