A popular greeting card attributes this quote to Henry David Thoreau: "Happiness is like a butterfly: the more you chase it, the more it will elude you, but if you turn your attention to other things, it will come and sit softly on your shoulder."

With all due respect to the author of Walden, that just isn't so, according to a growing number of psychologists. You can choose to be happy, they say. You can chase down that elusive butterfly and get it to sit on your shoulder. How? In part, by simply making the effort to monitor the workings of your mind.

Research has shown that your talent for happiness is, to a large degree, determined by your genes. Psychology professor David T. Lykken, author of Happiness: Its Nature and Nurture, says that "trying to be happier is like trying to be taller." We each have a "happiness set point," he argues, and move away from it only slightly.

And yet, psychologists who study happiness -- including Lykken -- believe we can pursue happiness. We can do this by thwarting negative emotions such as pessimism, resentment, and anger. And we can foster positive emotions, such as empathy, serenity, and especially gratitude.