I have two friends, Seth and Michael, and one of them is a lot happier than the other.

Seth is chronically unhappy. He is often glum, frequently irritable, and sometimes hopeless, though he has never been clinically depressed. By contrast, Michael is a remarkable happy person. Although he has his low moments and periodic stress, he manages to find joy in his days and is quite content with the way his life is going. To understand why these two men are so different, let me tell you a little bit about them.

Both are in their early 40s and doing well in their careers. Michael is a professor of psychology at a prestigious university, who has reasonably bright students, a fair amount of autonomy in his work, and many opportunities for travel. His research program has been successful, garnering attention from all over the U.S. Seth is a deputy city attorney in a small but beautiful city right on the Pacific Ocean. He specializes in landlord-tenant disputes and other civil matters, and his success as a litigator has led to occasional media appearances, in which he is asked to speak about his latest cases. He gets a kick out of doing that.

Both have close-knit families. Seth is married to Allison, whom he met while on sabbatical in the Netherlands, and they have 5-year old twin boys. Michael is married to Holly. They started dating in law school, and now have a boy (age 6) and a girl (age 3).

Both men own homes in the suburbs of a major metropolitan area, about half an hour from the city and their jobs.

So, why is Michael happier than Seth? Was he simply lucky to be born with a sunnier disposition? Or, is he more fortunate with regard to the events and circumstances of his life?

Knowing them, I would be hard-pressed to assert that the life situation of one is clearly superior to the other. On balance, neither seems to have the better job, wife, kids, house, or car. Furthermore, scientific research has shown that prosperity, health, and physical attractiveness are only minimally related to one’s overall happiness. For example, a study by Ed Diener from the University of Illinois demonstrated that the richest Americans – those earning more than $10 million annually – report levels of personal happiness only slightly greater than the people who work for them. So, even if Seth had fewer of life’s “goods,” this shortfall wouldn’t explain his acute unhappiness.

What about genetics? Growing research done with identical and fraternal twins suggests that each person is born with a particular “happiness set point” – that is, a baseline or potential for happiness to which he or she is bound to return, even after major setbacks or triumphs. The set point for happiness is similar to the set point for weight. Some people are blessed with a “skinny disposition.” Even when they’re not trying, they easily maintain their weight. By contrast, others have to work extraordinarily hard to keep their weight at a desirable level and the moment they slack off even a bit, the pounds creep back on. So, Michael may simply possess a higher set point for happiness, a higher potential for well-being. He doesn’t have to work hard at it – he just is happy.

So if Michael’s happiness is due to genetics, what is left for Seth to do? Are we all doomed to obey the directives of our genes?

The answer is “no.” I am an experimental social psychologist who has conducted the first controlled experimental intervention studies to increase and maintain a person’s happiness level over and above his or her set point. In broadest terms, this research suggests that sustainable happiness is attainable regardless of genetics, if one is prepared to do the work. Much like permanent weight loss and fitness, becoming lastingly happier demands making some permanent changes, requiring effort and commitment every day of one’s life.

My two colleagues – Ken Sheldon at the University of Missouri and David Schkade at UC San Diego – and I developed a theory that describes the most important factors determining happiness. (This theory lies at the heart of my book, The How of Happiness.) In sum, we argue that the set point determines just 50% of happiness, while a mere 10% can be attributed to differences in people’s life circumstances – that is, whether they are rich or poor, healthy or unhealthy, married or divorced, etc. This leaves a surprising 40% of our capacity for happiness within our power to change. This means that Seth can be a great deal happier and Michael could be even happier too.