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(CNN)Happiness -- you know it when you see it, but it's hard to define.
You might call it a sense of well-being, of optimism or of meaningfulness in life, although those could also be treated as separate entities. But whatever happiness is, we know that we want it, and that is just somehow good.
We also know that we don't always have control over our happiness. Research suggests that genetics may play a big role in our normal level of subjective well-being, so some of us may start out at a disadvantage. On top of that, between unexpected tragedies and daily habitual stress, environmental factors can bring down mood and dry up our thirst for living.
Being able to manage the emotional ups and downs is important for both body and mind, said Laura Kubzansky, professor of social and behavioral sciences at Harvard School of Public Health.
"For physical health, it's not so much happiness per se, but this ability to regulate and have a sense of purpose and meaning," Kubzansky said.
Why be happy?
Many scientific studies, including some by Kubzansky, have found a connection between psychological and physical well-being.
A 2012 review of more than 200 studies found a connection between positive psychological attributes, such as happiness, optimism and life satisfaction, and a lowered risk of cardiovascular disease. Kubzansky and other Harvard School of Public Health researchers published these findings in the journal Psychological Bulletin.
It's not as simple as "you must be happy to prevent heart attacks," of course. If you have a good sense of well-being, it's easier to maintain good habits: Exercising, eating a balanced diet and getting enough sleep, researchers said. People who have an optimistic mindset may be more likely to engage in healthy behaviors because they perceive them as helpful in achieving their goals, Kubzansky said.
Lower blood pressure, normal body weight and healthier blood fat profiles were also associated with a better sense of well-being in this study.
For now these studies can only show associations; they do not provide hard evidence of cause and effect. But some researchers speculate that positive mental states do have a direct effect on the body, perhaps by reducing damaging physical processes. For instance, another of Kubzansky's studies found that optimism is associated with lower levels of inflammation.
If what you mean by happiness is specifically "enjoyment of life," there's newer evidence to support that, too. A study in the Canadian Medical Association Journal found that people ages 60 and older who said they enjoyed life less were more likely to develop disability over an eight-year period. Mobility was also related to enjoyment of life. This study does not prove that physical problems are caused by less enjoyment of life, but suggests a relationship.
Where happiness comes from: genes + environment
There is substantial evidence that genetics play a big role in happiness, according to Nancy Segal, psychologist at California State University, Fullerton, and author of "Born Together -- Reared Apart."
Research has shown that identical twins tend to have a similar level of happiness, more so than fraternal twins. And in identical twins, one twin's happiness is a better predictor of the other twin's current or future happiness than educational achievement or income, Segal said.
"If you have happy parents and happy children, I think that people usually assume it's because the children are modeling the parents," she said. "But that's not really so. You need to make the point that parents pass on both genes and environments."
What's more, there seems to be a certain level of happiness that individuals have generally, to which they usually gravitate, Segal said. That level depends on the person, and the situations he or she is in.
Even if genetics has a big influence, though, that doesn't mean anyone is biologically stuck being unhappy, she said. It might take more work if your baseline mood is low, but certain therapies have proven useful for elevating psychological well-being.
The environment is still quite important for psychological well-being, too, Kubzansky said.
"To say to someone, 'Don't worry, be happy,' is kind of not looking at the whole picture of, what are the environmental constraints on things they can do?" Kubzansky said.
Money and time
You might be thinking: "Maybe I would be happier if I had more money." There's that old cliché "money doesn't buy happiness" -- but is it true? A 2010 study in Proceedings of the National Academy of Sciences found that emotional well-being rises with income up to a point, which seems to be a household income of $75,000. Day-to-day happiness did not increase with higher incomes.
But when participants were asked about overall satisfaction with their lives, that did continue to rise in conjunction with income, even after $75,000, Princeton University researchers Daniel Kahneman and Angus Deaton found. Their results show a sharp distinction between how people see themselves in terms of happiness "today" vs. life satisfaction.
"More money does not necessarily buy more happiness, but less money is associated with emotional pain," Kahneman and Deaton wrote. "Perhaps $75,000 is a threshold beyond which further increases in income no longer improve individuals' ability to do what matters most to their emotional well-being, such as spending time with people they like, avoiding pain and disease, and enjoying leisure."
Would you be happier if you bought the car you always wanted? Several studies suggest experiences make us happier than possessions. That's partly because once you have purchased something, such as a new car, you get used to seeing it every day and the initial joy fades, experts say. But you can continue to derive happiness from memories of experiences over time.
Experiences form "powerful and important memories that I wouldn't trade for anything in the world," Thomas Gilovich, professor of psychology at Cornell University, told CNN in 2009.
But if you're in the market for a birthday present for your sweetheart, a material object can still be meaningful, becoming a keepsake with sentimental value that increases over time, Gilovich said.
Or maybe you'll be happier once you've lived longer. Research has also found that some sense of happiness may come with age.
Older adults may be able to better regulate their emotions than younger people, expose themselves to less stress and experience less negative emotion, Susan Turk Charles, a psychologist at the University of California, Irvine, told CNN in 2009. More science needs to be done on whether the diminished negative response is also associated with a feeling of happiness.
Happiness: Living in the moment
But what about right now -- what can we do to make ourselves feel more positive?
If you're seeking to increase your own sense of happiness, try mindfulness techniques. Mindfulness means being present and in the moment, and observing in a nonjudgmental way, Susan Albers, psychologist at the Cleveland Clinic, told CNN in 2010.
Mindfulness comes from Buddhism and is key to meditation in that tradition. Therapies for a wide variety of conditions, including eating disorders, depression and PTSD, incorporate mindfulness. Focusing on the here and now is a counterbalance to findings that mind-wandering is associated with unhappiness.
Activities such as keeping a gratitude diary and helping other people are also associated with feelings of well-being, Kubzansky said.
A variety of smartphone apps are also available that claim to help you monitor and enhance your moods. But don't feel you have to face emotional challenges alone; a professional therapist can help you get to where you want to be.
If a sense of well-being makes a healthier person, then policy-makers should also promote large-scale initiatives to encourage that, Kubzansky said. Creating parks to encourage exercise and insituting flexible work-family initiatives are just some of the ways that communities can become healthier as a whole.
So remember: A glass half full might be healthier than a glass half empty.