Among the many wise things said by President Abraham Lincoln is this take on happiness: A person will be just about as happy as he makes up his mind to be.
That sentiment is somewhat controversial among psychologists and other scientists. Some experts believe that long-term happiness is essentially unchangeable, thanks to one's genes and personality. (One study has pegged DNA as the source of 44% to 52% of one's happiness.) Many economists and other experts believe, like Lincoln, that people can take steps to become more -- or less -- happy.
A study published online Monday by the Proceedings of the National Academy of Sciences provides evidence that the latter group is right.
Read on to find out the characteristics of happiness, and those which make for an unhappy life.