The adage that money can't buy happiness is rubbish, considering that not having enough of it certainly can make a person miserable. Just ask anybody who hasn't been able to pay their bills, or had to raid a change jar to buy milk or fuel. But statistically speaking, researchers can put numbers on how much money affects life satisfaction and emotional well-being.

1. There is an optimal income, and it's probably not as high as you might think.

Researchers analyzed more than 450,000 responses to the Gallup-Healthways Well-Being Index and looked at two components of subjective well-being. They defined emotional well-being as how often and strongly respondents felt things like joy, stress, sadness, anger and affection on a daily basis. Alternately, they characterized life evaluation as how people think about their life overall. They found that a person's income affects the two dimensions differently, with higher life evaluation correlating with higher income. And while emotional well-being also rises with income, it caps at around $75,000, after which point it doesn't rise with more money. And looking at the other side of the spectrum, low income--which can be linked to divorce, ill health and loneliness--increases emotional pain.

2. People who earn their own money are happier.

While a modest income may be all that a person needs for a feeling of emotional well-being, there's a difference between being rich, and really rich. Researchers have found that when it comes to people who are loaded, those whose wealth exceeds $8 million are happier than run-of-the-mill millionaires. And people who make their own money are more satisfied with their lives compared with those who inherited their affluence.

3. Adequate income is less important than other factors when it comes to having a successful marriage.

According to a study conducted by the Pew Research Center, only 42 percent of married adults believe that having an adequate income is "very important" to a successful marriage. Factors that ranked higher: Having shared interests (64 percent), a satisfying sexual relationship (61 percent) and sharing household chores (56 percent).

4. The "American Dream" is in decline.

Researchers from Stanford, Harvard and UC-Berkeley have crunched the numbers on the "American Dream," which they define as the notion of children achieving a higher standard of living than their parents. Sadly, they've found it's an ideal that's increasingly difficult to achieve, with 90 percent of children born in 1940 able to do it, and only 50 percent of the people born in the 1980s making more than their parents.