Does money buy happiness? If so, how much money do you need to be happy? And what is happiness anyway?

Harvard Business School researchers Grant E. Donnelly and Michael Norton penned their answer in the Wall Street Journal.

Donnelly and Norton reviewed the literature and found that money contributes to happiness to meet basic needs -- but above a certain level, more money does not yield much more happiness.

What made their research unique was their survey of 4,000 millionaires who are clients of a financial institution -- such a survey had never been done on people with that exalted level of wealth. What they found is that people with a net worth of $10 million are significantly happier than those in the $1 million to $2 million range.

But not all decamillionaires are equally happy. The one factor that makes some of them happier than their equally wealthy peers comes as no surprise -- making the money themselves instead of inheriting or marrying into it.

The Harvard researchers say that their findings have an important implication for the wealthiest -- they should give it away. They argue that doing so is better for the wealthy and their heirs.

Why $50,000 May Be Just About Enough For Happiness

Researchers have found that above a certain point more money does not yield much more happiness. As Donnelly and Norton wrote, "The relationship between money and happiness has been studied for decades, and typically shows that money matters for well-being, but with diminishing returns: the difference in happiness between people with incomes of $50,000 and $75,000 is larger, for example, than between people with incomes of $75,000 and $100,000."

"The more we have of it, it seems, the more money wears off. Indeed, research by Nobel laureates Daniel Kahneman and Angus Deaton suggests that the happiness benefits of increased income diminish around $75,000--in part because increases beyond that point likely don't exert as large an impact on people's ability to live comfortably," they continued.

Except...Millionaires Are Happier Than You And Me

The Harvard researchers found a weakness in this research -- millionaires are not as well-represented in these surveys. So they and their "colleagues Tianyi Zheng at the University of Mannheim and Emily Haisley at BlackRock, surveyed a financial institution's high-net-worth customers--a sample of more than 4,000 millionaires--about their wealth and happiness."

The researchers got answers to questions about "their happiness with their life in general, and about their current net worth, which we calculated as the total value of their savings, investments and assets, minus any debt."

Using a 10 point scale of happiness, they found that respondents with more than $10 million were happier. As they wrote, respondents with "a net worth of roughly $10 million or more--reported greater happiness than those with a net worth of 'only' $1 million or $2 million. The effect is significant, but small, with the very wealthy roughly [0.25 points] happier on a 10-point scale. Additional millions are associated with additional happiness, but not in life-changing magnitude."

They also found that how you make your money makes a difference in how happy you are with it. They asked respondents to report whether their wealth was earned -- through investing, business profits, wages and bonuses --  or unearned through inheritance or marrying into wealth.

In general the researchers found that the more money both groups had, the happier they were.

One Thing Makes Some Millionaires Happier Than Others

It is better to make millions than to inherit or marry into them. "Those who earned their wealth reported significantly greater happiness than those who primarily inherited or married into it. Of course, there are likely other differences between people who earned versus inherited their wealth that may contribute to these different levels of happiness," they wrote.

Self-Made Decamillionaires Can Buy More Happiness...By Giving It Away

In an ironic twist, the researchers found that there is one thing that decamillionaires can do to get even more happiness -- give it away.

As the researchers wrote, "Andrew Carnegie came up with one solution: He donated the vast majority of his fortune to charities, foundations and universities during the last few years of his life, keeping it from his heirs in an apparent effort to lead them to useful, worthy lives. And his solution has greater wisdom as well: Because research shows that giving to others leads to greater happiness than spending on oneself, Carnegie was also employing his wealth in a manner likely to maximize his own happiness."

Bill Gates And Warren Buffett are among over 170 other millionaires and billionaires are following in Carnegie's footsteps. These folks have signed on to "The Giving Pledge, a campaign started in 2010 by Bill Gates and Warren Buffett to encourage the wealthy to contribute a majority of their wealth to philanthropic causes. Our research suggests that this strategy has benefits not only for the recipients of that charity, but for the wealthy and their heirs as well," write Donnelly and Norton.