In a recent column in Scientific American, Brad Hines summarizes the latest science of happiness. Some surprising results are reported. It seems that once individual income reaches $20,000.00, an increase in income does not add happiness. The same is true with other basic needs.  

Once a basic level of any commodity or need is reached, the ability of that product to increase happiness by its quantity alone diminishes.

To quote from Brad Hines' column:

Happiness is better equated with satisfaction than pleasure, says Emory University psychiatrist Gregory Berns in Satisfaction (Henry Holt, 2005), because the pursuit of pleasure lands us on a never-ending hedonic treadmill that paradoxically leads to misery. "Satisfaction is an emotion that captures the uniquely human need to impart meaning to one's activities," Berns concludes. "While you might find pleasure by happenstance--winning the lottery, possessing the genes for a sunny temperament, or having the luck not to live in poverty--satisfaction can arise only by the conscious decision to do something. And this makes all the difference in the world, because it is only your own actions for which you may take responsibility and credit."

For the entire article, refer to:
http://www.sciam.com/article.cfm?chanID=sa006&colID=13&articleID=C73C7109-E7F2-99DF-31EB094AF750C3C3