From David Mulligan:

Re: Money CAN buy happiness, within limits: study (Sept. 8): This piece is neither new nor well researched. The study is comprised of only US citizens. The US is characterised by a high-level of inequality of income and as such has a large amount of conspicious consumption. In more unequal societies purchasing goods as status symbols is the method to achieve high social status. From high social status comes high respect and happiness. If this study was to be carried out on an international scale and in countries with much higher levels of income equality (eg. the Scandanavian trio) then I would expect to see a much flatter curve with increasing income providing much less marginal happiness. A much more full answer is to be found in the book 'The Spirit Level' which covers this in great detail and encompasses many studies. Also I'd like to say how much I like this site as it allows me to keep in touch with areas of science that I wouldn't be researching myself.