We often hear that money can't buy you happiness.

Bo Derek famously once said, "Whoever said money can't buy happiness simply didn't know where to go shopping."

Research in the US, carried out by Princeton University's Woodrow Wilson School, has found money can indeed buy you happiness - but it has to be the right amount.

The optimum number, according to the study, is $75,000 a year. In British terms, this means a salary of £62,000 a year.

The researchers found people earning less than this magic number felt unhappier, and those earning more failed to feel any happier.