Stop smoking, eat less, exercise more, pay your taxes on time. So many things governments want us to do; so hard to get us to do them.

Shaun Ley speaks to behavioural economist Richard Thaler who thinks he has the answer. It's called 'nudge' theory, but it's not just an academic idea. Britain's Prime Minister is so impressed, he's set up a whole 'nudge unit' in the heart of his government.

If you live in Britain, you may unwittingly already be part of a nudge experiment. So is the nudge guru teaching those in power how to encourage us to live better; or helping politicians to control us?