Freakonomics guru Steven Levitt joins us to talk about an unusual experiment – getting people to agree to make major life decisions based on the toss of a coin. Is this really good social science? And what do the results tell us about decision making and happiness? And with 365 days in the year, it feels like a huge coincidence when we meet someone with the same birthday. But you only need 23 people to have a better than even chance that two will share a birthday. This counter-intuitive result is known as the birthday paradox, and the best place to look for proof is the World Cup, where 32 squads of 23 players provide an ideal data-set. Alex Bellos crunches the numbers for us. This programme was first broadcast on the BBC World Service.