Two years ago this month, Wal-Mart CEO Mike Duke dropped a bomb on the retailing world by announcing that the company would spearhead an audacious effort to create a "sustainability index" that would reassure the environmental and social impact of every product sold in its stores. Though the move was generally praised by environmentalists, Wal-Mart had not suddenly turned green -- it turns out a vast amount of money is to be made by reducing energy and waste up and down the supply chain.

In the last fifty years, the value of internationally traded goods has increased from less than a fifth to more than half of world GDP. A couple of years ago, a shipping container followed by the BBC went twice round the world in a year, stopping at Scotland, Shanghai, Brazil and Los Angeles along the way. Whereas a century ago we might have known where, how and who produced the things we eat, wear and use, in so many instances today all we know is what we're told. And how can we be sure that what we're told can be trusted?