Imagine you're a government minister in a developing country, with responsibility for improving the lives of the poorest 20 per cent of the population. Given a blank slate, it's not hard to make a list: get everybody a basic bank account; pay a small cash sum to the poorest households; enrol every poor child in primary school, using sticks or carrots to make sure the children show up; provide handouts of cash or food to those hit by natural disasters; provide free basic healthcare and vaccinations. Such a list is ambitious, but not because it's too expensive. The real constraint is that to implement any of these policies, you need to be able to identify your own citizens.