The usual second part of that phrase is that economics shows us what we don't know. But in this case it is in fact to tell us what we do know:
An infant school has launched a new crackdown on parents who pick up their children late from school – by fining them £6 if they turn up more than 15 minutes late. Germander Park School in Milton Keynes wants to discourage parents from treating it as a 'childcare provider' by charging whenever it has to look after pupils outside school hours.
Ah, no, that won't work. On the basis that humans are contrary beings:
In an example made famous by Freakonomics, when parents at an Israeli kindergarten were fined a small amount for showing up late to collect their children, their punctuality actually declined.
Because if we're paying for baby sitting anyway then why not take advantage of the babysitting?
Or, an alternative explanation was that the social pressures of not imposing upon the staff were stronger than the monetary fine. Once the fine was introduced those social pressures lost much of their force.
However you want to explain the finding is one thing: that the result is there is another. Charging parents for being late is likely to make more parents late: and also to make those parents who are late later. For once you've incurred the fine why hurry?
Then again perhaps we shouldn't be all that surprised by this. No one has ever found evidence that the British school system is over burdened with those who understand economics.