Behavioural psychologists use the concept of “mental accounting” to refer to violations of the fungibility principle in thinking about money. We tend to put different value on money depending on how we label it — “savings” is much more important to us and we resist spending it, compared with, say, getting a tax refund, which is easy to blow. So we do stupid things such as run up credit card debt while putting money into savings accounts at the same time.

Money, however, is fungible. A rand is a rand no matter where it comes from. This is deeply embedded in the national budget framework. All money goes into the central revenue fund. Ring-fencing for specific purposes, depending on the source of the cash, is forbidden by the constitution. The decision on how to spend revenue is separate and unconnected to its source. But this makes it hard to clarify the consequences of choosing one spending priority over another. Our cognitive machinery is not wired to see the things that are n...

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