Walk into any fast-food restaurant, and you can watch a small crowd of ordinary people doing something that is utterly irrational: eating junky, excess-weight-inviting food likely to leave them feeling bad about their bodies and open to a host of serious illnesses. We literally trade our health with self-image for a few minutes of pleasant mouth feel and belly comfort, as the latter is right here, right now, as opposed to the former, which is months, years and decades away. This foolish exchange reflects a problem in our brains that may cause more damage in our lives and in society than any other. Known as ‘temporal discounting’, it is our tendency to view small rewards available now as more desirable than even much bigger benefits down the road. Scientists think this trait may have been programmed into us by evolution at a time when the environment, with its many threats to our survival, favoured those who grabbed whatever they could whenever they could get it.