The pedestrian-crossing lights in New York City, like most other cities, required pedestrians to press a button if they wanted the lights to change.

Then, over the course of many years, and without giving it any publicity, the city authorities disabled most of the control buttons that once operated the lights. They had decided that computerised timers almost always worked better. However, the city government did not take the disabled buttons away. Initially, the buttons survived because of the cost of removing them. But it turned out that even inoperative buttons serve a purpose. Apparently, pedestrians who press a button are less likely to cross before the green man appears.

(a) Why do you think this is?

(b) Does this mean that so-called

‘placebo buttons’ should be used everywhere?