They were then given a choice of carrying out a relatively easy task – tapping the spacebar on a computer keyboard 30 times in seven seconds – or a hard task – the same thing, but 100 taps in 21 seconds.

The easy task paid out 50p, while the hard one resulted in a payment of between 80p and £2.

One of the researchers, Professor Val Curran, of University College London, said: “Repeatedly pressing keys with a single finger isn’t difficult, but it takes a reasonable amount of effort, making it a useful test of motivation.

“We found that people on cannabis were significantly less likely to choose the high-effort option.