In American high schools, black students typically perform worse than their white peers, which can damage their self-esteem and their future prospects. Studies have found that the fear of living up to this underachieving stereotype can cause so much stress that a child’s performance suffers. Their teachers may even write them off as lost causes, and spend less time on them.

With so many students caught in this vicious cycle, where the stereotype of poor performance strengthens itself, it might seem absurd to suggest that you could turn things round in less than an hour. But try telling that to Geoffrey Cohen from the University of Colorado.

In 2007, he showed that a simple 15-minute writing exercise at the start