Children make better progress if they are grouped in classroom ‘sets’ according to ability. Taking juvenile offenders to visit adult prisons helps to ‘scare them straight’. The best way to fix torn knee cartilage is with keyhole surgery.

You may think that all these statements are logical, perhaps even obvious. But when researchers conducted rigorous studies, they actually found the opposite. Setting students has negligible overall effect, visiting prisons increases the risk of future offending and physiotherapy is often as good as surgery for common knee injuries.

If you find these results jarring, then you may be experiencing the ‘Semmelweis reflex’. Named after Ignaz Semmelweis – whose 1847 discovery that handwashing could reduce deaths during childbirth was roundly rejected – this is the tendency to dismiss findings that contradict our pre-existing beliefs. This phenomenon, and what to