“ Our ancestors would not have lasted long if they had assumed that a rustle in the grass was caused by wind when there was even a small chance it was a lion. And it is worth making false-positive mistakes to get these relationships right. Kevin Foster of Harvard University and Hanna Kokko from the University of Helsinki, Finland, used mathematical modelling to show that whenever the cost of believing a superstition is less than the cost of missing a real life-or-death association, superstitious beliefs will be favoured by evolution.