If you saw someone punching a stranger in the street, you might think poorly of them. But if you found out that the stranger had slept with the assailant’s partner, had kicked a kitten, or was Justin Bieber, you might think differently about the situation. You might even applaud the punch-thrower.

When we make moral judgments, we do so subtly and selectively. We recognise that explicitly antisocial acts can seem appropriate in the right circumstances. We know that the enemy of our enemy can be our friend. Now, Kiley Hamlin from the University of British Columbia has shown that this capacity for finer social appraisals dates back to infancy – we develop it somewhere between our fifth and eighth months