A hungry pigeon given food at frequent but irregular intervals will develop weird rituals – tics, dances, erratic head jerks – in the hope of summoning another morsel. BF Skinner, the psychologist who first demonstrated this effect in 1947, described the birds adopting “a sort of superstition … as if there were a causal relation between its behaviour and the presentation of food, although such a relation is lacking”. It would be unkind to compare that bird-brained affliction to the behaviour of European leaders trying to make sense of Donald Trump’s erratic distribution of favours. Their diplomatic manoeuvres are more...