As if Western politics were not volatile enough, a wave of recent elections seemed to offer contradictory evidence as to whether populism is advancing or receding.

It triumphed in the British vote to leave the European Union and in the American presidential race, fell short in the Dutch elections, and won its greatest-ever success in France’s first presidential round and faces likely humiliation in the second round.

But these results may not be as contradictory as they seem. Populism, research suggests, has been steadily growing since the 1960s. It is now reaching a size that is often too small to win outright, but is large enough to shape and, at times, to upend the politics of a country.

Whether populist parties win or lose depends not just on the level of popular support — which appears surprisingly consistent across countries — but also on the nature of the political system.