While we British smile by pulling our lips back and upwards and exposing our lower teeth, Americans are more likely simply to part their lips and stretch the corners of their mouths.
So distinct is the difference that the scientist behind the research was able last week to pick out Britons from Americans from close-cropped pictures of their smiles alone, with an accuracy of more than 90%.
The study by Dacher Keltner, a psychology professor at the University of California in Berkeley, near San Francisco, analysed the 43 facial muscles used by humans to charm, smirk and appease.
He found the British were also more likely to raise their cheeks when they smile, showing the crow’s feet at the corners of the eyes. This produces a more sincere, hard-to-fake smile.
But what happens when Irish eyes are smiling?