The reason Frida Kahlo never smiled

The face of twentieth-century Mexican painter Frida Kahlo has become iconic. In somber self-portrait after self-portrait, she presented an unflinchingly honest depiction of imperfect beauty: bushy eyebrows that meet to form a unibrow, an unmistakable moustache on her upper lip, and a body increasingly wracked by injury and infection.

In her paintings, she exposed every inch of her body to public scrutiny. With one notable exception.