What do Mark Wahlberg, Tilda Swinton, Lily Allen, and Bill Paxton have in common? For one thing they're all very, very famous, but so are lots of people, and you already knew that. Another thing these four individuals have in common is that they have extra nipples. A third one, to be exact, though there are humans who have had as many as six additional nipples. Eight nipples! The mind boggles.
The first formal description of the condition was made in a German scientific journal in 1878 by a researcher called Leichtenstern. He estimated that around one in 500 humans, or 0.2%, have "supernumerary nipples" – that is, a number of nipples greater than two. There are almost as many guesses as to the prevalence of extra nipples as there are studies on the matter.