In The Man with the Golden Gun, Francisco Scaramanga is a typical Bond villain, flamboyant and menacing, with a supposedly unusual feature — a third nipple. But it's actually not that unique an identifier. One in 18 men have third nipples, while one in 50 women have them — and many people may have more than three.
It may be a shock to the people who have them, though. Supernumerary nipples come in eight classifications, the most mild of which can be as inconspicuous as an odd patch of hair on the skin. Others resemble moles, because they have no areola, and so are just nubs on the skin. The most developed nipples are fully-formed, with glandular tissue and fat tissue.
Extra nipples can form anywhere one the body — in one case, a woman grew an extra nipple on the bottom of her foot — but there are some areas where nipples are much more likely to grow. Those areas explain why third nipples aren't that unusual. Raise your left arm, and trace a line from your underarm, through your nipple, to your inner thigh. That is one of your "milk lines." There's a corresponding one on your right side.
In humans, milk lines usually are nothing more than very slight thickenings of the skin. In animals, spots on these milk lines develop to form nipples. An animal can have anywhere from two to 12 nipples. For some the nipples are high up, almost under the armpit. Others have inguinal nipples, meaning nipples down near the groin. For humans, two nipples high up on the chest are the norm, but milk lines show us we always have the potential for more.
Because third nipples don't pose any real danger to their possessors they don't get a lot of attention. Still, we have some idea as to why extra nipples start growing along milk lines. Recently scientists at the Breakthrough Breast Cancer Research Centre found, and nicknamed the Scaramanga gene after the villain. It produces a protein that the researchers call NRG3 which signals the body and causes it to develop breast tissue. Release the signal in the wrong place, and there's an extra nipple. The cancer center researchers hope that understanding the Scaramanga gene and its protein might help them find out more about breast development and breast cancer. In the meantime, check your milk lines for moles. You might get a surprise.