Obsessed as we are with the regular ones, it’s no wonder that extra nipples remain a constant source of curiosity.

But while we’re plenty entertained by simply knowing which famous celebs (Mark Wahlberg) and iconic TV characters (Chandler Bing of Friends) are carrying around a little more nippage than usual, it turns out there’s a lot more to know about them. So let’s go ahead and take a peek.

Crossing The Milk Line

Just why do extra nipples, a condition also known as polythelia, happen in the first place? As it turns out, we can blame evolution.

Sometime around the fourth week of being stuck inside our mothers’ wombs, we all start growing breast tissue. Cordoned off like a construction site, this development typically only happens along two thick symmetrical ridges of skin called the milk lines which run from just above our armpits to the very beginning of our legs. The milk lines are thickest at around week 6 and fade away by week 9, ample enough time for our breasts and accompanying nipples to reach full growth (at least until puberty for women).

But sometimes the milk lines don’t go away completely. When that happens, pockets of extra breast tissue can start growing in other areas, like a rogue train that insists on making stops to long-abandoned stations. From there, this growth can give rise to anything from a tuft of hair to a completely operational breast (lactation included) with its own glands, areola, and nipple. The system doctors use to classify these bumbling boobs of fetal development, in place since 1915, has eight different levels, depending on how far the growth progresses. Nipples are a key feature in four. And, while extra nipples can be discovered early on in childhood — provided someone doesn’t mistake them for a mole or any number of skin bumps, it usually takes puberty for a person’s extra breasts to make their presence known. The latter condition is also called polymastia.

Milk Lines The same process of fetal development that helps us grow our regular breasts can also give us extra ones. The Geneva Foundation for Medical Education and Research, CC BY-SA 2.0

Even stranger and rarer still, breast tissue can also grow in places completely off the milk line track, such as when doctors found “a mass that clinically resembles the normal female breast” on the back of an elderly man’s left thigh in 1980. Whether any close friends or lovers of his had ever discovered it beforehand and spared him any embarrassment by not mentioning it was left unsaid. Considering that the man never bothered to have it removed, it’s safe to say that he handled the news just fine.

Perhaps he felt he was better off than if he had grown a nipple on the bottom of his foot. Or five extra nipples — the current world record — just about anywhere.  

Rare But There

Like most genetic anomalies, the number of people who grow extra nipples/breasts varies across different ethnic populations. But they’re pretty rare wherever you go. According to a 2012 study, 2 to 6 percent of women and 1 to 3 percent of men develop some kind of extra breast tissue growth, with nipples more likely to show up than breasts. And fewer than 200,000 Americans are born with extra nipples annually, according to the National Institutes of Health.

While extra nipples haven’t been linked to any health problems, the same isn’t necessarily true for extra breasts. Some research has found a strong association between polymastia and other conditions involving the kidney and urinary tract, including cancer. More recently, though, other researchers have found no added health risks at all. And although fully developed extra breasts are capable of developing the same types of cancer seen in their regular counterparts, a person’s overall risk of breast cancer isn’t any higher than it would be otherwise. On the flip side, there’s no evidence that they’re a sign of added fertility or virility as ancient civilizations like the Romans and Phoenicians once thought.  

Nope, as fascinating as they are, extra nipples are nothing more than a harmless, neutral quirk of humanity.