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.

One study found a prevalence of 0.22% in a Hungarian population; another estimated 0.6% of white American infants; another claimed that 1.63% of Black American infants have them. For Israeli infants, the rate was estimated to be 2.5%; for Israeli-Arabic children, 4.7%; and 5.6% among German children. Another study reported that 5% of Japanese women but only 1.6% of Japanese men have additional nipples, though most other studies find them to be more common in men than in women. Some evidence has suggested that they more commonly erupt on the left side than the right.

In other words, while the estimated prevalence varies wildly according to ethnicity, sex, geographic region, or (most likely) detection methods, extra nipples probably occur fairly rarely. Indeed, the National Insitute of Health’s Office of Rare Diseases considers supernumerary nipples a "rare disease", by which it means fewer than 200,000 people within the United States have them.

One early voice in the drive to scientifically understand "supernumerary nipples" was a researcher named Kajava. In 1915, based on a study of Finnish people, he classified supernumerary nipples into eight discrete categories, according to the types of tissue present. In "polymastia," or category one, the extra nipple comes with an areola as well as underlying breast tissue. In essence, it's a complete additional breast.

Embryonic development

Other categories describe areolas with breast tissue but without a nipple (category three), for example, or an areola by itself sans nipple and breast tissue (category seven). In one category, a nipple and areola are present but fat tissue replaces the typical glandular breast tissue (category five).

The most common type is category six, in which an extra nipple forms, but without an areola. It's more specifically known as "polythelia". Despite being more than a century old, Kajava's classification scheme is still in use today by doctors and researchers.

To understand how, where, and why extra nipples might be found on a human, it helps to understand where the more typical pair comes from in the first place. After a sperm attaches itself to an egg, the pair combine to form an embryo. (Starting after the eighth week after fertilisation, human embryos are referred to as fetuses.) Sometime around the fourth week of embryonic development, two strips of the ectoderm (part of which will eventually become skin) become slightly thicker. These strips, known as milk lines or mammary lines, stretch from the armpits down the chest and stomach and towards the upper thighs near the groin. In the following weeks and months, the strips become even thicker, and become known as mammary ridges. Eventually, the mammary ridges regress except for on the chest area, where the breasts and nipples continue to grow and develop.

But sometimes the mammary ridges don’t disappear completely. When that happens, a supernumerary nipple might form, whether as a complete extra breast as in Kajava's category one, or just a nipple, as in category six. Most are located below the regular nipples, but according to one estimate, as many as 13% are found above the regular nipples, such as on the armpits. "Such an occurrence can cause discomfort after childbearing when it lactates in unison with its two sanctioned colleagues," writes Dr Norman A Grossl in the Southern Medical Journal.

The knowledge of extra nipples long predates the advent of modern medicine. The ancient Greeks sometimes depicted their goddess Artemis with multiple breasts, and followers of her Phoenician counterpart, Astarte, believed that extra breasts and nipples were indicators of abundant fertility. Men with extra nipples, it was once believed, were more virile than their double-nippled brethren. While it is now known that additional nipples are not associated with enhanced fecundity, the idea was still taken seriously by medical practitioners as recently as the 1800s.

Unexpected places

Charles Darwin himself wrote about the presence of additional breasts or nipples in The Descent of Man, speculating that extra nipples were atavisms, or remnants of our evolutionary past that sneak in to the final human form during the development process. It's not a crazy idea: as an embryo develops it resembles our more fish-like and reptilian ancestors before becoming more obviously mammalian, and finally a primate. Many other mammals have more than two breasts along their milk lines – cows have four, dogs have eight to 10, rats have 12, and pigs have a whopping 18 – so he reasoned that when additional nipples appeared among humans, they were ancestral traits and, for some reason or another, were retained as the fetus developed. But because extra nipples can also occur beyond the milk lines, we now know that most are simply flukes of embryonic development.

If extra nipples are rare, extra nipples that fall outside the milk lines (they're called ectopic supernumerary nipples) are even rarer, but when they do occur, they can be found on the back, shoulder, limbs, neck, face, and even on the vulva (for women) or the perineum. Some have theorised that ectopic nipples are modified sweat glands, or are evidence that the mammary ridges became somehow displayed during embryonic development and wound up somewhere unexpected, where they went on to develop nipples, as is their tendency.

In most cases, extra nipples are hardly even noticeable, or may be mistaken for moles or birthmarks. But if there is glandular tissue – the same sort of breast tissue that comes with normal nipples – then they can suffer the same sorts of hormonal changes and diseases that can afflict normal breast tissue. Even if they don't reveal themselves to their owners until puberty or pregnancy, they can change pigmentation, swell, become tender, or even lactate. In those cases, they act like regular nipples and breasts, only they're found elsewhere on the body. They can also develop cancerous tumours, just as regular breasts can.

Extreme examples

While extra nipples and breasts themselves aren't atypical (aside from their number), some research has suggested a correlation between supernumerary nipples and kidney or urinary tract malformations. It isn't exactly clear what could cause the connection. It could be that the urinary, renal, and mammary systems grow around the same time during embryonic development. But while the frequency of urinary or renal tract defects in the general population is between 1-2%, it's thought to be as much as 14.5% among those with additional nipples, or even higher, though studies like these are often plagued by low sample sizes. Indeed, other researchers claim that the connection is spurious, saying that physicians do not need to go about looking for urinary or renal complications just because their patients have extra nipples.

Most owners of third nipples are unfazed by this quirk of their bodies. Others have them removed for cosmetic reasons. But some historical case reports offer more extreme tales. A case reported in 1827 refers to a French woman who had a complete third breast on her left thigh. After she gave birth, it began to produce milk. "It was offered to her infant who took it willingly," writes Grossl. "She apparently nursed five children during her life from all three of her breasts." And a 1980 report of an ectopic supernumerary nipple described a "well-formed mass resembling a female breast on the back thigh of a 74-year-old man." The "fatty tumour," as he called it, never really bothered him and he refused an offer to have it removed.

Share this story on FacebookGoogle+ or Twitter