My bus tour through the Andes of southern Peru took an unexpected stop. We were in the cold, dry highlands, less than 100 miles from Arequipa, when the tour guide insisted that my fellow travelers and I get off the bus "to take a small hike." We walked through a small farm with some rocky ruins of indeterminate age. But then the guide pointed to a big rock positioned over a hole and told us to look inside.

Image: Alexandra Ossola

There were a number of skulls in the hole, and they didn't look quite right. The crown was too dome-shaped, taller and more cylindrical than usual, it seemed. The guide said these skulls were made to look this way intentionally; these individuals wore bandages wrapped tightly around their heads up until about age five, while their skulls were still soft.

Artificial cranial deformation—or the practice of intentionally changing the shape of a person's skull—has been practiced by Neanderthals of 40,000 years ago until very recently, maybe even still today. People on every continent except Antarctica have done it, making heads more cylindrical, cone-shaped, ridgier, bumpier or flatter depending on the region. The reason, most archaeologists believe, was pretty much the same reason we modify our bodies today: to show an association with a particular social group.

"Cranial deformation had to do with ideas of beauty, what would be socially acceptable and desirable to look like, and that differed between groups around the world," Mercedes Okumura, an anthropologist at the Federal University of Rio de Janeiro in Brazil, said in an interview.

Since cranial deformation was so widespread, archaeologists don't think it originated in just one place. "In a small region like the Andes, it's easy to see how the practice could spread from one group to the other," Okumura said. But when scientists find evidence of skull deformation in Australian aborigines from 30,000 years ago and eastern Europeans from 5,000 years ago and Melanesians from a generation ago, it's harder to pinpoint just how the practice could have traversed so many barriers.

The first cranial deformations may have been by accident. Babies are born with lots of different bones in their skulls, which enables them to exit the womb more easily and for their brains to grow. By age five, the gaps between the bones start to fuse together to make the skull more contiguous, like we see in adults. It's easy to imagine that a baby with a soft skull that lays for a long time while his mother is working would get a partially flattened skull as a result, Okumura said.

In fact, one of the methods used to deform the skull among Native Americans when was essentially a cradle that had one piece of wood that rests under the infant and another that came out at an angle on the other side of the skull. "Native Americans strapped infants to a board so the kid isn't flopping around while you're working, and over time that flattens the head," said Carl Feagans, who is now an archaeologist for the U.S. Forest Service in Grand Prairie, Texas but published on artificial cranial deformation as a graduate student.

Okumura agreed: "With those [cradle board] deformations, we don't know if they're intentional or an accident."

A painting from around 1850 of a Chinook woman with her child. Image: Paul Kane, uploaded by Lupo/​Wikimedia Commons

Different cultures used different methods to shape the skull, which archaeologists know from some of the historical documentation that various cultures left behind. In the Andes, bands of cloth bound tightly around the widest part of the skull gave it a more cylindrical shape. In Europe, the same method was used to create a series of bumps and ridges along the head.

Skulls deformed for the sake of fashion. Image: Popular Science Magazine, 1880. Uploaded by Jesper7/​Wikimedia Commons

Other cultures, like the Maya, attached wooden boards to children's heads. Some other methods that would have less dramatic results included manual manipulation (where the baby's parents would press the head with their own hands) or putting large stones around the child's head in the cradle. "But most of the research and most of the cranial deformation has been focused on wooden boards and bandages. That's when you get the most extreme deformation," Okumura said.

Even within the Andes, the shape of the skull varied widely by region and by group affiliation. Researchers aren't sure if deformation was used to show socioeconomic class based on the objects people with deformed skulls had in their graves, but they are doing more work now to assess whether those with deformed skulls had better nutrition (an indicator of better health and higher class) than those who did not.

Mayan methods for skull deformation. The two heads on the right were shaped with wooden boards. Image: Fruitpunchline/​Wikimedia Commons

Sometimes the deformation wouldn't go as planned. "Sometimes you would see bone necrosis, which happens when the bone lacks blood. Then the bone sort of rots—it's quite ugly," Okumura said. "In the Andes, especially because we have a lot of mummified remains, you can see infants with this, they would die from it." It's hard to tell how often this happened—or, for that matter, what percentage of people in any given community had their skulls deformed.

Surprisingly, all this manipulation probably didn't have an impact on how well the brain worked. "The skulls [anthropologists] are finding are fully functional adults," Feagans said. "You would think if there was any neurological damage done, the practice would die out."

Okumura agrees, although she notes that it's impossible for researchers to test today other than in simulations, for ethical reasons. "The cranial bones and the brain will grow and accommodate themselves according to the desired shape. Most researchers agree that these deformed people would be normal in neurological terms," she said.​

Skull deformation in France, which was practiced through the 1800s. Image: Didier Descouens/​Wikimedia Commons

Of course, understanding all of these practices and methods leads to one big question: why? "It has to do with the identity of the group," Okumura said. Humans are very social—we constantly feel the need to be included and to feel like we belong, even just to survive, she said. "The social part really played a role in terms of identifying yourself with the rest of your family and social relations."

The question of how cranial deformation went out of fashion is more complicated. Okumura theorizes that new cultural influences or the destruction of the previous social order could have been the cause. She noted that when Europeans arrived in the Andes, the Incas saw their cities destroyed, their population ravaged, and their social structure totally upended. "In a situation like that, it's quite common to have an interruption of many cultural practices, and artificial cranial deformation could have been one of them." Sometimes, though, cultural conquest can be less dramatic. No one is really sure if cranial deformation continues today; Melanesians are wearing Polo shirts, Feagans said, but this particular practice may still live on.

People who had a reaction like mine—that this is simply not how skulls should look—may find it hard to see artificial cranial deformation just for what it is. But imposing our modern perceptions of beauty and ethical practices don't give us a fair assessment; we modify our bodies in all sorts of ways today, from plastic surgery in Brazil to sharpened teeth in various African countries to the bagel heads of Japan to foot-binding in China. Surely some of those would be horrifying to future (and, in some cases, present) cultures.

"The sense of what is acceptable to perform on your kids changes across cultures," Okumura said. "Plastic surgery is horrible but we really don't care, no one gets really horrified by it even though you are really cutting and taking out chunks of a person or putting things in. The concept of beauty varies across groups, so does the idea of what is acceptable or horrible also varies."

In order to better understand artificial cranial deformation, archaeologists need to find new specimens. "Time is the enemy of the researcher. The longer we wait, the harder data will be to find, and the less data it's going to be," Feagans said. Specimens are disappearing for natural and human reasons, he said, from natural decay to bulldozers making way for development around the world.