Some weeks ago, during a bleary-eyed subway ride to work, I found myself staring at a young woman on the other side of the car. She wore business attire with a North Face jacket and flip-flops, and she had an infinity symbol tattooed along the outside of her left foot, only a portion of the loop had been left out to make room for the word Love. Next to her, a scruffy guy in t-shirt and jeans had ornate black and gray murals inked on each arm, one of which seemed to depict an alien fight scene, the other some sort of robot love story. To his left, squeezed in at the end of the bench, was a man thumbing his phone with quick, nervous jabs. When he turned his hand over, I saw the word Jasmine tattooed above his knuckles and a date printed beneath it.

Then there was me, a blank canvas, wondering if I was missing something. Each inked-up person on the train appeared to be in the same age group—Millennials, to use the much-maligned descriptor. Being of the same generation, presumably we all post to social media on a regular basis, through profiles and accounts that compel us to confront the question, Who are you? For some, that choice is liberating: It’s a chance to start from scratch. For others, the sheer volume of options can be paralyzing. In either case, modernity compels us to declare our identity with conviction, whether we’ve found it yet or not.

Growing up in a rapidly changing and challenging world, most young people have struggled at some point or another with figuring out who they might be. Tattoos, recent research suggests, don’t just express identity: They help define it.

Although tattoos have been around for millennia, they’re more popular now than ever. In 1960, there were approximately 500 professional tattoo artists operating in the United States. By 1995, that number had risen to over 10,000. Nearly 20 years later, demand continues to surge, and by the latest estimates, roughly 20 percent of Americans have a tattoo. What’s more, 40 percent of the people in that group are Millennials, which some academics argue isn’t a coincidence.

“We’re living in this world that’s so fragmented and so chaotic,” says Anne Velliquette, a professor at the University of Arkansas who studies the relationship between consumer behavior and popular culture. Velliquette argues that we’re more able now than ever before to “recreate identities very easily,” both online and in real life.

In 1998, Velliquette and colleagues conducted an interview-based study that found people use tattoos as a way to cement aspects of their current selves. “We were hoping to look at the postmodern identity, and really what we found is that we were in this modern era where people did know who they were,” she said. “They had a sense of their core self.” Eight years later, the team revisited the idea. The second study, like the first, found that people used tattoos as a means to express their past and present selves. But the people interviewed in the second group also seemed to need proof that their identities existed at all. They relied on tattoos as a way to establish some understanding of who they actually were.

“We continue to be struck by rapid and unpredictable change,” study co-author Jeff Murray said at the time. “The result is a loss of personal anchors needed for identity. We found that tattoos provide this anchor. Their popularity reflects a need for stability, predictability, permanence.”

For people who study identity, this permanence is key. We define who we are by the elements that stick with us—people, stories, places, memories—and we measure ourselves in relation to them, patching the highlights together into what sociologists call a “personal myth.” These myths make sense of often-turbulent lives, integrating our “remembered past, perceived present, and anticipated future,” as Velliquette wrote in her 2006 report. Some people use institutions such as religion, work, and family to create this myth. Others use material objects like houses and cars to define it. But Millennials are something of a breed apart. Without access to many of the anchors their parents had to create their personal myths, that sense of stability and permanence is often harder to find.

People rarely get just one tattoo. About half of the inked-up population has between two and five, and 18 percent have six or more. In other words, tattoos aren’t just snapshots. They’re part of the ongoing narrative of the personal myth. Unlike material objects, part of what makes them so meaningful is the degree of sacrifice involved in the process. A tattoo’s acquisition “involves a painful ritual that may take hours,” Velliquette writes, and in fact “becomes a part of the object, since the experience adds meaning and becomes embodied in the tattoo.” And unlike trucks or apartments, which get manufactured en masse, “every tattoo is unique from the beginning.” People age with their tattoos and can chart the timeline of their personal myth from start to finish, just by running a finger over their skin.

Tattoos weren’t always a tool for seeking out the self. They first emerged in the U.S. as a way for sailors to avoid being forcibly recruited into the British Royal Navy in the years following the American Revolution. The protection papers seamen carried, essentially passports of the day, were meant to prove their newfound citizenship, but the Royal Navy exploited the papers’ vague descriptions and quickly began rounding up as many brown-haired, brown-eyed sailors as they could find. Tattoos helped add a dash of specificity by signaling individuality in the same ways a birthmark or scar might.

More recently, tattoos, which were typically symbols of various subcultures in the 1970s and ‘80s, have evolved into works of art that are broadly acceptable in the mainstream. Their transformation coincided with the explosion of the Internet in the late ‘90s and early 2000s, and its accompanying changes to the ways in which people work and play.

The traditional model of spending a lifetime with one employer has eroded over the past few decades. Today the average stay is closer to four years: Employees sell their skills, not their  loyalty, and businesses comply. Outside of work, the fragmentation of popular culture has enabled people’s interests to splinter across millions of different niches. The 1960s were defined by bowling leagues and block parties: come-all events that encouraged big groups of people to congregate. Today, people find solidarity in micro-communities, which can be ordinary—soccer leagues, running groups, poetry readings—or offbeat (the League of Professional Quirksters is one of many thriving meet up groups in Portland, Oregon).

With new frameworks in place, tastes and taboos changed. Tattoos started to look different and to mean different things, because the people getting them started to want something different—something more—out of their ink.

Though little research exists on when certain tattoos tend to spike in popularity, anecdotal evidence offers insight into trends. The most popular works used to be “flash” tattoos: simple one-off pieces that took no more than an hour, if that. They’re the stock images you can still find in any tattoo-parlor catalog: Chinese lettering, tribal lower-back tattoos, flames, music notes, a rose. They’re as simple as they are safe, letting people have ink on their skin, but discreetly. (My mother has two, for this exact reason.) It wasn’t until the turn of this century that clients started getting really creative, demanding that tattoo artists prove the artist part of their titles.

There’s been a nostalgic return to old-school designs that recall Sailor Jerry pin-up girls and retro-chic iconography. More people are getting hyperrealistic portraits and three-dimensional tattoos, which appear to literally leap off the skin. While there’s still interest in simpler imagery, such as anchors, feathers, text, and birds, much of the creativity comes in the application. Tattoos of flat hearts or plain text have been reinvented as the perfect recreation of a grandmother’s handwriting or an anatomically correct 3D human heart.

Why do people go to such great lengths to make their tattoos so perfect? The stakes are higher: Body art has taken on a greater significance, and people want their ink to say something about who they are. A good example is text. “A word, like faith or hope, is very easy and concrete,” said Gene Coffey, a resident tattoo artist at Brooklyn’s Tattoo Culture. It declares in the most literal way what a person finds important. “And a more abstract version of that, like a feather, is more of a free-spirited symbol.”

These considerations are fairly new, even if the images themselves aren’t always original. Many of the same requests keep popping up, Coffey says. “People think it’s unique to them because it spoke to them on this personal level. But to think outside of that box is very, very difficult.” Despite this limitation, which Coffey refers to as a “vocabulary of the art form,” in his experience the majority of people are getting tattoos to infuse meaning into their lives when a major event or feeling hits. “It’s like a time capsule for that feeling.”

It may also be a time capsule for identity. At the same time that people crave a sense of limitlessness, turning toward popular images like feathers, arrows, birds, and infinity symbols, they also want stability, which gets represented by dreamcatchers, anchors, a relative’s handwriting, or religious imagery. The most popular location for anchor tattoos, for instance, is on the feet, Coffey explains, owing to their symbolism. Tattoos give people, Millennials in particular, a way to prove to themselves and to others that a changing world is no match for them. The proof is right there, for all to see.

As people evolve and grow up, Anne Velliquette says, tattoo remorse isn’t always the go-to response. Instead, some people see old tattoos as a valuable reminder of a past self, although by Velliquette’s own admission, the gray cat she got tattooed on her ankle in her twenties as a way to remember a family pet is incongruous now. “I can’t say that I love that I have a cat on my ankle,” she said. “I don’t dislike having tattoos. But everyone sees it, and I’m 47 and I have a cat on my ankle.”

Even though tattoos might seem ubiquitous, 80 percent of the total population, and even a majority of Millennials (roughly 49 million people) are still without any ink. Many have no trouble establishing their identities via their families, religions, social lives, or careers, despite statistics that say modern influences should be steamrolling them all.

But if Velliquette’s research says anything, it’s that the people who do get tattoos have similar reasons for doing so. The motivations behind a particular decision aren’t something people can always easily express, or even know. Why, exactly, are so many people between the ages of 18 and 33 getting highly realistic, specific, and personal tattoos permanently inked onto their skin? Because the meaning behind art representing infinity, robot love stories, and names—like Jasmine—carries more weight for us now than ever before.