The buccal smear came all the way from Utah. A company called Affiliated Genetics, which specializes in confirming the zygosity of twins—whether they've been born miraculously from one egg, or coincidentally of two—had provided their "Sample Collection Kit" free of charge. (For non-journalists, the test costs $150.) It lay on my cluttered desk, impassively, a slightly bulging manila envelope the size of a fancy invitation. I considered it with mild disdain and called my twin brother, Russell, whom I had not seen for several days. I wouldn't open the package without him. We planned to meet outside my office, head to Brooklyn, and swab.

You say identical and fraternal, science says monozygotic and dizygotic. Russell and I were born 12 minutes apart at Union Hospital in Terre Haute, Ind., 27 years ago. I'm the elder. According to the family myth, we were something of a windfall: My father, having already raised three sons under a different regime, had set the limit for my childless mother—his new wife—at one. The presumption all along has been that Russell and I are identical, as if our whole lives conformed to that paternal decree. In photographs from our childhood, we are tied with blue and red satin ribbons, and costumed in corresponding French sailor's suits. There are several photos of us dressed in white, hovering over a book of Mother Goose, and nobody—not my mother, not me, not Russell—can tell the two big-headed infants apart. We swapped fourth-grade classes while inviting detection from neither friend nor frau.

And yet: We are not strictly identical. We have our petty discordances, which in their accumulation conspire against us. Suspicion is lured by doubt. In a giant senior-class photo of Russell and me that hangs framed in my old bedroom, we might as well be cousins. I am perched at his left shoulder, looking like my head is balancing on a drinking straw, while his own neck threatens to split his collar. We had long ago compared our fingerprints in vain. He was an All-State high-school quarterback (and an All-American in college), while I sat on the bench for a beleaguered basketball team that couldn't win even five games. Though we've both escaped the primordial sludge of the Ohio River Valley for New York City, no one confuses us anymore. For two years, I've spent $80 a month on a pharmaceutical that will keep me from going bald. We've noticed that I'm slightly taller. That our noses have a slightly different bent. Our penmanship is at odds and his hair is falling out.

Hence the buccal smear! "Over 99% accurate"! Hi-ho!

***

Twins these days are an enlightened bunch. For tens of thousands of years—from the first Cro-Magnon birth to roughly the American Civil War—identical and fraternal twins were born unto mothers that hadn't a clue there were essentially two varieties. And even if they did notice that some pairs were as alike as two siblings, and others way more alike than that, they didn't have the language to say it out loud. The idea of wondering what kind of twin you are, let alone testing it in some objective or scientific way, simply didn't exist before, say, the automobile. All twins were beheld under the same banner: unusual, unexplained, and undifferentiated.

The confusion was bound with our ignorance of knowing exactly what was happening in utero. It wasn't until the latter decades of the 19^th century that embryologists figured out the basic twinning process: Either two sperm fertilize two eggs, or one egg splits in two. (An earlier notion held that twins arose from two sperm that fertilized an ovum in separate places; obviously a red herring.) In 1875, the statistician Francis Galton interviewed 100 pairs of same-age, same-sex siblings, along with their close relatives, and concluded, "Twins is a vague expression." Though not quite a zygotic eureka, he found that extreme similarity among twins was just as common as a moderate resemblance, or hardly any resemblance at all. Even through the embryonic fog, it was clear something elemental divided at least the extremely similar from all the rest. By 1911, the usage of fraternal, as it relates to twins, had entered English usage, and the lexicography of mono- and dizygotic followed five years later.

Galton's work led to the establishment of the twin method, which proved the foundation for investigations into those dubious sciences called behavioral and eugenic. It also corroborated something that would have been apparent to the era's midwives and cowboys: Not every pair of twins comes into the world trailing the same debris. If you're witness to enough twin births—among humans or cattle—you're likely to notice certain differences from one to the next; that some pairs are born with a single placenta, that others have two placentas fused together, and still more spring from the womb with one placenta each.

For twins ourselves, realizing we came in two distinct types no doubt encouraged us to glom onto studies that could say who and what we were. There was an element of flattery (I'm gonna be an experiment!), and—as genetics took hold—of practicality ("I'm gonna give my brother a kidney!"), but perhaps also a spirit of competition: Who among us can truly lay claim to all the mystical mumbo-jumbo about a shared identity? Will the real mutation please stand up?

Early twin tests spanned the mundane to the violent: Are your earlobes attached or unattached? Do you share the same blood type? Do your palm-prints match? Your fingerprints? Your handwriting? Will your body accept a chunk of your sibling's skin, or will it repulse the alien, the fraternal, tissue? (The Nazi doctor Josef Mengele's torturous prodding of identical twins perverted a curiosity sweeping the globe).

And then there was the old cowboy's trick: Beginning in the 1940s, parents were told of zygosity in the delivery room, judging by the placenta(s) and the amniotic sac(s). It's more complicated than it seems, though: Every "monochorionic" twin birth is monozygotic, but only 70 percent of "dichorionic" births are dizygotic.

As science looked for more cost-effective ways to divine zygotic history, blood tests and other lab work gave way to surveys that combined objective measurements—height, weight, tone of voice, etc.—with questions about how the pairs were perceived. Were they confused for each other by teachers and friends? Parents? Strangers? But even that proved more in-depth than necessary. In a 1961 study by a Swedish scientist named Rune Cederlof, the whole exam hinged upon a single, probing question: "When growing up, were you and your twin 'as like as two peas' or of ordinary family likeness only?"

It turned out that whether twins thought they'd been "as like as two peas" could predict the results of every other available test with surprising accuracy. Cederlof found that the twins' answers to this one item on the questionnaire matched overwhelmingly with five independent measures of blood type. After nearly 100 years, our finest scientists realized that discerning a man's zygotic origin was about as easy as discerning whether he was ill by asking if he had a runny nose.

The examination of DNA, then, may be an entirely superfluous reassurance: like searching for witnesses to a murder when the act itself was caught on tape.

***

My mother claims her placenta never received the appropriate inspection before Russell and I were carried home, swaddled and bruised. No blood test, either. She used to tell us the gruesome detritus had been, of all things, frozen. We have grown up with the image of our birth matter languishing in an ice-cube tray, and wrung our hands in secret. What would it mean if we weren't who we thought we were?

On the walk around the corner from the subway to my Brooklyn apartment, with Russell pawing the lush crown of my fertilized head, I ask him if he thinks we were as alike as two peas in a pod, and he says yes. Did people confuse us? Yes. Did we suffer similar physiological pratfalls (acne, chicken pox) at roughly the same time? We did. By all accounts, we don't need Affiliated Genetics. By all accounts, it's inconceivable that we are something other than what we suspected as children.

Indeed, that's the only comfort when staring down the buccal smear: treating the thing as mere confirmation, a technological novelty. The idea that we might be fraternal makes me feel guilty and unnerved, like we've stolen a painting and hid it hastily in the attic. My god, our whole lives we've thought ourselves superior to that bovine herd of same-age siblings. We've built our personas around the assumption that the two of us were once the same collection of cells. How could I face anyone, even Russell, knowing we weren't?

I protest too much. My temple throbs as we sit at my dining room table and read the instructions for the sample collection. No one wants to have lived a fiction. A faulty premise patterns a thousand more lies. We scrub the inside of our cheeks, laughing for the absurdity, and nervous as hell.

# # #

Slate contacted seven pairs of twins who underwent commercial zygosity testing, including the author and his brother. Photographs of each pair are presented below, along with the twins' guesses for how their tests would turn out. Make your own predictions, and then look for the results in Part 2.

Click here to read Part 2, and see the results of the twin tests.