Monica and Erika Hoffman stand barefoot, side by side near a sign that reads “Twin Studies Center” at California State University at Fullerton. Their glasses removed, both have auburn eyes, softly jutted chins, light freckles, and perky noses. Both wear black shirts and small sparkly earrings (Erika’s are flowers, Monica’s, bows). The identical twin sisters turned 39 the day before this lab visit.

“You are the 101st twin pair we’ve had in this study,” Nancy Segal, a shrewd, spirited professor in a sequined black hoodie, tells them, as a cluster of graduate students shadow her through the halls. Segal, a fraternal twin herself, is a walking Wikipedia of twin science. She specializes in evolutionary psychology and behavioral genetics, and has studied thousands of twins and their families around the world. Nearly three decades ago, Segal founded the Twin Studies Center to learn what twins like the Hoffman sisters—far from the same, despite appearances—have to teach us about the complex interplay of forces that impact our health and shape who we are.

Segal takes out a tape measure and begins with Monica, then moves to Erika. “Sixty-eight inches,” Segal says. “That would make Monica three-quarters of an inch taller.” At first glance, the Hoffman twins’ physical differences are as difficult to notice as their slightly mismatched heights. It’s tricky to tell them apart at all, except that Monica wears a gray beanie, wispy baby hairs peaking from beneath its knitted edges. Erika wears her brown hair loose, falling past her shoulders. Monica used to have the same hairstyle—before four-and-a-half months of chemotherapy and six-and-a-half weeks of radiation treatments left her bald.

In September 2015, doctors discovered a tennis-ball-size tumor in Monica’s left breast. It turned out that she had stage two breast cancer, which had already spread to her lymph nodes. But the specialists were surprised to learn that Monica had an identical twin—and three years later, they continue to be baffled. Monica is now in remission after a partial mastectomy, while Erika continues to receive regular mammograms and ultrasounds, but has never tested positive for cancer. (A year before Monica’s diagnosis, a mammogram also detected early stage breast cancer in the twins’ mother.)

Identical twins (also known as monozygotic twins) come from a single egg that splits in two, and share 100 percent of their genes. Neither Hoffman twin tested positive for BRCA gene mutations, which account for between 5 and 10 percent of breast-cancer cases. There are other mutations that may be involved in breast cancer, but with the Hoffman twins, the question loomed: What could have led to their divergent diagnoses, when their bodies contain the same roughly 20,000 genes?


Twin studies have historically been some of the most valuable genetic research tools in the world—contributing a century of data to our knowledge of human behavioral, medical, and physical traits.

“All twins are valuable to research,” says Jeffrey Craig of the Center for Molecular and Medical Research at Deakin University in Australia. “But identical twins, I think they are the most valuable. Because what we control, or hold still, is the genetics. The mother, the father, the date of birth, the season of birth, the shared environment, the family, the mother’s diet [are all the same].”

“Twin studies are a simple, very elegant design,” Segal adds. Holding the genetics constant allows researchers to study the age-old question of nature versus nurture—what aspects of a person come from their DNA, and which come from their environment? There was a time when scientists tended to think one or the other factor was more important to development, but they have since come to realize how limiting it is to confine our understanding of behavior, health, and identity to this either-or dichotomy. “Nature and nurture work in concert,” Segal says, “affecting every measurable human trait.”

To her, twins are not just curious spectacles, but unique individuals born in tandem, living gifts to science and to humanity. Segal has investigated some of the world’s most fascinating twin cases. She’s traveled to Brazil to spend time with twins who belong to a family that includes 22 sets of identical twins born across five generations. Like many researchers, Segal used to believe that while fraternal twins run in families, identical twins are just random occurrences in nature. But cases like the Brazil family and a study of 13 sets of identical twins in Jordan challenge that idea, along with findings like a 2009 study that found twins from seven different families shared similar alleles. Each of these families produced at least two pairs of identical twins.

But Segal may be most known for her studies on twins separated or switched at birth, such as the case involving Begoña and Delia from the Canary Islands in Spain. As a baby, Delia was accidentally switched with another infant in the hospital, and the girls and their families grew up oblivious that it had ever happened until they were 28. They were the sixth switched-at-birth pair of twins ever identified.

In 2014, Segal stumbled on the eighth and ninth publicly known switched-at-birth cases, an even more bizarre story of two sets of brothers in Colombia. Each family believed their two sons were fraternal twins. But everything the men thought they knew about their families and selves blew up when a co-worker of one of the brothers, Jorge, in Bogotá went to the butcher shop on the far side of town. She was sure she saw Jorge working behind the meat counter, which was particularly odd because she knew Jorge worked at an engineering company, designing gas and water lines. It was actually Jorge’s identical twin, William, who he’d never met.

This mistaken identity led to the discovery that two of the four brothers had been switched at birth. The young men were actually part of two identical twin sets. Two of them had been raised in the wrong families altogether, about 150 miles away from their identical brothers. Segal traveled to Colombia to study the four young men, and write about their journeys in a new book, Accidental Brothers.

Though switched-at-birth twins are exceedingly rare, there are more instances of twins who are separated at birth. Since 1922, there have been 1,894 cases of sets of twins reared apart, according to a study by Segal. Today, there are more documented cases of twins separated at birth and later reunited than ever before, largely because the internet has helped connect these siblings. Segal has studied 150 reared-apart twin pairs, and is currently studying 22 cases, mostly from China, whose one-child policy of the 1970s led to the abandonment of tens of thousands of infants. Over a dozen twin sets from China were adopted (since the 1990s) and raised separately.

Though all twins, through their similarities and differences, offer insight into the effects of genetics and the environment, twins who were reared apart offer particularly powerful case studies.

Nancy Segal

In 1979, Jim Springer and Jim Lewis, “the Jim twins,” were reunited at age 39 after not knowing the other existed. As described in Segal’s book on the identical Jim twins, Born Together—Reared Apart, both had been adopted and raised by different families in Ohio, just 40 miles apart from each other. Despite their separate upbringings, it turned out that both twins got terrible migraines, bit their nails, smoked Salem cigarettes, drove light blue Chevrolets, did poorly in spelling and math, and had worked at McDonald’s and as part-time deputy sheriffs. But the weirdest part was that one of the Jim twins had named his first son James Alan. The other had named his first son James Allan. Both had named their pet dogs “Toy.” Both had also married women named Linda—then they got divorced, and both married women named Betty.

The Jim twins inspired the Minnesota Twins Reared Apart study, which Segal also worked on from 1982 to 1992. This research once again showed surprising similarities in identical twins’ habits, interests, intelligence, and religion despite their separate upbringings. Still, even the Jim twins had differences. For starters, one divorced Betty and married a woman named Sandy, which, as Segal jokes, must have caused worry for the other still-married Betty.

Even the most strikingly similar identical siblings can also differ in deeper ways. In the 1960s, researchers studied a set of four identical sisters known as the Genain Quadruplets, all of whom were diagnosed with schizophrenia at 24. As a graduate student at the University of Chicago, Segal worked on the case one summer at the National Institutes of Mental Health in Bethesda, Maryland. The quads’ shared diagnoses might have seemed like a vote for nature over nurture. But it wasn’t so simple. “The Genains did have a very abusive, paranoid father,” Segal says. But the quads were especially remarkable because “despite their identical genes, they all showed varying symptoms.”

One sister, known as Myra in the study, had mild features, and might not have even been diagnosed with schizophrenia had it not been for her three sisters whose symptoms ranged in paranoia, hallucinations, catatonia, and incoherence. Genetics obviously played a role. But how the disease manifested within each sister may have been influenced by something else.


The reasons why identical twins have differences at all—not just in health outcomes, but temperament, taste, and physical traits—can come down to random chance. But it can also be traced to how each sibling’s (identical) genes are expressed. These microscopic variations can lead to radical differences in a person’s health, personality, and even appearance. The study of how this works is known as epigenetics.

This research field is often misunderstood. Confusion over gene expression contributed to recent widespread fake news claiming that the identical twin astronauts Mark and Scott Kelly no longer had identical DNA, after Scott’s record-length stay on the International Space Station.

What actually happened to the astronaut? “Some of Scott’s genes changed their expression while he was in space, and 7 percent of those genes didn’t return to their preflight states months after he came back,” as Marina Koren writes in The Atlantic. “If 7 percent of Scott’s genetic code changed, as some of the stories suggested, he’d come back an entirely different species.” Gene expression would be expected to change as one’s body reacts to life in space, a drastically different environment from earth. But genetic code itself would not.

With epigenetics, gene activity reacts in response to various mechanisms at the cellular level. In Greek, the prefix “epi” means “on top of” or “above.” So referring to “epigenetics” or the “epigenome” implies a process occurring on top of the genes. A common analogy used to describe the epigenome is to consider genes as instruments in the “symphony” of life. But they don’t play themselves. They need musicians. Epigenetics would be the musicians that help express (or silence) the performance of our genes. Exercise, sleep, trauma, aging, stress, disease, and diet have all shown significant effects on the epigenome.

Studies suggest that some changes to the epigenome may be passed on to our future grandchildren. Meanwhile, scientists are actively working on epigenetic editing—finding a way to hack gene expression. Others are developing drug treatments that target the epigenome. There is hope that the science of epigenetics will one day help doctors detect and disrupt diseases earlier and more effectively.

Whether a gene is active or not can depend on chemical compounds that click onto the DNA structure, toggling the gene’s on-off switch (think of it as a biological lock and key). These changes can be clicked into place, and they can also be undone. Environment and lifestyle can influence gene activity, and it is within this segment of the field of epigenetics that twin studies can play a key role.


My own identical twin boys are now 16 months old. One has a strawberry-shaped birthmark on his left ankle. The other has a thick em-dash-shaped birthmark on the back of his right thigh. One has a hair whorl that swoops to the right. The other’s swoops left. Without those defining markers—for the first six months of their lives especially—my husband and I might have mixed them up and never straightened out the mistake.

It is not as hard to tell my sons apart now, but we often recognize them more based on personality differences than looks. One is adventurous, daring—the first to nosedive off a sofa, the first to fall down stairs. He also crawled, stood, cruised, and walked first. He hollers and cries when we leave the room. Our other boy is an observer. He can be laser-focused, able to spend 30 minutes trying to click together a buckle as his brother marches around with his chest puffed, in need of constant movement and entertainment.

In life, they’ve shared bottles, diets, sleep schedules, and common colds. In the womb, they shared a placenta (but not umbilical cords). Still, they were starkly different long before most of these experiences, from the instant the doctor sliced me open and yanked them out. One screamed all night long in those first days on Earth. The other, in the same bassinet, dozed right through his brother’s cries.

Erika Hayasaki

I wondered: What the heck then was going on inside my body that may have influenced their different personalities in their first shared year of life? No doctor or scientist can possibly tell us for sure. But the epigenetic changes that can take two identical strands of DNA and turn them into two unique individuals are thought to start in the womb. Some studies of twin newborns have shown that intrauterine and postnatal environments lead to differences in gene expression, and some of these divergent patterns are detectable at birth.

These differences may widen as twins grow up. While infant identical twins can be almost indistinguishable, some can begin to look more unique as they age (though friends and family still confuse adult twins all the time, Segal says, as with the Colombian sets). But Segal studied a pair in which one twin was always heavier growing up. “Mom said he always ate more—so there is a subset of twins like that. And adverse prenatal factors can intervene, making identical twins somewhat different in height—the average difference is two inches,” she says. For some pairs, their different environments change them. One may spend more time in the sun. One may smoke or experience greater stress. All of these things could influence their epigenomes.

Twins share many environments—a room, a religion, a family. But for twin researchers, understanding what they call “non-shared environments” is of special significance. In life, a non-shared environment “could be a college course. A great teacher. A trauma,” says Segal. “Say one twin took an exotic trip around the world, or one twin had a terrible disease, or won the lottery, or had an accident. It’s those unshared experiences that affect behavior.”

In the womb, non-shared experiences could be slight differences in placenta size, or in umbilical cords, and the fetuses’ placement in the womb. “If it’s a really long cord, the idea is that you need more pressure to actually get the nutrients and oxygen from mother to baby,” says Craig, of Deakin University, who believes this is one area of research that is understudied but potentially very important.

“There are some twins in Brazil, where one twin has microcephaly due to the Zika virus infection and the does other not,” he adds. “And you’d really think, ‘Hold on a minute, how does that happen if the mother gets infected, why does only one twin get infected?’” As a recent study on twins exposed to Zika in pregnancy suggested, infection risk could be related to epigenetic mechanisms.

In 2015, Segal collected cheek-swab samples from the Colombian twins doubly switched at birth, and sent them to Craig’s lab for an epigenetic analysis. This was the first published epigenetic comparison of identical twins raised apart.

Two of the identical twins raised apart (Jorge and William) ­shared the same bump on the same spot on the bridge of their nose, and until they were reunited both had been convinced that it was from an injury. They also both preferred only eating the drumsticks of chicken. But one wore glasses and the other did not. The other identical pair (Carlos and Wilber) had both been smokers, and both had a speech impediment (Carlos’s was corrected but Wilber’s was not). Segal also noticed that Wilber is more strongly right-handed than Carlos, who borders in ambidexterity.

The identical, reared-apart Colombian twins doubly switched at birth (Nancy Segal)

Two of the young men grew up in the city of Bogotá, where they had access to strong educational resources and were working toward graduate degrees when Segal met them. The other two grew up on a remote farm in Vereda El Recreo, and left school after fifth grade. “The Colombian twins really made me think hard about the environment,” Segal says. “The separated twins were raised in extremely different environments, more so than most separated pairs.”

Craig specializes in reading one kind of epigenetic marking known as methylation patterns. With the Colombian twins, it turned out that one identical pair was still epigenetically similar, despite being raised apart. But with the other pair, the epigenome of one brother raised in the city seemed to differ significantly from his identical twin raised in the country. This could be because of genes affected by ultraviolet rays, radiation, or pesticides—factors that may have differed from city to country, Craig and Segal hypothesized in their study, published in December 2016. But it’s unclear why these twins diverged so greatly while the other pair—who also grew up in these different environments—had such similar epigenomes.

One possibility is that epigenetic changes could have been triggered long before the twins were separated. “What happens if one had a big placenta, and other had a small one? Or a thin umbilical cord, and the other had a fat one?” Craig asks. There is also, as Craig and other researchers emphasize, happenstance. There are spontaneous, unpredictable variations between all cells, and all people, including identical twins.


For twins raised together in similar settings who share the same genetic profiles, it isn’t surprising that one’s illness could befall the other, like identical twin girls diagnosed with a rare leukemia at three months old; or identical twin brothers who each received an ALS diagnosis within weeks of each other; or the tragic story of identical teen boys who developed a deadly form of liver cirrhosis last year (one survived and his twin did not).

But by comparing differing gene expressions in identical twins, researchers are beginning to understand a variety of conditions. Rare pairs like the Hoffman sisters with “discordant” diagnoses (in which one has a disease, but the other does not) may help physicians determine risk factors for diabetes, autism, schizophrenia, cerebral palsy, thyroid disease, and ALS.

Manel Esteller, the director of the Cancer Epigenetics and Biology Program at the Bellvitge Biomedical Research Institute in Barcelona led one of the earliest efforts to identify differences in gene-expression “portraits” of monozygotic twins. “We saw that different lifestyles were able to create divergent epigenomes,” Esteller says. And the changes grew more contrasting in the twins as they aged.

Esteller’s lab then set out to look for epigenetic markers of cancer risk. They studied twin pairs with discordant breast-cancer diagnoses, and found that epigenetic changes signaling higher breast-cancer risk could be detected in the sick twin several years before doctors would be able to make an actual clinical diagnosis. (The sample had been previously studied before the diagnosis, which allowed Esteller to look at their epigenomes over time.) These changes “can be detected by a biopsy of the breast. Sometimes the epigenetic defects can also be observed in DNA circulating in the blood,” Esteller says. “Nowadays it is widely accepted that in all human disorders there is a genetic and epigenetic component.”

The number of academic papers in the field of epigenetics has exploded since 2000, which has also led to hyped-up promises and overblown results. It is a relatively new field, with some problematic studies, says Andrew Feinberg, who directs the Center for Epigenetics in the  Johns Hopkins Institute for Basic Biomedical Sciences. But for some diseases, like cancer, “it is already well established that most of the mutations disrupt the epigenome, so epigenetics is at the very heart of malignancy.”

In April’s New England Journal of Medicine, Feinberg called for doctors to integrate genetic and epigenetic information into their practices, arguing that the field “can lead us at last to an era of comprehensive medical understanding, unlocking the relationships among the patient’s genome, environment, prenatal exposure, and disease risk in time for us to prevent diseases or mitigate their effects before they take their toll on health.”


After Monica Hoffman’s breast-cancer diagnosis, doctors probed the twins’ medical and life histories, forcing them to think about where exactly their environments had diverged.

When Erika and Monica were newborns, the only way their mom could tell her babies apart was by a freckle on Monica’s lip. But sleep-deprived or in the dark of night, it was impossible to keep her kids straight. “My mom was so tired that she still couldn’t remember who she fed, who she bathed, who she burped,” Erika says. “She would just cry. So our grandparents came over. They put my mom to bed. They washed both of us. They fed both of us, and they painted Monica’s toenails.”

The red toenails kept the confusion to a minimum at home. At school, classmates called Monica and Erika the “Twin Towers.” They played softball and volleyball, were natural leaders, and had similar taste in clothes (surfing tanks and flip-flops even in the winter). They both attended California State University at Long Beach. They went on to start a pharmacy wholesale business together and, after living apart for a while, are now roommates again in Huntington Beach, California.

The sisters wondered if stress may have played a role in Monica’s illness. Erika was in a long-term, stable relationship. Monica struggled to find the right romantic partner. Did it come down to diet? Monica ate salmon three times a week for a year. Erika ate far less fish. The sisters told doctors about Monica’s irregular, low-pain periods, which started when she was 10. Meanwhile, Erika had regular menstruation cycles since the age of 11, accompanied by excruciating cramps. Monica developed breasts by fifth grade, much earlier than Erika.

The truth is doctors may never be able to tease out any environmental triggers that may have been involved in Monica’s diagnosis. Many environmental-epigenetic findings so far are based on correlation, not proven causation. In the field of twin studies, there is so much still left to untangle and explore, as the research continues to veer more toward epigenetics and also the human microbiome, Segal says.

One key question for scientists to consider may be not just if and how the environment toggles with gene expression, but how humans could flip genes on or off ourselves. This is where technology such as epigenetic gene editing—which focuses on turning the volume knob of gene expression up or down without changing the underlying DNA—holds great promise.

The Hoffman twins enrolled in Segal’s study (which is still underway) to better understand themselves, but through their involvement, they will also help researchers like Segal understand more about all humans. Today, the cancer-free twin, Erika, is focusing on disease prevention. She receives regular mammograms and ultrasounds. She thinks back to Monica’s bouts with chemo, which caused terrible bone pain, migraines, and neuropathy. Erika cradled her sister though sleepless nights and helped carry her when she needed to move off the couch. “I always thought I was a burden,” Monica told her twin, to which Erika replied: “I would rather have you be a burden, than have you not be around.”

After her battle with cancer, Monica had a hysterectomy at her doctors’ advice, as a preventative measure. It was bittersweet, because although she is still in remission, she had always wanted to be a mother. Erika, on the other hand, knew that she did not want children. The twins still wonder why they ended up on these contrasting trajectories.

“I suspect that there are some answers that will always elude us,” Segal says.