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Editor’s Note: For Twosday (Tuesday 2/22/22) We’re sharing a story we did a few years back about identical twins. Twins are endlessly fascinating for us as humans, and their stories are embedded in our folklore and mythology. But twins and the study of twins is also a cornerstone for genetic research helping us better understand the heritability of different traits, conditions and diseases. When we look at identical twins, we often are drawn more to the their similarities than their differences, and we’ve written about identical twins separated at birth and how meeting as adults reveals how uncannily much they have in common. But the differences can often be just as illuminating. Check out these stories.

Greg and Alex

The twins Greg and Alex Abbas were always really good in math and science. They sounded exactly alike on the phone. They could each play the piano, and they both became Eagle Scouts.

“We’ve always been more alike than different,” said Greg, a software engineering manager at Apple Inc.

$\"A$

Alex and Greg

In high school, they looked enough alike that they could switch places and fool the teachers. But despite all those similarities, the brothers were always told they were fraternal, not identical twins.
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\n\n\n\n\n\n\n\n“That was mom’s assertion,” said Greg. “The doctor told her that there were two placentas.”\n\n\n\n\n

\n\n\n\n\n\n\n\nThat old assumption, and others, led many twins and their parents to wrong conclusions about whether they were identical or fraternal twins. As many as one-third of identical twins and all same-sex non-identical twins have separate placentas. Even when there are two placentas, that might not be obvious to the delivering doctor.
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\n\n\n\n\n\n\n\nIt’s no wonder correctly identifying twins as identical or fraternal is harder than you’d imagine. One recent study found that 32 percent of parents of identical twins were confused or plain wrong about whether their children were identical or not. About 29 percent of parents of fraternal twins had it wrong.
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\n\n\n\n\n\n\n\n\nIn hindsight, it explains a lot to Greg and Alex. The two had always wondered if maybe the doctors had it wrong. Not long ago, Alex, a bioinformatician at Genentech with a Ph.D in biology, decided they should look at how similar they were genetically by testing with 23andMe.
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\n\n\n\n\n\n\n\n\n“I thought there was a possibility that we might be semi-identical,” said Alex, referring to scarce sets of twins.

In those cases, the twins are half fraternal and half identical, having the same genes from one parent but different genes from the other. While there are plenty of critical medical reasons for knowing whether twins are identical, there are also less concrete reasons that are no less important.
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\n\n\n\n\n\n\n\n\n“I wouldn’t say it’s a huge deal, but it is special to learn about yourself in this way,” said Alex.
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\n\n\n\n\n\n\n\n\nWhat the brothers found out from testing is that they are indeed identical twins, not fraternal.

\n\n\n\n\n
\n\n\n\n\n\n\n\n\n\n“The results were 100 percent shared DNA,” said Greg. “I said, ‘Wow, that’s not fraternal.’” While the brothers said they can’t claim that their lives were changed by the news or did anything differently because of what they learned, both said the information was extremely valuable.
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\n\n\n\n\n\n\n\n\n\n“Being a twin is an intrinsic part of the way I look at my own identity, so getting it right and understanding myself is important,” Greg said.

The Abbas brothers are not the only twins who learned through using 23andMe that they are identical twins.

Mike and Matt

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\n\n\n\nWhen Mike Convente, a Ph.D. student in Philadelphia, learned through 23andMe that he and his brother were identical twins, it made their bond somehow more special.

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$\"A$

Mike Convente (left) and his brother Matt.

\n\n\n“The true essence of being a twin is to be identical,” Mike said. “Otherwise, you’re just siblings born at the same time.”

`\n\n`Tim and Nick

Tim Dang, a product manager here at 23andMe, and his brother Nick also grew up being told that they were fraternal twins but learned that they were identical twins after testing with 23andMe. Like the Convente twins and Alex and Greg, learning about being identical was an important detail for Tim and his brother.

\n\n
\n\n\n“Knowing we’re identical made us think hard about our similarities and differences, identity and the life choices we made up to this point,” he said.

$\"\"$

Tim Dang (left) and his brother Nick.

Erika and Kristin

Sometimes twins only learn that they are identical twins later in life, as in the case of Erika and Kristin.

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In a recent paper, Southern California researchers announced that political involvement has a genetic component. $\"\"$

Though they stopped short of identifying a gene or genes at work, the researchers concluded that the decision to go out and vote was genetically determined by up to 50 percent. Genes, they also noted, were partially involved in other activities such as attending rallies, donating to a campaign, and even running for office.

UC San Diego professor James Fowler and his colleagues published their results in The American Political Science Review — not your typical genetics journal. They studied 396 same-sex twins, culled from a list of nearly four million registered voters in Los Angeles County, who had voted in eight state elections between 2000 and 2005. They then replicated their results using an independent twin population taken from the National Longitudinal Study of Adolescent Health.

Twin Study

Aside from finding identical twins more likely to vote the same way than fraternal twins, their data also suggested that genetics accounts for 53 percent of variance in going out to vote.

Fowler and his colleagues aren’t the only ones who use twin studies for their research. In a way, twins can provide researchers with perfectly matched test subjects and controls. Because identical twins have the same genetic information, for example, scientists can do one-on-one comparisons to find out why one is left-handed and the other right-handed, or why one has cancer but the other doesn’t. Such twin studies allow researchers to estimate how much of behavior is due to genes and how much can be attributed to environmental factors. They can also make such comparisons over larger groups of twins to look for a gene or genes that could explain these common traits.

Twin studies have indicated a genetic component underlying an amazing variety of traits, including:

Not So Fast

But not all researchers are impressed.

“We suggest that no scientific purpose is served by the flood of heritability estimates generated by these studies,” psychologist Leon J. Kamin and economist Arthur S. Goldberger wrote in a 2002 paper.

Kamin and Goldberger looked at two population samples used as the basis for several twin studies. One twin population known as MISTRA (Minnesota Study of Twins Reared Apart) is composed of identical twins separated early in life. Researchers wishing to study behavioral differences who used this group might think that any similarities between the twins would be genetic (attributed to nature) and any differences would then be due to environmental influences (nurture). However, Kamin and Goldberger found some cases where statements the twins had given indicated they might have compared notes prior to being interviewed. Such interactions, they argued, could skew the results in studies using MISTRA twins.

Other Studies

A second twin population is known as SATSA (Swedish Adoption/Twin Study of Aging). This sample is composed of four groups of twins: identical twins separated in childhood; identical twins reared together; fraternal twins separated before the age of 11; and fraternal twins reared together.

Kamin and Goldberger questioned the definition of separation as in some cases the twins were raised within the same extended family though they didn’t live in the same house. The environmental influences would be too similar, they argued, and that again could skew any conclusions reached by those conducting studies using these twin populations.

One twin study where researchers seem to have been able to successfully separate and track twins and multiples for years was conducted between 1953 and 1997. Known as the Child Development Center’s Study of Twins Reared Apart, the data are part of the records donated by lead researcher Peter Neubauer to Yale University, with the stipulation that the information remain sealed until 2066. For those unwilling to wait, some tidbits may be gleaned from a book by two of the study’s unknowing participants, Elyse Schein and Paula Bernstein, in their book Identical Strangers, as well as in Neubauer’s own book, Nature’s Thumbprint.

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