Twinning 101: Sorting Twin Facts from Folklore

twin facts, fraternal twins, identical twins

Identical twins have piqued curiosity for centuries: secret twin languages, ESP, twin trickery. Let’s dig into some twin facts as we attempt to tell the world’s most infamous pair apart: fact and fiction.

“They like the same clothes. They like the same food. They like the same boys. But they need different doctors.” This was the 1990 Health Chicago ad campaign that my identical twin sister and I posed for at age 12. It was in some business magazines and on a few Chicagoland billboards. While the ad included cute phrases and identical curly-haired girls, it is clichéd with one of the most common misconceptions about twins: that they like the same things (though it does accurately point out that twins may have different health concerns).

twin facts, fraternal twins, identical twins

Another Health Chicago ad featuring the author and her twin sister. Courtesy of Joy Thomas

Growing up, my twin sister and I were frequently asked certain questions to which we had ready replies:

“Do you feel each other’s pain?”

“Yes, when we pinch each other.”

“Do your parents ever mix you up?”

“Only over the phone and from the side and behind.”

“Do you ever trade places?”

“Once in second grade. For five minutes.”

“Who’s the evil one?”

“She is!”

In a very unscientific survey (aka a post in an identical twin Facebook group), fellow twins reported being asked similar questions and added other familiar ones:

“If you’re identical, why are your hairstyles different?”

“Why aren’t you dressed alike?”

“Who’s the louder/nicer/funnier/smarter twin?”

“Who’s the oldest/youngest?”

“Are there other twins in your family?”

twin facts, fraternal twins, identical twins

The author and her twin sister. Courtesy of Joy Thomas

Some of these are reasonable questions, and the answers often vary from one set of twins to another, but plenty of studies on twins offer factual insight.

Here are some common, widespread beliefs about twins, corrected when necessary with twin facts based on research.

1. Twins have the same preferences.


According to studies of identical twins (monozygotic twins, from the same egg with the same DNA) raised apart, 50% were found to have identical interests despite being raised in different environments. Yet even identical twins raised in the same households do develop different personalities and interests. Researchers say this is because part of our brain development is based on our individual experiences. So even though twins have the same genetic makeup, the genes are expressed differently. Geneticists refer to a subfield known as epigenetics to explain the differences. While identical twins’ gene sequencing is the same, there are structures attached to the genes that differ. These structures change as they are affected by the environment over time. Some of this occurs in utero, with factors such as one twin’s closer proximity to the mother’s placenta, influencing the infant’s hormone levels and even weight and height. This may explain differences in a variety of preferences, and even different sexual orientations despite shared genes (though research shows that if one twin is gay, the likelihood that their twin is gay increases).

2. Twins are best friends.

“You’re so lucky! You have a built-in playmate,” people often remark to twins. While it’s true there is someone your age to spend time with, it’s similar to regular siblings in that there is also rivalry. Twins often struggle with comparison and finding their own identity, especially during adolescence. But most adult twins learn to work out their differences. According to twin expert Dr. Nancy Segal, twins tend to be extremely close. In a National Geographic interview, Dr. Segal referenced a study she did on people who had lost a twin. Using a grief-intensity scale, she found that the grief associated with losing a twin was equivalent to that of losing a spouse.

3. Twins are so connected that they can feel one another’s pain.

When my twin sister was in labor, I woke up around 4:00 a.m. with a sharp stomach pain and received a text shortly after, saying that my sister had just delivered her baby. Since I knew she was in labor earlier that night, was it my mind inducing the stomach pain, or was it the legendary shared twin pain? While many twins report similar stories, shared pain has yet to be confirmed, despite countless studies that have attempted to prove it. This belief continues to be one that people love to believe, along with mental telepathy between twins, which also has not been validated in conducted studies.

twin facts, fraternal twins, identical twins

Courtesy of Joy Thomas

4. There is a “good twin” and a “bad twin.”

This is a very Hollywood-invoked idea. And like many Hollywood portrayals, this one has very little merit. Since twins are complex, whole people — not two halves of a whole — both individuals come with their own sets of positive traits and flaws.

5. Twins are rare.

Yes and no. In Western nations, about one in 80 births are twins, and only one-third of those are identical twins. Fraternal twins are on the rise in the U.S. and other countries where assistive reproductive technology is increasing. From 1980 to 2009, the rate of twin births increased 76%, to an astounding one in 30 births in the U.S. Interestingly, fraternal twins can run in families and are more prevalent in certain parts of the world, such as Central African nations, while Latin America and Asia are least prone to twinning.

twin facts, fraternal twins, identical twins

Courtesy of Joy Thomas

So statistically speaking, most beliefs about twins are sprinkled with fact and fiction. As an identical twin, I understand that the similarities and differences among twins are as complex as the genome patterns that make up our DNA. In line with current research, my sister and I seem to have more similarities than mere siblings do, yet we’re far from mirror images. As the data confirms, there’s no such thing as a clone. We are all individuals. And while my sister and I may have fun dressing alike on occasion, anyone who knows us well can tell us apart in an instant.

It turns out we like many of the same clothes, most of the same foods, but definitely not the same boys. end



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