This article first appeared in Issue 10 of our free digital magazine CURIOUS.
Speak to anyone with siblings and they’ll likely have a rich backlog of dumbassery they’ve witnessed in the company of their brothers or sisters. The quirks of genetics mean that sharing the same parents isn’t enough to guarantee any morphological consistency among siblings, and personalities can be even more wildly disparate. That all changes, however, in the case of identical twins.
Monozygotic twins are the only “identical” twins but research has found that even among these siblings there can be some differences. While some identical twins may look, sound, or behave a little differently, others seem to share a unique connectedness that extends beyond our understanding.
“The Twin Thing" is a term often given to twins who share these kinds of special relationships. They may be able to finish each other’s sentences to a degree that has you questioning if they’re reading minds, or they might have a way of knowing when something’s happened to their twin without being there. The anecdotal evidence of twin telepathy is all over the Internet, but has science found anything to explain, confirm, or deny the existence of this kind of twinship?
The cracking of eggs
It’s first important to note that not all twins are created equal, and by that we’re not implying a twin hierarchy but stating the fact that different twin types involve a different number of sperm and egg cells. In the case of what’s known as identical or monozygotic twins, one fertilized egg cleaves in two at a very early stage of development to create two embryos. These babies are born looking very similar to one another on account of sharing the same genomes.
Non-identical twins are known as dizygotic because they are created from separate egg cells getting fertilized by separate sperm cells at the same time in the same uterus. Typically, a human female only releases one egg per menstrual cycle, but this isn’t a hard and fast rule. Hyperovulation can happen naturally or be brought on by fertility drugs (something that’s standard practice during in-vitro fertilization), giving sperm more than one target to aim for.
We know that men produce between 2 and 5 milliliters of semen per ejaculation event. With around 100 million sperm believed to be in each ml, it’s easy to see how those extra eggs have a shot at fertilization.
As fans of This Is Us may recall, The Twin Thing isn’t exclusive to monozygotic twins (that is, as a self-reported experience rather than a proven scientific fact). The old “nature versus nurture” debate comes into play here, as non-identical twins who shared a childhood but not an egg may have more personality-based similarities than a set of monozygotic twins who were separated at birth (hello, The Parent Trap?).
What do the twins say?
There are remarkable stories of twins sharing a connectedness that seems to defy science’s understanding of the senses. Even British Olympic athlete Mo Farah shared in his autobiography Twin Ambitions: “Whenever Hassan is upset, or not feeling well, I’ll somehow sense it. The same is true for Hassan when it comes to sensing how I feel. He’ll just know when something isn’t right with me. Then he’ll pick up the phone and call me, ask how I am. Or I’ll call him.”
IFLScience Editor Laura Simmons’ grandmother, Joyce Siney, is an identical twin. Now in her 70s, Siney and her sister Shirley have gone through their lives sharing identical genomes.
“We've not had a time where we've not been together,” Siney told IFLScience. “We've never been separated by more than maybe 10 miles [16 kilometers].”
The twins have similar interests, including church activities, knitting, and sewing, and both pursued careers in nursing. However, when asked about The Twin Thing, Joyce’s experiences raised a pivotal question about the theory. “Maybe if you asked Shirley a question, and then you asked me, I’d give the same answer… but I’m not sure that’s telepathy.”
As it turns out, science isn’t sure either.
What does the research say?
To explore the concept of twin telepathy, researchers are faced with the challenge of coming up with a study design that can definitively test for exactly that. “Telepathy” specifically refers to the exchange of thoughts and ideas through extrasensory means, so not using the known senses of sight, sound, touch etc.
A 2013 pilot study tried to settle on an objective method for investigating this kind of extrasensory connectedness using four pairs of monozygotic twins. They were separated and then exposed to surprising stimuli to see if the other twin could pick up on anything. It used five-minute windows to apply the stimuli, ranging from a jump-scare like dropping plates to touching something painfully hot, and considered it a positive synchronous response if the other twin reacted within 15 seconds.
Their results were of “marginal statistical significance,” wrote the authors, “but the outstanding performance of one of four pairs of identical twins is cause for further interest.” A similar result came from a Copenhagen study that used a similar methodology and also found that only one of the four pairs of monozygotic twins appeared to show significant signs of connectedness.
Interestingly, there was something unique about the “outstanding performance” twin pair in the 2013 pilot study: “The pair of twins who produced most of the hits in this series was the youngest in the sample (25 years), and one of them was 7 months pregnant. In the interview, the nonpregnant twin told how they had led independent lives but her belief in twin telepathy had suddenly increased because of her remarkable sensitivity to her twin sister’s state during pregnancy.”
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Synchronous experiences have led some scientists to describe twin telepathy as an analogy of macro-entanglement. You may be more familiar with the phrase quantum entanglement, which maintains a connection between separated particles to the extent that if you change one you’ll change the other, even if they’re far apart.
A more tangible type of entanglement that could contribute to the connectedness of monozygotic twins is the stage at which one egg becomes two. Most identical twins split within four to eight days of fertilization, but for some it happens a few days later. This is something parapsychology writer Guy Lyon Playfair explored in his 2017 paper titled “Monozygotic twins and macro-entanglement” after finding reports that twins that split in the day eight-to-12 window of development show greater connectedness. Could this be linked to the fact that they remain literally entangled until a later stage of pregnancy?
The twin cohort for the Copenhagen study included a pair of female identical twins who Playfair says were born “bound to each other by their umbilical cords and had to be disentangled by hand”. They gave several anecdotes of what they believed to be twin telepathy, including one twin collapsing in severe pain when her twin underwent surgery for kidney stones. They also happened to be the best performers in the synchronous response experiment.
University of Greenwich twin studies investigator Göran Brusewitz, whose research interests include parapsychology, tackled the concept of twin attachment in a 2019 thesis. It defines incidences of twin telepathy or remote emotion sensing (picking up on the state of a twin from a distance) as “exceptional experiences”, sifting through the research and studies that have investigated their frequency among different twin types.
There are notable mentions for both sides of the argument, but one includes a King’s College survey that found almost half of twins studied (46 percent) denied having any ability to sense how their twin was feeling when they weren’t around. Identical twins were twice as likely to say they could compared to non-identical twins, but this still represented the minority of responses.
Twin telepathy, a special relationship, or just similar ideas?
Professor Tim Spector started the UK Twin Registry in 1992, creating one of the richest collections of genotypic and phenotypic information on twins worldwide. It hasn’t, however, included much in the way of testing mind reading among twins, instead focusing on more Earthly concepts like “Do our genes tell us how to vote?”
A poll of voting preferences was carried out by the Department of Twin Research, which hosts TwinsUK, to explore how much nature and nurture influence party political alliances in the UK. Unlike the experiments that used just eight participants to search for signs of The Twin Thing, the poll included responses from a large sample of 2,355 mono- and dizygotic twins.
It found that there was a genetic influence for voting preference: when voting Conservative, 57 percent of the voting similarities and differences were linked to genetic effects. This indicates that identical twins are more likely to vote similarly compared to non-identical ones, raising questions as to how far this kind of genetically linked thought processing extends. As Siney put it, simply answering a question with the same answer doesn’t amount to telepathy, so could it come down to genes?
As far as the science is concerned, it seems that for now, the jury’s still out on twin telepathy and the mechanisms that could explain The Twin Thing. But as far as special relationships go? We’d say Joyce and Shirley are all the evidence you need.
“All I would say is I thank God that I am a twin, it’s just been wonderful,” Siney told IFLScience. “I think it’s a very special relationship. Ours is, anyway.”