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Twinning: How twins can help unlock the mysteries of the mind

Published on Psychology Today

My friends Karen and Kelly are identical twins. Needless to say, they look a lot alike, both with dirty blond hair and friendly blue eyes. But they also act alike; they are voracious readers of mostly non-fiction, they laugh at the same jokes, they are both vegetarians, they both have doctoral degrees, they are both runners, hikers, and they are both prone to anxiety. However, despite these striking similarities, they are also different people: Karen is a college professor, and Kelly is a physical therapist; Kelly likes camping while Karen doesn’t like to sleep in tents; Karen is enjoys drinking wine, while Kelly doesn’t care much for alcohol.

Seeing Karen and Kelly together is an extraordinary experience. If you’ve ever met or even seen a pair of twins, you would probably agree that they are undeniably special. Indeed, twins have dazzled us since the beginning of time. They held a special status in ancient Greek and Roman mythology, often representing the forces of good and evil. The fabled twins Romulus and Remus were even credited with founding the Roman Empire. But it wasn’t until the late 1800’s that Sir Francis Galton recognized the significance of twins, not for the study of good versus evil, but for the classic question of nature versus nurture. Over the course of the next 150 years, twins have made amazing contributions to what we know about the body and the brain, giving us important insights into who we are and how we get there.

As I’m sure you already know, there are two types of twins: identical and fraternal. Identical twins are born when a single fertilized egg splits in two. That means when a single sperm and a single egg come together—like they would for any other baby—it splits in half, producing two identical babies instead of one. We call them identical twins because they literally share all of the same genes. Scientists also call them monozygotic twins, because they come from a single zygote or embryo. As Karen likes to point out, she and Kelly were once a “single cell.”

Karen and Kelly

Fraternal twins come from two different sperm and two different eggs. Usually, women only release a single egg during each monthly cycle, which is why most mothers get pregnant with one baby at a time. In rare cases, she might release two eggs instead of one, and if they both get fertilized, what results is fraternal, or dizygotic (from two zygotes) twins. Fraternal twins are like siblings in every way except that they share a womb. They have some of the same genes, but only about 50% since they come from two different sperm and two different eggs.

Because both fraternal and identical twins theoretically share the same environment—the same family, the same home, and the same community—looking at the differences between identical and fraternal twins can tell us something about the effects of nature versus nurture on a number of factors. Let’s say you want to know whether eye color is heritable. You would likely find out that in identical twins, there is a 100% chance that they will share the same eye color, but for fraternal twins, it is closer to 70%. Since the percentage is higher in identical than in fraternal twins, this tells you that genetics plays a role (and in this case, a major one) in determining eye color. In the case of IQ, the relationship between IQ in identical twins can be over 80%, where it is closer to 50% in fraternal twins. This tells us that, like eye color, heritability plays an important role in IQ, there’s more room for the environment to play a role as well, since the relationship isn’t 100% in twins who share all of the same genes.

Using this strategy, studies on twins have given us a lot of important information about human behavior, the brain, and the body. For example, twin studies have shown us the approximate heritability of type 1 and type 2 diabetes, schizophrenia, and different types of cancer (e.g., Castillo-Fernandez, Spector, & Bell, 2014). In fact, a group of researchers from Great Britain recently studied twins to find out what the likelihood is that if one twin was diagnosed with autism that the second one would be too. They found that genetic factors (and not vaccines or bad parenting) accounted for the majority of the incidence of autism in their twin-based population.

Despite what we know about the heritability of various illnesses based on twin studies, twin studies have also told us a lot about the impact that the environment can make on who we are. This research has shown that sometimes the heritability of a certain trait or behavior can vary based on variations in the environment in which we grow up. For example, as mentioned above, several twin studies have reported that IQ can be highly heritable, but it turns out that this is only true in certain environments—namely the environments of children who are raised in middle class, or wealthy households. For impoverished families who live in more high-risk neighborhoods, the heritability of IQ is nearly zero (Turkheimer et al., 2003). In other words, if given every opportunity for your genetic endowment to flourish, IQ is mostly based on your genetics. But, if the environment is riskier, any advantages your biology might bestow on you get overwhelmed by the circumstances in which you’re surrounded.

Further, while we assume that both identical and fraternal twins share the same environment, there are always aspects of the environment that are unshared, and these unshared experiences can have a major impact on our behavior. Indeed, even identical twins are never treated in exactly the same way, and don’t have exactly the same experiences. One of them might go to a basketball game with a friend while the other stays home, which piques a new interest in playing basketball for one twin and not the other. One twin might get more positive feedback in school for their math ability, which leads one twin to excel more in math than the other. These are just a few examples, but all of these tiny unshared experiences add up over the course of the lifetime, shaping different individuals with different wants, needs, likes, and dislikes.

There is no better example of the impact of the unshared environment than of conjoined twins. Conjoined twins start out just like identical twins, where a single fertilized egg splits in two. However, for conjoined twins, the egg doesn’t fully separate into two individuals, and instead, remain physically connected, most often at the chest, abdomen or pelvis. Like monozygotic twins, conjoined twins share the same genes, but they also share parts of their bodies, or in essence, their physical and social environments.

Perhaps the most well-known pair of conjoined twins in the mainstream media are Abby and Brittney Hensel. Abby and Brittney were born in 1990, and each has a separate head, heart, lungs, spine, stomach, and spinal cord, but they share two arms, legs, large intestine, bladder and reproductive organs. Given that they share a body, and most importantly, a single pair of arms and legs, they have to coordinate everything they do. In fact, each twin manages only one side of their body, making all movements an amazing act of teamwork, yet they can walk, run, swim, play basketball, and even drive a car. What is most interesting about the Hensel twins is that even with a shared “environment”—or in this case, a shared body—Brittney and Abby are different. They have a seamstress to make clothes for their unique body, each outfit containing separate necklines to emphasize their individuality. One twin would prefer to live in a city, while the other would opt for the calmness of a suburb. Although they both majored in education in college, they each had a different focus. And while they sometimes share meals out of pure convenience, they like different foods, and often prepare themselves different meals.

Altogether, this suggests that while twins may share the same genes, there are parts of their lives that are also unshared, leading to two distinct individuals. Thus, studying twins can tell us about how to predict and potentially treat various genetic illnesses and how our genes might mold our behavior, while at the same time, also shed light on how the environment might work hand in hand with genetic makeup to make us individuals. So the next time you see your twin friends or twin family members, or pass by a set of identical twins on the street, you can not only marvel at their amazing likeness, but also at the extraordinary circumstances that have made them each unique.

Photo by 3194556/Pixabay


Castillo-Fernandez, J. E., Spector, T. D., & Bell, J. T. (2014). Epigenetics of discordant monozygotic twins: implications for disease. Genome medicine, 6(7), 1-16.

Turkheimer, E., Haley, A., Waldron, M., d'Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological science, 14(6), 623-628.


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