Twin Studies: A Scienitific Approach to Learning What Makes Us Who We Are

Siblings share half of their genes, and for the most part, their upbringing is very much the same as well (Roderick). It would seem that with so much in common, brothers and/or sisters would find themselves to be very alike. Often, though, that is not that case. What accounts for that? Why am I so different from my sister, yet so very similar? How can our opinions and lifestyles contrast so greatly, while our interests and talents remain the same? Were we always so dissimilar, or did something spark a change in direction along the path of our youth? Did I even have control over the changes? Many people have pondered such questions over the years, and their curiosity has spawned some very interesting research.

Why are we the way we are? Is it because of genetics, or is it because of our environment and the way we were raised? This issue is often referred to as the Nature/Nurture Debate. The term was originally coined in the early twenties by Sir Francis Galton when he snatched the phrase “nature/nurture” from Shakespeare’s The Tempest, but the question of heredity vs. surroundings is something that has piqued our curiosity for some time. A cousin of Charles Darwin, Galton conducted most of his research in an attempt to prove that nature (or heredity) was the key factor in shaping humanity. To help prove his theories more quickly, he decided to study twins, something scientists hadn’t yet incorporated in the pursuit of unveiling the source of our individuality (Berreby). Although Galton wasn’t able to prove whether we are a solid result of genetic code or of a more moldable design, he made many contributions to science-twin studies are still used today.

Carol Cooper, the author of Twins and Multiple Births, tells us that only one in seventy births brings a set of twins, and only one third of those are identical twins, so the opportunities for studying identical twins are prized (24). The book also includes information about how the different types of twins come about. She writes, “Identical twins, or monozygotic twins, develop from the splitting of a single egg, and are always the same sex. Non-identical twins, also known as fraternal twins or dizygotic twins, are about twice as common as identical twins. They are no more alike or dissimilar in looks and personalities than any other two siblings. They are conceived by the union of two different sperm with two separate eggs” (27). The book also states that dizygotic twins could have separate fathers or even be conceived on different days. If parents are of a mixed race, it is possible to have dizygotic twins of different races (28).

Identical twins are understandably better candidates for twin studies, because although fraternal twins and identical twins ideally have very similar environments, fraternal twins only share half of their genes. Fraternal twins are as different from each other as any non-twin siblings, except for the fact that they are closer in age, which suggests that their environments will be slightly more alike. Identical twins share their genes completely, so any difference in them is entirely due to variances in environment. In an article for Psychology Today, Angle Roderick says of identical twins: “They are nature’s handmade clones, doppelgangers moving in synchrony through circumstances that are often eerily similar, as if they were unwitting dancers choreographed by genes or fate or God, thinking each other’s thoughts, wearing each other’s clothes, exhibiting the same quirks and odd habits. They leave us to wonder about our own uniqueness and loneliness, and whether it’s possible to inhabit another person’s being.”

The ability to work with two people that are physical replicas of each other has begun to answer many questions about the impact of our surroundings. Those rare (and unfortunate) occasions in which identical twins are separated at birth and raised in different families also provide scientists with clues, perhaps even more so than the more common occurrences of identical twins raised together. In different ways, researching twins of either kind has helped us to make progress. In the past, twins were considered to be a symbol of good luck. We may no longer see them as such, as least not in the superstitious sense, but at least as far as science is concerned, they’re bringing luck in plentitudes.

We may never know for sure which percentage of what gene contributes to what personality trait, or exactly how intelligent a child will be based on his/her parent’s IQ scores. We will probably never be able to predict, in detail, what will become of a child merely by knowing who his/ her parents are and what type of environment he/she will be raised it. In some ways, that would take away the mystery of humanity, and maybe rob us of our impressions of self. We are learning something, though, and that age-old question of Nature vs. Nurture is unfolding in a way it never has before. Studies have been done on many aspects of what humans consider to be part of their individuality, from intelligence to psychological disorders, physical attributes, and even personality traits. We’ll take a look at the results of those studies, as well as what they imply concerning the debate of Nature or Nurture.

According to Don Hockenbury, co-writer of the Discovering Psychology textbook, identical twins that were raised together almost always have similar IQ scores. Interestingly enough, even identical twins that were raised apart appear to be closer in intelligence than fraternal twins that were raised together. This information would suggest that heredity is the strongest factor in terms of determining levels of intelligence, but further research implies that environment plays a role as well. The identical twins that were raised together were still closer in intelligence than those that were not, indicating that a shared childhood, too, has an impact on intellect. The study also revealed that fraternal twins raised together were more alike than regular siblings raised together. This is because although fraternal twins share no more of their genetic makeup than other siblings, the fact that they are the same age usually results in closer experience in life. The conclusion of this particular study was that in matters of intelligence, environment and heredity were equally responsible (266).

The same author (Hockenbuy) gathered data on the results of tests that investigated the role of heredity and environment on psychological disorders. These findings were slightly different from those of the studies on intelligence. It was discovered that major depression and bipolar disorder were two of the most common disorders acquired by genes. If one identical twin has bipolar disorder or major depression, there is a seventy percent chance that the other identical twin will have it as well. That remains true for identical twins that were raised together and those that were not. Schizophrenia is another disorder that seems to be largely genetic. With schizophrenia, there is a forty-eight percent chance that both identical twins will receive it, a chance greater than if both parents had the disease. Non-twin siblings only have a nine percent chance of sharing schizophrenia. Unlike intelligence, it seems that psychological disorders are mostly a result of genetics (Hockenbury 491).

Most physical attributes are a result of genes. We’ve known for some time that eye color, facial features and body type are mostly determined by the biological parents. Even aside from the obvious differences twins might have, like weight and scars and other marks acquired over time, it seems that environment has a role in appearance. Sharon Blake wrote in an article about differences in twins, “The average difference in height between one identical twin and his or her co-twin was less than an inch. The average difference in height between fraternal twins was 2.3 inches.” The differences between identical twins are always due to environmental influence, because genetically, they do not differ at all. Blake goes on to say, “Although identical twins have the same genotype, or DNA, they have different phenotypes, meaning that the same DNA is expressed in different ways. Traits determined by phenotype, such as fingerprints and physical appearance, are the result of the interaction of the individual’s genes and the developmental environment in the uterus. Thus, a DNA test can’t determine the difference between identical twins, while a simple fingerprint can.”

Perhaps the most interesting bits of information gathered by twin studies are the answers to questions posed by the study of twins in regards to personality. Don Hockenbury writes, “Evidence gathered from twin studies and adoption studies show that certain personality traits are substantially influenced by genes.” He says that the “Big Five” or the “Five Factor Model of Personality” is considered to be one of the most universal ways of approaching personality differences and similarities in psychology. The traits measure by the Big Five are neuroticism, extroversion, openness to experience, agreeableness, and conscientiousness. Of those five traits, extroversion and neuroticism were found to be mostly influenced by genetics, and only agreeableness proved to be mostly influenced by environment. That’s not to say that one’s personality is predetermined. The same book also says, “Behavioral variability is due at least as much to environment as to heredity.” By that he means this: As a whole, our surroundings have just as much of an effect on our personality as our genes do, if not more (Hockenbury 395).

Despite the results of studies that point to environment as the more dominate factor in determining various aspects of individuality, there have been some almost unnerving incidents of twins that were separated at birth, exposed to very different circumstances, and still were alike in more ways than most siblings. Those cases open up questions that go beyond genetic similarities, even. Usually, both of the twins felt that they were missing something throughout their lives-as if they themselves were not complete. It is difficult for science to explain these occurrences, but we can certainly study them to obtain even more answers about what makes us, and what makes us unique.

As of November of 2002, only nineteen cases of twins being reunited after separation at birth have been reported. One of the most famous of such cases is the “Jim Twins.” Adopted into separate families, Jim Springer and Jim Lewis each spent 39 years feeling like something was missing in their lives-something that they couldn’t clearly define. In 1979, that definition became abundantly clear, when for the first time, they were discovered each other and stood face to face, seeing in each other what most of us can only glimpse in a mirror. As if the discovery of an identical twin brother weren’t enough, more uncanny similarities began to reveal themselves.

Aside from the fact that each of their adoptive parents had named them “Jim” (a coincidence that was clearly out of the twins’ control), they uncovered a much more bizarre parallel in names. As they were growing up, they apparently both had dogs named Toy. Both Jims had been married twice, and their first wives had each been named Linda. Their second wives were both named Betty, and both Jims had sons that they named James. Jim Springer chose the middle name Allen for his son, while Jim Lewis opted for the variant in spelling-Alan (MacLeod). Aside from the names, other correlations arose. Both of the men enjoyed drinking Miller Lite, and they both smoked Salem cigarettes. Both men drove Chevrolets. They both enjoyed carpentry, and had designed and built nearly identical benches around trees in the back yards of their houses. They both chewed their fingernails, and spent time as sheriff’s deputies. Neither of them excelled as students in high school. They both voted for the same presidential candidate in the last three elections. They both had vasectomies, and both Jims left love notes around the house for their wives, Betty (MacLeod).

The “Jim Twins” raise more questions about the influence of our genes, but they are a very unique case. Another set of reunited twins has been interviewed thoroughly, and apparently have only about as much in common as one might imagine. Adriana Scott and Tamara Rabi were also adopted by different families and were lucky enough to discover each other at the age of twenty-one, through friends of friends that said they bore a shocking resemblance to one another. They are identical in height and appearance, but their preferences in things like food and clothing vary greatly, even contrasting in some ways (Pressman). With the instances of twins reunited after separation at birth being so rare, it is a limited way to study the issue of nature vs. nurture, but it is certainly one of the most intriguing.

So what makes us unique? Is it nature, nurture, or both? The evidence points to both, and not always in equal measure. From the early moments of conception, we are granted potential to grow in certain ways, and in the years that follow, we are shaped by our lives as we shape our lives. It is interesting to consider the power of the world around us, and to think about what pieces of us it has affected. It is also interesting to think about the influence of genetics, about just how much of us was predetermined before birth. We can only become exactly the way we are by our specific set of genes coming together and experiencing the precise sequence of events that make up our lives. Identical twin or not, no one is quite like us. At the same time, our genes, our blueprints, connect us to our parents, our siblings and our children. We’re cut from different cloth, but sewn to the quilt that is our family. Each of us represents a unique pattern, boasting of our individual textures. Stringing together the colorful patchwork are the threads that unify us, stitches of kinship entwining those pieces of distinct fabric. Comforting, it is.

Works Cited

Angle, Roderick, and Jill Neimark. “Nature’s Clones.” August 1997. 24 October 2003. .

Berreby, David. “Multiplicity.” 02 March 1997. 24 October 2003. www.slate.msn.com>.

Blake, Sharon S. “A Groundbreaking Resource for Genetic Research.” The University of Pittsburg. 24 October 2003. .

Cooper, Carol. Twins and Multiple Births: The Essential Parenting Guide from Birth to Adulthood. London: Vermilion, 1997. 24+

Hockenbury, Don, and Sandra Hockenbury. Discovering Psychology. 2nd ed. New York: Worth, 2001. 266+

MacLeod, Scott. “Jim Twins.” 09 November 2002. 24 October 2003. .

Pressman, Gabe. “Twins Tamara Rabi And Adriana Scott With Their Mothers.” 09 March 2003. 24 October 2003. .