How Twins Help Us Understand Nature and Nurture



As identical twins, 19-month-old Sol and Luna share 100 percent of the same DNA. Both girls have soft oval faces, tufts of curly brown hair, slight noses, and large almond-shaped eyes. If Sol’s coat is taken off, Luna will notice, and she will eagerly mimic for hers to be removed. If one is dancing, the other jumps in without hesitation, and if one drops her cup you can count on the other to follow posthaste — neither likes to be left out of a potentially fun situation.

Their mother, Grace Castillo, says they have different personalities. “Sol is adventurous,” says Grace. “She was the first to reach milestones and is much more of a troublemaker. She doesn’t like to be smothered, and only will offer affection on her terms. Luna, on the other hand, is more sensitive. She’ll cry when Sol gets in trouble, and she is oddly more independent in public, showing less fear of strangers.”

Castillo admits that although she knows that both girls are developing noticeably different personalities, she still believes that genes are playing a major role in how each child’s behavior is progressing. Especially when she compares their behavior to her own when she was their age. “It has to come from somewhere, and genetic inheritance makes sense when your child starts doing things you used to do at that age.”

When researchers seek to determine how personalities are developed, many look to twin studies to see how relative factors such as nature (genes) and nurture (environment) influence or dictate the way behavior and traits are established. The case centers on the dispute of what exactly drives us to be us. Twins offer a glimpse into shared family alikeness and allow scientists to decipher to what extent similarity is caused by genes and how much of it is shared environment. Identical twins demonstrate that while an individual might share the same physical characteristics as another person, many other contributing factors are in motion when uncovering the complexities of human behavior.

In the 1960s, child psychologists Dr. Peter Neubauer and his colleague Dr. Viola Bernard, also a child psychologist and consultant to the Louise Wise adoption agency in New York, sought to find an answer to the impasse of the “nature versus nurture” theory. Both doctors strongly believed that twins should be raised separately and used the adoption agency to conduct long-term studies of twins and triplets in order to see how each individual would develop without the interference of being treated exactly the same, by the same guardians.

“Neubauer believed at the time that twins posed such a burden to parents — and to themselves in the form of certain developmental hazards — that adopted twins were better off being raised separately, with no knowledge of their twinship,” writes Lawrence Wright, author of “Twins: And What They Tell Us About Who We Are.” “Here was an opportunity to look at twins from the moment they were separated, and to trace them through childhood, observing at each stage of development the parallel or diverging courses of their lives.”

This is why some researchers study twins that have been adopted into separate homes. Adoption studies help scientists see the impact of two identical individuals raised in different environments. Just because twins share the same DNA, will they both automatically be as studious as their mothers or as gregarious as their fathers? Are we born with a blank slate that is only molded through environment, or do genes decided what type of person we will ultimately become?

It would not be until decades later that a set of twins from the study would finally be reunited. Remarkably, both reported sharing strikingly similar traits that could have only been passed down through genetic inheritance. Both recounted childhood habits of sucking the same finger, and both felt that they had the same basic personality. “Despite the difference in their environment, their pathology was fundamentally the same,” writes Wright.

However, because of the radical nature of the twin-separation study and the backlash received from critics, Neubauer never completed his study and did not publish any of his findings. Any results that were theoretically discovered were filed at Yale University, to remain sealed until 2065.

Dr. Scott Barry Kaufman, a cognitive psychologist and popular science writer, says that both genetics and environment need to be considered when trying to understand behavioral development. “There always is a dynamic interplay between our genes and our environment. Genes aren’t deterministic things; they are entirely there to help guide us to choice. If our genes didn’t influence us at all and we were completely at the whim of external forces, we would never make any of our own decisions, we would never have any of our own inclinations.”

This idea is important when researchers look at whether certain disorders like alcoholism and mental illness are heritable, because both genetics and environment affect and alter the synaptic mapping and organization of the brain. Twin studies help decide the impact of predisposition and determinism. “Even if you have the genes that dispose you to being an alcoholic, but you never in your entire life encounter alcohol, you will not become an alcoholic,” says Kaufman.

However, the variance in some twins is so profound that many researchers are pointing to a third element at work: epigenetics, the study of inherited changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence. For example, one can be influenced by lifestyle and diet. Some researchers are also looking at epigenetics to explain exactly why some twins start out with the same characteristics but dramatically diverge as they age; and perhaps why one twin might suffer from an illness while the other is not affected.

In a study published in the Proceedings of the National Academy of Sciences, an international research team lead by Dr. Manel Esteller of the Spanish National Cancer Center in Madrid, studied 80 pairs of identical twins ranging in age from 3 to 74. They discovered that 35 percent of twins revealed a difference in genes, especially the twins who were older. The study suggests that genes are regulated by epigenetics, and that as twins aged their DNA also changed. Researchers believe that such external factors as smoking, drinking and eating habits could dramatically change certain expression of genes by causing them to become irregular. This discovery has led some scientist to believe that we are more responsible for certain illness than once was thought.

While the debate of what makes us who we are is still undecided, researchers lean to the idea that all external and internal influences encourage and affect our behavior. And, because of twins, science is taking closer steps in uncovering the truth of human behavior. “No longer is it nature vs. nurture, but nature via nurture,” writes Dr. Matt Ridley in “The Agile Gene: How Nature Turns on Nurture.” “You will have to enter a world were your genes are not puppet master pulling the strings of your behavior but puppets at the mercy of your behavior; a world where instinct is not the opposite of learning, where environmental influences are sometimes less reversible than genetic ones, and where nature is designed for nurture.”

This article is updated from its initial publication in Brain World Magazine’s print edition.

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