Twins and Nature vs. Nurture

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What is Nature vs. Nurture?

Nature vs. Nurture is the debate that seeks to answer which component–genes or the environment–has a greater influence on the development of an individual’s personality. This article will explore aspects of nature and nurture, how twins studies are used to differentiate the two, and philosophical implications of the debate.

DNA is the blueprint for you. Information-bearing segments of DNA are called genes, and these will determine your gender, the color of your eyes and hair, everything about you down to the tiniest detail. The genes you inherit from your parents are your nature–your genes are, and they are largely unchangeable.

Nurture, on the other hand, refers to the environment in which you were raised, but it also covers any factor that is not genetic. For example, the release of new Grand Theft Auto titles is always accompanied by calls for bans by parents worried that the game will negatively influence their children’s behavior. In this case, the video game is in the child’s environment. Other environmental factors include where and with whom a child grew up, how they were treated in school, even what books they read and movies they watched.

How Twins Contribute to the Debate

Twins come in two varieties: fraternal and identical. Fraternal twins arise when two separate eggs are fertilized by two different sperm simultaneously. According to Mendel’s Law of Segregation and Law of Independent Assortment, no two eggs or sperm are completely alike in genetic makeup; therefore, fraternal twins are just as likely to share the same amount of genetic material as siblings born from different pregnancies–about 50%–but just happen to share a womb.

On the other hand, identical twins arise from one egg fertilized by one sperm. During embryogenesis, the single fertilized zygote splits into two zygotes, and both continue forming embryos. Identical twins share 100% of their genetic material because each resulted from the same egg-sperm union.

Twins are natural experimental models, because each set of comes with its own control. Identical twins share 100% of their genetic material, i.e. they have the same nature, so any differences between the two in personality can be attributed to nurture. This is especially evident when identical twins are separated early on in life and raised in different environments. The situation gets more complicated when identical twins are raised in the same environment, yet have different personalities.

Fraternal twins share roughly 50% of their genetic material. If they are raised in the same environment, any differences in their personalities can be attributed to differences in their genes. However, fraternal twin studies are best carried out when they are raised in the same environment, so as not to introduce more variables.

Too Simple an Argument?

An interesting case to consider is that of Brian Dugan, a man who admitted to abducting, raping, and killing a 10-year-old girl in 1983. His defense lawyers used brain scans and the testimony of prominent neuroscientists to argue that Dugan’s brain had been hardwired to commit violent acts, that it was in his nature to kill and he was unable to control his behavior. This argument falls in sharp contrast to the idea on which laws are based, that all people have free will and can choose whether or not to follow the law. The environment may influence the ease of that choice–a starving man might have little regard for laws against theft if it means he can eat–but ultimately the individual has the ability to choose. To say that nature has the most influence in determining personality, then, provides a scapegoat for a person when he does something considered unlawful or morally unacceptable: I couldn’t help it, it’s in my nature. Like an animal, such an individual would exist outside the realm of right or wrong, innocent or guilty, because he would be acting purely on instinct. How could that person justifiably be punished? The converse, saying nurture has the most influence in determining personality, similarly frees a person from accountability for his actions by placing blame on those who raised him.

The sci-fi film Gattaca also addresses the danger of relying too much on nature in determining a person’s ability or worth. The world of Gattaca creates a culture obsessed with genes. Individuals who were not genetically engineered are called “degenerates,” relegated to custodial work and other blue-collar jobs, and are the first suspected when laws are broken. The protagonist, a degenerate, is forced to cloak himself in the genetic makeup of another man in order to follow the dreams that would otherwise be out of reach. The film’s theme, as well as its tagline, is that “there is no gene for the human spirit.”


Nature and nurture cannot be pitted against each other; it is impossible to separate the two. An individual cannot be separated from who he is, and the lack of an environment is still an environment. Psychologist Donald Hebb illustrates this point by asking, “which contributes more to the area of a rectangle, its length or its width?” Few things in science are so simplistic as to be solely determined by one component.