In this blog post, we explore the debate over whether human behavior and personality are determined by genes or shaped by the environment.
For hundreds of years, across both the East and West, humanity has engaged in a constant debate over whether humans are determined by nature—that is, by their genes—or shaped by nurture—the environment of upbringing, including experiences and education. “Nature” is represented by genetic determinism, a view that holds that the behavior of not only humans but all organisms is inevitably determined by a combination of genes. Simply put, this position holds that the behaviors humans exhibit from birth are determined by chromosomes and DNA and are already predetermined at birth. On the other hand, “nurture” is represented by environmental determinism, which, contrary to genetic determinism, holds that humans are “shaped” by their surroundings after birth. For example, it views education as a key factor in shaping human development. The key point is that neither genetics nor the environment determines everything 100%; rather, they play a leading role in a ratio of roughly 80:20.
In the class-based societies of the past, genetic determinists held an overwhelming advantage because individuals could not escape the class into which they were born; however, since we no longer live in such a class-based society, the conflict between the two sides is now evenly balanced. This debate over “nature versus nurture” has intensified as science has advanced. In the 1800s, with the emergence of Charles Darwin’s theory of evolution and the eugenics of his cousin Francis Galton, genetic determinism gained even stronger support. The universality of animals, humans, and humankind that Darwin researched and demonstrated allowed us to surmise, albeit faintly, that specific genes play a role in making animals animal-like and humans human-like. However, with the emergence of Pavlov’s theory of conditioned reflexes in the 1920s, the claim that human personality could be altered through training alone gained traction. As eugenics was used to justify Hitler’s genocide of the Jews, it became a source of trauma within the scientific community, leading to a renewed focus on environmental determinism.
Even when faced with the same experimental results, genetic determinists and environmental determinists offer diverse interpretations. In a 1997 experiment conducted by Professor Michael Minney in Canada, pups born to two mice with different temperaments were swapped and raised by the other mother. The results showed that the pups adopted the temperament of the foster mother rather than that of their biological mother. In response to these results, environmental determinists emphasized the influence of the foster mother, arguing that the environment plays a more dominant role than genes. However, genetic determinists explained that the mice did not change simply due to the influence of the foster mother, but rather that specific genes were expressed depending on the environment.
However, by the end of the 20th century, an event occurred that put an end to this debate. Advances in science and technology made it possible to analyze DNA, providing genetic determinists with theoretically sound evidence. Phenomena that had previously been analyzed inductively could now be interpreted on a theoretical basis. In particular, the analysis of human DNA provided decisive support for genetic determinism, with twin studies being a prime example. Twins are classified as monozygotic and dizygotic. Identical twins are formed when a single egg and a single sperm are fertilized, and the resulting zygote splits into two; they are composed of cells with identical chromosome pairs, and their DNA sequences match. In contrast, fraternal twins are formed when two eggs are fertilized by different sperm; their chromosomes and DNA sequences do not match, and they may even be of different sexes. Simply put, fraternal twins are siblings born at the same time.
A notable recent example in twin research is the story of French fashion designer Anaïs Bordier from February 2013. After hearing from a friend that there was an actress in Los Angeles who looked like her, Bordier discovered that she and the actress, Samantha Futterman, were identical twins who even shared the same birthday. It is remarkable that, despite growing up in completely different environments in geographically distant locations—France and the United States (more than 8,000 km apart)—without ever knowing each other, the two shared very similar traits, such as digestive issues and eating habits, and both pursued careers in the arts. There is also the case of the “Jim” twins, studied by Thomas Butcher in the United States. They, too, were adopted into different families within weeks of birth and reunited at age 40; reportedly, they shared not only medical histories but even the habit of biting their nails. Their gardens, which reflected their professions and personal tastes, as well as their family names and even the names of their pet dogs, were identical. This is a remarkable example of how twins with identical genes share even their most personal preferences.
However, the first factor to cast doubt on genetic determinism—which had received strong support from twin studies—is the “Flynn effect.” The Flynn effect is a theory stating that the average IQ has risen steadily by at least 5 points every 10 years, attributing this to advances in education and improvements in upbringing and environmental conditions. However, the Flynn effect is insufficient to refute genetic determinism. Genetic determinism also acknowledges the influence of the environment to some extent, while asserting that genetic factors play a leading role. Furthermore, the IQ scores cited in the Flynn effect vary slightly depending on the time period, and even when measured using the same IQ test standards, IQ cannot increase indefinitely—there is an upper limit. Therefore, the Flynn effect is not sufficient evidence to refute genetic determinism.
The second factor is, paradoxically, the “Human Genome Project,” which began in the 1990s. Given the diversity of human behavioral patterns, it was expected that there would be approximately 100,000 genes, but the results revealed in 2001 showed there were only about 30,000. In response, Craig Venter, a key figure in the project, stated, “Genes alone are woefully insufficient to explain human beings.” However, another key figure, Dr. John Sulston, countered that “flipping a coin 33 times yields over 100 billion possible outcomes,” arguing that genetic determinism had not suffered a significant blow. For example, even if human behavioral patterns were determined by just two genes, the combination of 30,000 genes could still produce an infinite variety of behavioral patterns.
Finally, the effectiveness of education is a classic counterargument used by environmental determinists. They argue that humans are beings who actively evolve by changing their environment through education. However, this claim actually exposes a fundamental contradiction in their own theory of education. If humans are born as a blank slate, education would be impossible, and this fails to explain human evolution. Furthermore, it fails to explain human tastes or preferences—that is, “free will”—which lead to different outcomes under identical conditions. This is a phenomenon that cannot be explained by education alone.
Matt Ridley, author of *Nature vs. Nurture* and a renowned science writer, proposed a new framework called “nature via nurture” to transcend the nature-versus-nurture debate. However, the debate between genetic determinism and environmental determinism will likely continue. While environmental determinism has provided visible evidence both in the past and present, research on human genes—the basis of genetic determinism—has only recently begun; as research deepens, its evidence becomes increasingly solid. Furthermore, since the experimental and observational results presented by environmental determinism are ultimately based on genes, it can be said that “nature” reveals its identity through “nurture.” Genes are activated through the environment, and the distinction between “nature” and “nurture” may become meaningless.
It is ironic that all of this discussion ultimately negates the existence of true human free will. The interaction between the absolute control of genes and the external factors of the environment leads to the conclusion that human choices are predictable. However, advances in genetic research and psychology will deepen our understanding of human life and behavior, which will contribute to humans’ ability to recognize and develop their own free will.
