In this blog post, we’ll examine the nature versus nurture debate surrounding human behavior and explore a rational perspective on how these two viewpoints interact.
Nature versus Nurture: The Beginnings of an Age-Old Debate
Is human behavior rooted in genetics or the environment? Throughout history, the debate surrounding “nature versus nurture” has persisted for a long time. The more people tried to distinguish nature and nurture as opposing concepts, the more intense the clash between the two positions became. The empiricist philosopher John Locke likened the human mind to a “blank slate,” arguing that humans are born without any innate concepts and that the mind is shaped by the various experiences they encounter during their upbringing. In contrast, Charles Darwin emphasized the universality of human nature in his publication ‘On the Origin of Species’, while Francis Galton went so far as to overemphasize the role of heredity, asserting that “talent runs in the family.”
As such, the conflict surrounding nature versus nurture has at times led to tragic consequences. A prime example is Nazism. The Nazi regime distorted and abused biological concepts of human nature, and this obsession with biological determinism led to eugenics. The “biologically unfit” as defined by eugenicists included Jews, Roma (Gypsies), and Slavic peoples; passive eugenics—which sought to eliminate these groups from society—left behind the tragic consequences of mass murder and ethical dilemmas. Meanwhile, Vladimir Ilyich Lenin’s Marxism, influenced by environmentalism, held that human nature could be shaped. It posited that the human mind is a “blank slate” and that, because human nature is malleable, people are capable of transforming themselves in countless ways. Based on this belief, various attempts were made to reshape the social environment; however, these efforts ran up against the limitations of reality and were ultimately assessed as failed revolutions.
The debate over whether biological determinism or environmental determinism is correct is of little significance. What is far more important is exploring how nature and nurture interact to shape the human mind and behavior. In this article, I will introduce four experiments with different approaches to nature and nurture and present the most reasonable perspective on this long-standing debate.
What Do Twin Studies Reveal?
Eugenics is divided into “passive eugenics,” which seeks to systematically remove biologically unfit individuals—such as those with mental illness, intellectual disabilities, or a criminal history—from society, and “active eugenics,” which aims to increase the number of people with biologically superior traits.
“Twin studies” marked a major turning point in the development of behavioral genetics. This research began when Francis Galton sought to demonstrate that nature has a greater influence than nurture by showing that identical twins exhibit more similar behavioral traits than fraternal twins. His novel insight became the foundation for subsequent research in behavioral genetics and further intensified the debate between environmental determinism and genetic determinism.
Twin studies continued to evolve through the work of various behavioral geneticists, and they garnered even greater attention in 1979 with the introduction of a case involving identical twin men who had been raised apart. Separated immediately after birth and having lived in different environments for about 40 years, the two men were astonishingly similar—not only in their faces and voices but also in their medical histories, lifestyle habits, and even their likes and dislikes. They even had the same names for their pet dogs. This real-life case served as a catalyst for developing more systematic research methods to estimate the influence of genetics by comparing the similarities between monozygotic and dizygotic twins.
In a study examining religious inclinations, the correlation among monozygotic twins raised apart was approximately 62%, whereas that among dizygotic twins was only about 2%. Furthermore, identical twins showed much higher similarity than fraternal twins in areas such as personality, intelligence, and health. This is a representative research finding demonstrating that genetics has a significant influence on human behavior and psychology.
However, this does not mean that the influence of the environment can be ignored. In a study on body weight similarity, the correlation for identical twins raised together was about 80%, whereas for fraternal twins, it was only about 43%. This suggests that genes can have a greater influence than shared environmental factors, such as diet. However, body weight is not determined solely by genes. The trend of people becoming more obese in modern society than in the past is largely influenced by environmental factors such as excessive calorie intake and lack of exercise. On the other hand, genetic factors play a significant role in individual differences—such as why some people gain weight easily while others do not—even within the same environment. In other words, while genetics plays a major role in individual differences in body weight, the environment also has a significant impact on actual weight changes.
Behavioral genetics is the field of study that examines the respective influences of genetics and the environment on human development. Early research focused on identifying genetic predispositions, but it has since evolved to explain the interactions between genetics and the environment.
How does behaviorism explain human behavior?
“The true cause of all behavior lies outside the human being.” This is the claim made by extreme nativists. They believed that innate factors play a very small role in the human mind and that most behavior is shaped by the environment.
John Brothers Watson, a pioneer of behaviorist psychology, conducted an experiment to demonstrate that emotional responses can be formed through conditioning. The subject of the experiment was Albert, an 11-month-old infant who had been raised in a hospital since birth. Watson showed Albert various objects, including a white rat. At first, Albert was not afraid of the white rat at all. However, every time Albert touched the white rat, Watson struck an iron rod forcefully with a sledgehammer, producing a loud noise. After a few days, Albert began to feel fear just at the sight of a white rat. A so-called conditioned fear response had been formed. Subsequently, Albert’s fear spread beyond white rats to include all white, furry objects, such as white rabbits and fur coats.
Based on these results, Watson interpreted conditioning as the core principle by which humans learn about the world. He argued that, regardless of a child’s innate characteristics, various dispositions could be shaped through training and learning alone. Skinner, influenced by Watson, took this a step further by attempting to completely separate psychology from human nature, even going so far as to claim that humans have no instincts. Later, while he acknowledged that innate factors are partially reflected in human behavior, he concluded that these too must be accepted as a matter of fate.
Can human psychology and behavior truly be explained solely by the world experienced through learning, as these scholars claimed? It is not difficult to see that this is not the case. The twin studies examined earlier also showed that various traits, including intelligence, exhibit high heritability. Given these findings, it is difficult to explain human development without considering genetic factors. In some cases, an individual’s nature can have a more significant influence than a shared upbringing.
Ivan Pavlov, an environmental determinist, viewed the brain as acquiring knowledge through conditioning and association, and Watson also understood the brain as a mechanism that learns through conditioning.
A conditioned response refers to a response newly formed in response to a specific stimulus through the conditioning process. In other words, it is a response that emerges through learning in response to a stimulus that was not originally associated with that response.
How do nature and nurture interact?
Nature and nurture are not opposing concepts but rather interactive ones. We will now examine two experiments that clearly illustrate the intersection of these two perspectives.
The first is Francis Daly’s cross-fostering experiment. Using two different strains of mice—Strain A and Strain B—Strain A fertilized eggs were implanted into the uteruses of both Strain A and Strain B mothers, respectively, to allow for birth and rearing. The behavior and abilities of the mice were then compared as they grew.
Mice born within the same strain and reared by the same strain exhibited consistent behavioral characteristics specific to their strain. Mice of Strain A reared by a Strain A mother exhibited behavior similar to that of other Strain A mice. However, mice of Strain A reared by a Strain B mother displayed behavior similar to that of Strain B. In contrast, when mice of Strain A born in a Strain A uterus were reared by Strain B after birth, they retained the characteristics of Strain A while showing distinct differences from Strain B.
This experiment provides important insights into understanding the relationship between nature and nurture. The consistent behavior of mice bred within the same species demonstrates the influence of genetics. Conversely, the results from the cross-fostered mice show that environmental factors—which cannot be explained by genetics alone, such as development during the fetal period and immediately after birth—are also highly significant. Ultimately, this study refutes both extreme “nature” and “nurture” theories and presents a reasonable perspective that both factors interact.
The second example is Minneka’s monkey fear-learning experiment. Monkeys raised in a laboratory are not afraid of snakes, whereas those raised in the wild do fear them. This supports the hypothesis that monkeys learn to fear snakes by observing the behavior of other monkeys.
This hypothesis was confirmed in experiments. When baby monkeys born in a laboratory were placed with a wild mother, the young monkeys, having observed their mother’s fear of snakes, also came to fear snakes.
So what if the object of learning were a flower instead of a snake? To test this, researchers showed the young monkeys manipulated videos depicting monkeys fleeing in fear upon seeing either a snake or a flower. As a result, the young monkeys easily learned to fear snakes but did not learn to fear flowers.
This experiment demonstrates that the influence of upbringing is clearly present, as fear is learned through parents or other individuals. At the same time, the fear of snakes, high cliffs, and dark spaces—which is common to both humans and other primates—suggests the possibility of an innate predisposition formed over a long evolutionary process. In other words, evolution influences human nature through genes, and this nature is subsequently shaped more specifically through the environment and learning.
Which is more important: nature or nurture?
As Mel Caster said, “Nature determines where a person goes, but nurture propels them forward.” When exploring nature and nurture, it is more important to understand how these two factors harmonize within human psychology and behavior than to compare the relative importance of one over the other. Of course, depending on the situation, the influence of nature may be more pronounced, while in other cases, the effects of nurture may be more evident. However, it is difficult to conclude that either factor alone determines human behavior to an extreme degree. Even if someone claims there are such cases, it is necessary to closely examine whether there are any errors in the research process and methodology.
To illustrate this relationship, animal behaviorist Pat Bateson presented the following fable.
The process of behavioral and psychological development can be likened to cooking. While the ingredients used are important, the way those ingredients are combined is equally crucial. Furthermore, proper timing is a key factor that determines the outcome. In this analogy, the ingredients represent various genetic and environmental factors, while the cooking process symbolizes biological and psychological development.
This analogy offers an explanation capable of convincing both nature and nurture advocates.
Can a flower be frightening? To date, the human brain has not developed a genetic predisposition, through the process of evolution, to perceive flowers as objects of fear. This demonstrates the significant role played by nature. However, what if, over the course of a long evolutionary process, humanity had been repeatedly placed in environments where flowers posed a lethal threat to life? It is possible that information about flowers being as dangerous as snakes or high cliffs would have accumulated over the course of evolution, and humans might subsequently have come to fear flowers through new learning experiences.
Nature serves as the foundation for nurture, and nurture, in turn, operates on top of nature. Ultimately, humans grow and develop through a process of constant interaction between nature and nurture.
