Nature vs. Nurture: What Shapes Human Intelligence?

Introduction

While it is clear that many biological traits — such as eye color, height, skin color, and bone structure — are inherited from one's parents, it is far less clear whether genes predetermine talents and intelligence (Powell, 2003). For this reason, the exact roles that nature and nurture play in shaping intelligence remain contested. This paper aims to show that human intelligence is shaped more by environmental influences than by genetics.

Many scientists believe that people behave as they do according to genetic predispositions or "animal instincts" (Powell, 2003). This is known as the "nature" theory of human behavior. Other scientists believe that people behave in certain ways because they have learned to do so — this is known as the "nurture" theory of human behavior.

Increasing understanding of the human genome suggests that both sides are partly right. Nature provides human beings with inborn abilities and traits; nurture takes these genetic tendencies and develops them as we learn and mature. Nevertheless, despite these shared premises, scientists continue to argue over how much of who we are is shaped by genes and how much by the environment.

The Nature Theory

Up until the middle of the twentieth century, intelligence had been largely accepted as a trait that was biologically predetermined (Sticht, 1997). The "amount" of intelligence an individual possessed was believed to be present at birth and to remain constant despite personal experience or education.

An opposing perspective — that intelligence is due to experience and one's environment — was proposed as early as the seventeenth century (Sticht, 1997). These contrasting viewpoints comprise the classic "nature vs. nurture" debate: Is intelligence a biologically determined, innate potential, or is it an environmentally determined, acquired potential?

In the early twentieth century, the dominant view was that intelligence determined a person's potential for learning and success (Sticht, 1997). Because intelligence, and ultimately potential, was considered inherited, low intelligence was seen as something that simply could not be changed.

However, by the early 1950s, opinion had shifted from the "nature" side toward the "nurture" side of the argument (Sticht, 1997). Many scientists argued that intelligence was not genetically determined but was instead a product of individual experience. It was proposed that intelligence was especially malleable in early childhood, reaching approximately eighty percent of its adult level by around age five.

The developmental theory proposed by Jean Piaget is central to this debate (Sticht, 1997). Piaget's theory is significant in two major ways. First, it bridges both sides of the intelligence debate, arguing that biologically given intellectual structures unfold — much as embryological structures unfold — when placed in a nurturing environment. Second, Piaget proposes four mechanisms of cognitive development (maturation, experience, social transmission, and equilibration) through which the environment interacts with the individual's internal structures.

While Piaget's theory provides a foundation for understanding the structure and progression of cognition, it fails to address the role of knowledge in developing intellectual ability (Sticht, 1997). His theory predicts the same sequence of intellectual development as a person matures, regardless of the specific knowledge domains the person experiences. It also predicts that once a person reaches a certain level of intellectual development, that level applies across all domains of knowledge — a prediction that has been disproven over time.

It is widely accepted that specific genes embedded in human cells determine physical traits, including eye color and hair color (Powell, 2003). The nature theory, however, suggests that more abstract traits — including intelligence, personality, aggression, and sexual orientation — are also encoded in a person's DNA.

A 1998 article in LIFE Magazine titled "Were You Born That Way?" by George Howe Colt suggested that "new studies show it's mostly in your genes" (Powell, 2003). If genetics played no part, then fraternal twins reared under identical conditions would be alike regardless of genetic differences. Yet studies show that while fraternal twins do resemble each other more closely than non-twin siblings, they also exhibit striking similarities when reared apart — evidence that genes contribute meaningfully to intelligence.

Conclusion

However, Eric Turkheimer, a psychologist, published a study that challenged this conclusion (Pittman, 2003). Using IQ scores as a proxy for nature and socioeconomic status as a proxy for nurture, Turkheimer and his colleagues examined the influence of genes on intelligence among very poor, mostly minority children.

As Pittman (2003) describes: "Using a large national database, he identified more than 620 pairs of twins who were born in the 1960s and given IQ tests at age 7. By looking at the differences between identical and fraternal twins, he concluded that the importance of environmental influences on IQ is four times stronger in poor families than in affluent families. Conversely, the heritability of IQ is seven times stronger in wealthy families than in poor ones."

In other words, in environments of educational and social abundance, any additional effort wealthy families make to improve upon the genes they pass to their children has relatively little impact (Pittman, 2003). In environments shaped by poverty, however, there is much that parents, program providers, and policy makers can do to ensure children reach their potential. When it comes to socioeconomic class, it appears that nature matters more at the high end while nurture matters more at the low end. In this respect, the Turkheimer study highlights the importance of family circumstances in shaping intelligence.

Supporters of the nurture theory, while not completely rejecting the idea that genetic tendencies exist, believe those tendencies do not ultimately determine outcomes — that our behavioral characteristics originate mainly from the environmental factors present during upbringing (Powell, 2003). Studies on infant and child temperament provide some of the most compelling evidence for nurture-based theories.

American psychologist John B. Watson, best known for his controversial experiments with a young child referred to as "Little Albert," demonstrated that phobias could be explained through classical conditioning (Powell, 2003). A strong proponent of environmental learning, Watson famously declared: "Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select — regardless of his talents, penchants, tendencies, abilities, vocations, and the race of his ancestors."

If environment played no part in determining traits and behaviors, then identical twins should theoretically be exactly alike in all respects, even when reared separately (Powell, 2003). Yet numerous studies show that identical twins are never exactly the same, even though they are remarkably similar in most respects.

Numerous genomic tools have revealed the complex workings of the human cell, including interactions between genes and proteins within the cellular environment (Moore, 2001). However, the message that environmental factors inside cells play an important role in shaping traits has not received as much public attention. According to Moore (2001), media coverage of studies linking individual genes to traits ranging from sexual orientation to obesity has led many people to conclude that we are nothing more than the sum of our genes.

In The Dependent Gene: The Fallacy of "Nature vs. Nurture," Moore (2001) emphasizes the environmental dimension of the debate. "The viewpoint offered in this book," he writes, "frees both individuals and society from the chains of genetic determinism... no longer will people have to bear the thought that their destiny and the destiny of their children is preordained in their genetic endowment and forever beyond their dominion."

In essence, Moore argues that genes (nature) depend on the environment (nurture) to express themselves, and that even traits commonly assumed to be purely genetic are influenced by environmental factors. "All traits — from 'biological' traits like hair color and height to complex 'psychological' traits like intelligence — are caused by dependent interactions of genes and environments," Moore (2001) concludes.

Numerous studies have been conducted on the role of nature and nurture in intelligence, and the general consensus is that intelligence is somewhat hereditary but that environment can significantly enrich it (Mehrotra, 2004). Determining which factor plays the greater role, however, remains difficult.

Parents today tend to be more involved in their children's lives, partly in response to increasing pressure on children to perform academically at younger and younger ages (Mehrotra, 2004). Research in child development suggests that if parents provide the right kind of stimulation, even an average child can excel. Conversely, if stimulation is absent, even the brightest child may never reach his or her full potential.

Mehrotra (2004) emphasizes the importance of active parental involvement: "Parents should spend a lot of time with their children. They should not just give them toys — they should also play with them. There are age-appropriate toys available in the market. For instance, Lego is an excellent toy for developing attention and concentration. However, if you leave a child alone with a Lego set, he will probably just throw the pieces around, as he will not understand what he is supposed to do with them. Parents should demonstrate how to use Lego and also show their own enthusiasm for it. If parents are going to sit around watching movies and expect their children to play on their own, there is no point. Children need parental involvement and guidance."

In conclusion, whether nature or nurture more strongly influences intelligence remains a matter of debate among scientists. However, the available evidence suggests that nurture outweighs nature. Changes in the environment during early development can affect gene expression and produce dramatic effects on intelligence levels. Better education, engaged parenting, and increased opportunities for enriching activities are all believed to have a positive influence on intellectual development. Good nutrition likewise enables a person to function well both physically and mentally.