The Nature Vs Nurture Debate

[Slikk Huisenga]

RecentlyI was going through some old photos of my dad, Kurt Katch. He was an actor who escaped the Nazis and immigrated to the U.S. in 1937 with my mom (and two suitcases). Neither spoke English (but they did speak five other languages!) and once they left their families they never saw them again. All of their relatives perished in the Holocaust.

My dad died at age 65 when I was just 12 years old, so I never knew him when I (and he) got older. But a gander at these old photos are telling. Indeed, the physical resemblance is unmistakable. We are about the same age, 45, in this photo. I always have wondered if the dramatically different turns our respective lives took had an effect on our personalities, social intelligence, IQ, and other traits that made us who we are (and were).

While scientists have studied physical, behavioral, and intellectual inheritance throughout human history, the systematic investigation of the inheritance for most human traits formally began with Sir Francis Galton, cousin to Charles Darwin, in the 1869 book Hereditary Genius and in other writings.

Devising research to study the nature/nurture issue is complicated. It requires innovative approaches and new technological methods that can isolate and distinguish the cause-and-effect of human traits due to genetic or environmental influences. The two major approaches involve studying twins that have been raised together or who were separated at birth (or during early childhood). The second approach involves identifying how different genes correlate to specific traits, or how environmental influences (effects of circumstances of life and experience) affect how genes express. As noted above, this field of study is termed epigenetics.

Researchers worldwide have established twin registries where scientists systematically identify twin births and follow their development and life circumstances until death. The Swedish Twin Registry, founded in 1959 at the Karolinska Institute in Stockholm, contains information on 194,000 twins born since 1886. This largest twin registry in the world has made important strides regarding cardiovascular disease, cancer, and aging. The Twins UK Registry, which launched a database for studying arthritis, includes more than 14,000 twins, aged 16-100 years, throughout the United Kingdom.

By studying twins separated in infancy and raised apart, scientists can answer the question: To what extent do cognitive and psychological differences between people result from nature and/or nurture?

Research from the Minnesota Center for Twin and Adoption Research (The Minnesota Study of Twins Reared Apart) includes more than 137 pairs of separated identical and fraternal twins and triplets who participated in a battery of medical and psychological tests. Over the years, identical twins reared apart developed personalities and interests that showed about the same degree of resemblance as identical twins raised together.

Gene expression is the process by which the instructions in DNA are converted into a functional product, such as a protein. When the information stored in DNA converts into instructions for making proteins or other molecules, it is called gene expression and represents a tightly regulated process that allows a cell to respond to its changing environment. It acts as both an on/off switch to control when proteins are made, and also a volume control that increases/decreases the number of proteins made.

Fortunately, not all epigenetic changes are permanent. Some epigenetic changes can be added or removed in response to changes in behavior or environment. In fact, epigenetic changes often occur in smokers vs. non-smokers. After quitting smoking, former smokers can begin to have increased DNA activity in certain genes. Eventually, they can reach levels similar to those of non-smokers.

The nature-nurture debate has been cast as a contest between the idea that some characteristics are caused primarily by genetic factors and other characteristics are caused primarily by experiential factors. This premise is outdated and is not supported by current research. We now know that genes do not act independently of their contexts. Instead, genes do what they do because of their contexts.

(Lead image from Sports Illustrated. Comedians Randy and Jason Sklar are identical twins who happen to be Wolverines and rabid sports fans. This is a great video interview by then-student journalist Eric Cutter in which the twins share memories of seeing real-time game action by the legendary Fab Five, Desmond Howard, and more.)

In child development, the nature vs. nurture debate is evident in the study of language acquisition. Researchers like Chomsky (1957) argue that humans are born with an innate capacity for language (nature), known as universal grammar, suggesting that genetics play a significant role in language development.

Twin studies have provided valuable insights into this debate, demonstrating that identical twins raised apart may share linguistic similarities despite different environments, suggesting a strong genetic influence (Bouchard, 1979)

However, environmental factors, such as exposure to language-rich environments, also play a crucial role in language development, highlighting the intricate interplay between nature and nurture in child development.

The nature vs. nurture debate in personality psychology centers on the origins of personality traits. Twin studies have shown that identical twins reared apart tend to have more similar personalities than fraternal twins, indicating a genetic component to personality (Bouchard, 1994).

The nature vs. nurture debate in mental health explores the etiology of depression. Genetic studies have identified specific genes associated with an increased vulnerability to depression, indicating a genetic component (Sullivan et al., 2000).

However, environmental factors, such as adverse life events and chronic stress during childhood, also play a significant role in the development of depressive disorders (Dube et al.., 2002; Keller et al., 2007)

The diathesis-stress model posits that individuals inherit a genetic predisposition (diathesis) to a disorder, which is then activated or exacerbated by environmental stressors (Monroe & Simons, 1991).

Research by Flynn (1987) showed that average IQ scores have increased over generations, suggesting that environmental improvements, known as the Flynn effect, can lead to substantial gains in cognitive abilities.

Molecular genetics provides tools to identify specific genes and understand their pathways and interactions. However, progress has been slow for complex traits like intelligence. Identified genes have small effect sizes (Plomin & Spinath, 2004).

From this point of view, psychological characteristics and behavioral differences that emerge through infancy and childhood are the results of learning. It is how you are brought up (nurture) that governs the psychologically significant aspects of child development and the concept of maturation applies only to the biological.

Behavioral genetics has enabled psychology to quantify the relative contribution of nature and nurture with regard to specific psychological traits. One way to do this is to study relatives who share the same genes (nature) but a different environment (nurture). Adoption acts as a natural experiment which allows researchers to do this.

Empirical studies have consistently shown that adoptive children show greater resemblance to their biological parents, rather than their adoptive, or environmental parents (Plomin & DeFries, 1983; 1985).

Another way of studying heredity is by comparing the behavior of twins, who can either be identical (sharing the same genes) or non-identical (sharing 50% of genes). Like adoption studies, twin studies support the first rule of behavior genetics; that psychological traits are extremely heritable, about 50% on average.

For many environmentalists, there is a barely disguised right-wing agenda behind the work of the behavioral geneticists. In their view, part of the difference in the I.Q. scores of different ethnic groups are due to inbuilt biases in the methods of testing.

More fundamentally, they believe that differences in intellectual ability are a product of social inequalities in access to material resources and opportunities. To put it simply children brought up in the ghetto tend to score lower on tests because they are denied the same life chances as more privileged members of society.

Now we can see why the nature-nurture debate has become such a hotly contested issue. What begins as an attempt to understand the causes of behavioral differences often develops into a politically motivated dispute about distributive justice and power in society.

Thus, psychological traits follow a polygenic mode of inheritance (as opposed to being determined by a single gene). Depression is a good example of a polygenic trait, which is thought to be influenced by around 1000 genes (Plomin, 2018).

While still limited in predictive power, polygenic risk scores provide a way to quantify innate genetic risk, allowing researchers to study how this interacts with environmental factors to influence outcomes.

This complexity is further increased when you consider how these genes might interact with each other (epistasis) and with environmental factors. The same genetic profile might lead to different outcomes in different environments.

Nurture assumes that correlations between environmental factors and psychological outcomes are caused environmentally. For example, how much parents read with their children and how well children learn to read appear to be related. Other examples include environmental stress and its effect on depression.

People select, modify and create environments correlated with their genetic disposition. This means that what sometimes appears to be an environmental influence (nurture) is a genetic influence (nature).

Heritability statistics revealed by behavioral genetic studies have been criticized as meaningless, mainly because biologists have established that genes cannot

influence development independently of environmental factors; genetic and nongenetic factors always cooperate to build traits. The reality is that

nature and culture interact in a host of qualitatively different ways (Gottlieb, 2007; Johnston & Edwards, 2002).

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