The Roles of Nature and Nurture in Intelligence

Origins of Intelligence

Where does intelligence come from? The literature suggests that both nature and nurture affect intelligence. Although genetics seem to be in control of the level of intelligence, the environment influences the manifestation of cognitive abilities (Bartels et al., 2002). This idea is referred to as range of reaction, which suggests that each person responds to the environment in a unique way based on their genetic makeup. According to this idea, genetic potential is a fixed quantity (i.e., a range of based on genetics), but whether full intellectual potential is reached depends on the environmental stimulation experienced, especially in childhood. Socioeconomic status, family life, nutrition, and quality of education are all environmental factors that can impact intelligence.

Think about this scenario: A couple adopts a child who has average genetic intellectual potential. They raise them in a stimulating environment. What will happen to the couple’s child? It is likely that the stimulating environment will allow the child to perform at the higher end of their range of reaction. But what happens if this scenario is reversed? If a child with a strong intellectual genetic background is placed in an environment that does not stimulate them, it is likely that their intelligence will result in the child performing at the lower end of their range of reaction.

Keeping the concept of range of reaction in mind, we will discuss evidence for the role of nature and nurture in intelligence.

Genetic Contributions

Some researchers believe that intelligence is a trait inherited from a person’s parents. Scientists who research this topic typically use twin studies to determine the relative genetic and environmental contributions of intelligence.

Twin studies have found that between 40% and 80% of the variability in IQ is due to genetics, meaning that overall genetics plays a bigger role than does environment in creating IQ differences among individuals (Plomin & Spinath, 2004). The IQs of identical twins correlate very highly (r = .86), much higher than do the scores of fraternal twins who are less genetically similar (r = .60). And the correlations between the IQs of parents and their biological children (r= .42) is significantly greater than the correlation between parents and adopted children (r = .19).

The role of genetics gets stronger as children get older. The intelligence of very young children (less than 3 years old) does not predict adult intelligence, but by age 7 it does, and IQ scores remain stable in adulthood (Deary et al., 2004).

Environmental Contributions

But there is also evidence for the role of nurture, indicating that individuals are not born with fixed, unchangeable levels of intelligence. Twins raised together in the same home have more similar IQs than do twins who are raised in different homes, and fraternal twins have more similar IQs than do nontwin siblings, which is likely due to the fact that they are treated more similarly than are siblings.

The fact that intelligence becomes more stable as we get older provides evidence that early environmental experiences matter more than later ones. Environmental factors also explain a greater proportion of the variance in intelligence for children from lower-class households than they do for children from upper-class households (Turkheimer et al., 2003). This is because most upper-class households tend to provide a safe, nutritious, and supporting environment for children, whereas these factors are more variable in lower-class households.

Social and economic deprivation can adversely affect IQ. Children from households in poverty have lower IQs than do children from households with more resources even when other factors such as education, race, and parenting are controlled (Brooks-Gunn & Duncan, 1997). Poverty may lead to diets that are undernourishing or lacking in appropriate vitamins, and poor children may also be more likely to be exposed to toxins such as lead in drinking water, dust, or paint chips (Bellinger, 2008). Both of these factors can slow brain development and reduce intelligence.

If impoverished environments can harm intelligence, we might wonder whether enriched environments can improve it. Government-funded after-school programs such as Head Start are designed to help children learn. Research has found that attending such programs may increase intelligence for a short time, but these increases rarely last after the programs end (McLoyd, 1998). But other studies suggest that Head Start and similar programs may improve emotional intelligence and reduce the likelihood that children will drop out of school or be held back a grade (Morris et al., 2018).

While children from higher SES backgrounds continue to learn and maintain academic skills during the summer months–as a function of opportunities/experiences, parental involvement–children from lower SES backgrounds do not. In fact, there is sufficient evidence to indicate that they experience what is called the “summer slide,” which involves losing some of the academic skills and knowledge they learned during the school year (Vale et al., 2013). However, there is evidence to show that summer programs can maintain and even increase academic skills for students from low SES backgrounds (Harry, 2022).

The amount of education someone has is positively correlated with their IQ (Ceci, 1991). In part this correlation may be due to the fact that people with higher IQ scores enjoy taking classes more than people with low IQ scores, and they thus are more likely to stay in school.

A meta-analysis of quasi-experimental studies reveals that for each additional year of education, IQ increases 1 to 5 points (Ritchie & Tucker-Drob, 2018). Additionally, comparisons between children who are almost exactly the same age but who just do or just do not make a deadline for entering school in a given school year show that those who enter school a year earlier have higher IQ than those who have to wait until the next year to begin school (Baltes & Reinert, 1969).

Conclusions

It is important to remember that the relative roles of nature and nurture can never be completely separated. A child who has higher than average intelligence will be treated differently than a child who has lower than average intelligence, and these differences in behaviors will likely amplify initial differences. This means that modest genetic differences can be multiplied into big differences over time.