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Biological and Environmental Influences on Intelligence

Biological Influences on Intelligence

Research links biological factors such as brain structure and function with intelligence.

Brain structure shows a clear link to intellectual ability. Intelligence is correlated with a larger brain volume, more neurons, and a thicker cortex (outer layer of the brain). Brain function also predicts intellectual ability. The brains of people with a higher intelligence level operates more efficiently. They use less effort and exhibit lower levels of brain activity when working on a task. The brains of intelligent people also process information more quickly, essentially running faster than the brains of less intelligent people. People's speed on simple tasks, such as determining line length or pressing lit buttons, predicts intelligence. Working memory capacity, the ability to hold and process multiple pieces of information, also predicts intelligence. People who can work with more pieces of information simultaneously find it easier to solve problems.

Brain structure and function depend partly on genetic factors, in other words, the biology one inherits from parents. However, environmental factors such as good nutrition and strong educational opportunities help optimize brain development. Malnutrition, poor education, and chronic stress can impair brain development.

Intelligence is not located in any one part of the brain, but some areas show especially strong links to intellectual ability. Intelligence tests activate areas of the cortex involved in control, planning, and short-term memory. General intelligence (g) is not isolated to specific areas of activation. Instead, g seems to require the ability to integrate processing across many areas of the brain.

Brain Activation During Tasks Measuring Intelligence

The image on the left shows parts of the cortex (brain surface) activated during intelligence tasks. The image on the right shows the extensive activity happening beneath the surface. This reveals connections between areas that appeared separate in the picture on the left.

Genetic Research

Twin, adoption, and family studies have found that genetic factors have a significantly greater impact on intelligence levels than do environmental factors.

Psychologists use family studies, twin studies, and adoption studies to test the influence of genes, the biological units of heredity, on intelligence. These studies suggest that nature (genetic factors) has a greater impact on intelligence levels than nurture (environmental factors). People with strong genetic similarities are likely to have similar levels of intelligence. Identical twins have 100% of genes in common, whereas nonidentical (fraternal) twins have 50% of genes in common. In twin studies, identical twins have more strongly correlated intelligence test scores than do fraternal twins.

In adoption studies, the intelligence of children correlates more strongly with biological parents than with adoptive parents. Within adoptive families, parent intelligence correlates more strongly with their biological offspring than with their adopted children. Because adopted and biological children live in the same environment, this further suggests that biological factors strongly influence intelligence.

Twin and family studies also show that nurture influences intelligence. The influence of nurture proves that an individual's level of intelligence at birth is not completely fixed and unchangeable. Twins raised in the same home have more similar intelligence test scores than twins raised separately. Fraternal twins have the same degree of genetic similarity as ordinary siblings. However, fraternal twins have more similar intelligence scores than do other siblings, most likely because they are treated more similarly than other siblings.

IQ Correlations Between Groups

Many studies have explored intelligence score correlations between different groups. Groups with strong genetic similarity show larger correlations, indicating a genetic component to intelligence. Twins raised together have more similar scores, indicating an environmental component as well.

Environmental Influences on Intelligence

Environmental factors also play a role in intelligence, and children are more affected than adults by their environmental experiences.

While both nature (biological) and nurture (environmental) factors play a role in intelligence, the environment impacts children more than adults. This is because the environment can directly impact brain development during childhood and adolescence. Intelligence also becomes more stable over time once brain development ends.

Children living in poverty tend to have lower intelligence than children living with more resources. Poverty is associated with high stress levels and less time for parent-child interaction. Poverty also predicts less nutritious meals and lower-quality educational opportunities. All of these factors can impair brain development and reduce intelligence. Children living in poverty may also be more likely to be exposed to toxins such as lead. Lead impairs brain development, negatively impacting language, social skills, intellectual ability, and basic coordination.

Nature and nurture also interact to predict outcomes. For example, a highly intelligent child may get more attention from teachers and extra enrichment opportunities than a child with less intelligence. These educational differences may enhance the initial biological differences. Thus, slight differences in intelligence due to genetics can grow larger over time due to the environment.

Because the environment can have an impact on intellectual ability, early educational experiences matter. Early intervention, a system of services for very young children with developmental delays or disabilities, can help optimize intellectual development. Head Start is a government program for low-income families. It offers enhanced early childhood education, nutrition, and parenting support. Children who participate in Head Start get better grades and are more likely to graduate high school and attend college. As adults, they earn higher incomes and have better physical health. Government-funded after-school programs for older students have similar benefits. In particular, they reduce the chance that students will stay back a grade or drop out of school.