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Heredity and Environment’ Effects on Intelligence


Both heredity and environment play a role in the development of intelligence in adolescents. During childhood, the environment influences intelligence test performance by up to 60% while genetic factors influence it by 40% (Carter, 2011). Examples of environmental factors include diet, nature of the family, type of settlement, economic level of the family, and cultural practices.

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However, during adolescence, intelligence test performance is influenced more by genetic factors at 60% with the environment influencing it by 40% (Carter, 2011). For individuals to perform better in intelligence tests, they need to be brought up in an environment which is conducive for learning and development of various life skills.

According to Raymond Cattell, there are two types of intelligence namely fluid and crystallized intelligence. What is common in them is that they are part of general intelligence. The difference is that fluid intelligence, which is also known as fluid reasoning, is the ability to think and reason in a logical manner without using any knowledge or skills. It is mostly used in finding solutions to mathematical and scientific problems.

Fluid intelligence therefore is independent of a person’s level of education, age, and cultural background. A person using fluid intelligence uses logical reasoning to identify patterns surrounding a problem, analyzes the patterns and their relationships, and then comes up with a logical conclusion.

Fluid intelligence therefore is capable of generating knowledge through the scientific method of inquiry, which involves the proposition of a hypothesis and the subsequent clarification of the hypothesis. Fluid intelligence involves three types of reasoning namely abductive, inductive, and deductive reasoning. People use these types of reasoning to acquire new knowledge (Cattell, 2012).

Crystallized intelligence on the other hand has to do with application of skills, experience, and knowledge to provide solutions to problems.

In most cases, crystallized intelligence increases with age because people acquire more skills and experiences as they grow older. As a result, older people who are educated have more crystallized intelligence than young people. It is however important to point out that crystallized intelligence does not exclude fluid intelligence but fluid intelligence does exclude crystallized intelligence (Cattell, 2012).

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Views of Intelligence Development

There are various perspectives to the development of intelligence. For instance, empiricists argue that a child’s brain works exactly like that of an adult. They believe that a child’s brain only lacks the experiences to make associations of various events but that of an adult is capable of making associations because it is exposed to many experiences (Kaufman & Lichtenberger, 2005).

This perspective is opposed to the general idea that the environment has an influence on a person’s intelligence because intelligence is an innate ability which is present in all human beings at birth. As we grow older therefore, we do not acquire more intelligence but our intelligence gets expanded. The perspective therefore seems to lean towards fluid intelligence and views crystallized intelligence as an abstract concept which may not exist.

Psychological nativists base their argument about development of intelligence on the concepts of time, space, and numbers. They believe that these concepts are innate or “hardwired” into the brain at birth and as a result, babies are born with the ability to make use of them. The psychological nativists view intelligence as a product of the interaction of time, space, and numbers.

According to them, these concepts reflect the description of crystallized intelligence and as such, nativists do not believe whether fluid intelligence is an adequate criterion to explain development of intelligence (Kaufman & Lichtenberger, 2005). The reason is that according to them, people who live for many years and visit many places during their lifetime are more intelligent than those who are young and not widely traveled.

On their part, cognitive psychologists argue that the mind is a product of the brain, and the brain is shaped by genetics (Boeree, 2006). As such, the environment plays virtually no role in the development of intelligence. In his study of child development, Jean Piaget argued that the development of intelligence by children is influenced by their stages of development, meaning that at every level or stage of development, the child is capable of developing his or her intelligence faculties to match that stage or level of development.

He believed in the idea that development of intelligence precedes learning, meaning that children are able to learn only those things which match their age or development stage. He identified four intelligence development stages namely sensory motor, pre-operational, concrete operational, and formal operational stages (Boeree, 2006).

Another psychologist Steven Pinker argued that accepting that our intelligence is shaped by evolutionary psychology was tantamount to reducing our feelings, perceptions, motives, and emotions to mere processes of our genetic evolution, which gives biology an opportunity to “debunk all that we hold sacred” (Pinker, 2012).

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This argument is against the idea of attributing development of intelligence to environmental factors at the expense of genetic factors. As such, the biological factors play a bigger role in the development of intelligence than environmental factors because the brain, just like other body organs is able to develop irrespective of the prevailing environmental factors (Kaufman & Lichtenberger, 2005).

On his side, Charles Darwin in his theory of evolution laid out a framework to explain whether intelligence was fixed by genetic inheritance, or could be modified by circumstances. The framework views the development of intelligence as a product of the interaction of both genetic and environmental factors because non is capable of providing an explanation without mentioning the aspects of the other, that is, it is not possible to explain development of intelligence using genetics and fail to mention the environment at some point and vice versa (Bouchard, 1998).

However, one of the psychologists, Francis Galbon’s did believe that basic intelligence was fixed at birth and therefore the environment played no role. His study however revealed that acquisition of skills, knowledge, and age was capable of expanding basic intelligence.

How to Optimize the Development of Intelligence

While the debate of the role of nature and nurture in the development of intelligence has been there for long time, scholars have attempted to think of the ways of optimizing the development of intelligence. According to a New Zealand breast feeding study, there exists a link between high Intelligence Quotient (IQ) and breast feeding (Northrup, 2005).

The study followed up a group of children who were allowed to breast feed for prolonged periods and others who did not breast feed for prolonged periods. When all factors were held constant, the findings showed a positive correlation between breast feeding and high IQ because the children who breast fed for prolonged periods obtained higher test scores than those who did not breast feed for prolonged periods (Northrup, 2005).


Even though there are various arguments regarding the role of heredity and environment in the development of intelligence, many scholars and studies seem to be in agreement that both heredity and environmental factors are responsible for development of overall intelligence.

This argument is based on the fact that it is almost impossible to explain development of intelligence purely on heredity or environmental factors. Even if a person attempts to do so, he or she has to mention both in the explanation. One way to optimize the development of intelligence is through prolonged breast feeding of children.


Boeree, C.G. (2006). Personality theories: Jean Piaget (1896 – 1980).

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Bouchard, T. J. (1998). Genetic and environmental influences on adult intelligence and special mental abilities. Human Biology, 257-279.

Carter, P.J. (2011). IQ and psychometric tests: assess your personality, aptitude and intelligence. London: Kogan.

Cattell, R. (2012). Big ideas simply explained: The psychology book.

Kaufman, A. S., & Lichtenberger, E. O. (2005). Assessing adolescent and adult intelligence. Hoboken, NJ: John Wiley & Sons.

Northrup, C. (2005). Mother daughter wisdom: understanding the crucial link between mothers, daughters, and health. New York: Bantam Books.

Pinker, S. (2012). Big ideas simply explained: The psychology book.

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