16 factors that influences people’s intelligence


Ale notices a wide variety of individual differences in people’s intellectual abilities. Some are more intelligent than others. The differences are so much that some change the course of human civilization through their intellectual innovations, a few others even find it difficult to master a problem simple addition.

The individual differences that exist among all of us are the products of two general and broad factors: hereditary environmental.

Hereditary factors are based on the genetic make up of the individual influences growth and development throughout life. The offspring inherits genetic characteristics from his parents. The environmental factors cc of the influence of parents, family, friends, schooling, society, culture, and all other experiences to which the child is exposed right from the moment of conception.


The nature-nurture controversy has philosophical roots. In the 1600s, Locke, English philosopher, argued that the mind of a newborn is a blank: a tabula rasa on which the environment writes his life history. Locke believed that environment acts as the sole determinant of development. On the con the French philosopher, J. J. Rousseau suggested in the 1700s that people’s natural characteristics (i.e., genetic factors) mostly influence developmental process subject to the corrupting influences of the environment.

The nature-nurture question has been vehemently debated for decades. At present, psychologists agree that both nature and nurture interact to pro specific developmental patterns and competence. The question has changed from which (heredity or environment) influences behavior to how and to what extent heredity and environment shape the developmental proc No body is born without a genetic make up, or no one grows up free environmental influences.

Both heredity and environment are important, it is unwise to determine their relative influence. Even then, the de concerning their relative influence remains still active with different thee emphasizing either the role of heredity or the role of environment (Scarr, 1996). These factors are discussed below in detail.

1. Hereditary Factors


Heredity refers to genetically transmitted characteristics from generation to the next. We inherit genetic code from parents. Because of genetic code, a fertilized human egg never grows into a dog or a mouse or any other animal. Person’s genetic heritage is called genotype. The genotype expresses itself in observable appearance and behavior, which is called phenotype. The phenotype includes height, weight, eye color, and psychological characteristics such as intelligence, personality and creativity.

The genetic code provides the base on which phenotype grows and manifests. How can we know that human intelligence has a genetic basis? To do so, we have to infer the underlying genotypes from the observable phenotypic behavior.

A strategy to understand genotypes from phenotypes is to examine the similarities between the intelligence level of children, and their parents and relatives. Francis Galton (1885) was of the view that intelligence is largely inherited, and it runs in families. Researchers have found “that parents with high IQs tend to have children with high IQs, and parents with low IQs have children with relatively low IQs (Crooks and Stein, 1995).

Correlations between IQs of brothers, sisters, children and their parents, and their distant relatives indicate that intelligence has a strong hereditary component (Kagan and havemann, 1976). The correlation between the IQs of one child and another person selected at random would tend to be zero, while among the related individuals the correlations would be relatively high.


Correlation coefficient provides a measure of the strength of relationship between two variables, and; bound by limits from -1.00 to +1.00. The higher is the correlation coefficient; le higher is the strength of relationship.

In more recent times, Arthur Jensen (1969) raised a hostile controversy ‘hen he argued that intelligence is primarily inherited. He examined several studies on intelligence, many of which involved comparisons of identical and fraternal twins. Jensen found support in these studies for his argument in favor of the genetic influence on intelligence.

He claimed that clear-cut genetic differences are present in the average intelligence of races, nationalities, and social classes. He stated that blacks have lower genetic intelligence than whites, and that is why they do not perform as well as whites on intelligence tests.

He rid others have placed the importance of heredity’s influence on intelligence at about 80 percent, leaving only 20 percent to be manipulated by the environment, He believed that environmental manipulations can at best reduce group differences in intelligence, but cannot abolish it altogether. But Jensen’s views have been severely criticized, and he has been labeled as a racist.


Psychologists have drawn evidence from a number of studies to examine the relative influence of hereditary and environmental factors on intelligence The two kinds of studies discussed below are: (a) studies of twin children; and (b) studies involving adopted children.

2. Twin studies:

Twins are of two types: identical twins and fraternal twins Identical twins originate from a single fertilized egg and share the same gencode. The fraternal twins arise from two different eggs fertilized by two different sperm cells. While identical twins show a 100 percent genetic overlap, fraternal twins have 50 percent genetic similarity, which is no greater than that between ordinary siblings. If identical twins turn out to be more simile their intelligence as compared to fraternal twins, the evidence for the hereditary factors would be strong.

The average correlation coefficients between the IQs of children having different degrees of genetic similarity insightful analysis of the correlations will clarify several points regarding the relative role of heredity and environment in shaping individual’s intelligence.


3. Relationship Median Correlation

Identical twins reared together – .86

Identical twins reared apart – .72

Fraternal twins reared together – .60

Siblings reared together – .47

Siblings reared apart – .24

Parent and child living together – .42

Parent and child separated by adoption – .31

Genetically unrelated children reared together – .25

Genetically unrelated foster parents and foster child – .15

The correlation between the IQs of identical twins reared together is which is substantially higher than the correlation of fraternal twins reared together (.60). Furthermore, identical twins reared apart in different environments show a high degree of similarity (.72) in their intelligence as compared to the fraternal twins reared together (.60).

The finding that identical twins raised in different environments are similar in their intellectual abilities than fraternal twins reared in the same environment suggests a strong genetic influence on intelligence. However, the role of the environment cannot be ruled out, because the correlation drops from .86 to .72 as the environment changes for the identical twins.

4. Adoption studies:

Another line of evidence comes from the studies on adopted children. In one study (Horn, 1983), the correlation between the IQs of children and their biological mothers (whom they had never seen) was .28, which was much higher than a correlation of .15 between their IQs and their adoptive mothers.

Other studies have also shown that children’s intelligence is more similar to their biological parents than to their adoptive parents (Scarr and Carter-Saltzman, 1983). This pattern of similarity persists into adolescence. Those favoring an environmental position argue that children of poor and under-educated parents, when adopted into family of high socio­economic status, exhibit very large increase in their IQ scores.

But the findings showed that when the socioeconomic status of both biological and adoptive parents is equal, the IQs of adopted children do not rise; instead, it is found to be more similar to their biological parents. All these findings lend support to hereditary influence on intelligence.

5. Environmental Factors

Heredity alone cannot account for all the individual differences in intelligence. Environment also has a role to play. Environment consists of a wide range of stimulations that the child is subjected to. He lives and grows in his environment. It provides him the necessary input and experiential base for intellectual development. Enrichment or deficiency of the environment would obviously produce differences in his abilities.

The information given above can be used to support environment’s role. Though fraternal twins and siblings share the same genetic similarity (the genetic overlap in both cases is 50 percent), the correlation between the IQs of fraternal twins is a bit higher than that for ordinary siblings (.60 versus .47).

This is because environmental opportunities and experiences are more similar for fraternal twins than for ordinary siblings. When researchers have manipulated child’s environment by providing extra intellectual input, they have observed a remarkable improvement (up to 30 IQ points) in the IQ scores of children.

Hence, the role of environment cannot be underestimated. Furthermore, we can change the environment the child, not his genetic make up. Thus, irrespective of genetic make adults should carefully monitor child’s environment to help him perform at optimal efficiency level.

Environment starts showing its actions right from the moment of chi conception. Both the prenatal environment (when the baby/fetus is in mother’s womb), and the postnatal environment (after the child is born) influence intellectual capabilities of the child.

6. Prenatal Environment

The prenatal stage is extremely important as a fertilized egg is shaped the form of a human being during this period. Rapid development takes place in major organs and brain cells. If things go wrong during this period, the effects are nearly irreversible or are very difficult to correct. The major prenatal environmental influences are: (a) mother’s nutrition, (b) mother’s emotional state, (c) illness of the mother, (d) mother’s use of drugs, and (e) birth complications.

An undernourished mother cannot provide adequate nutrition to the grow baby. As a result, the baby is likely to be underweight, and more susceptible to diseases. Lack of nutrition would have an adverse impact on the mental development of the child. Mothers who are anxious and tense are also likely deliver infants who would be irritable and show problems in sleeping and eating

Maternal diseases like rubella, syphilis, AIDS, diabetes, and high blood pressure may produce permanent adverse effects on the baby. The brain d of the baby would either be damaged or not grow properly. The intellect development may be arrested. The consequences may be devastating to such an extent that later environmental enrichment programs for the child may totally ineffective.

Drugs taken by mothers can have tragic effects on the unborn child. Alcohol and nicotine are very dangerous for pregnant mothers. The unborn baby may develop fetal alcohol syndrome, which is a condition of retarded physical and mental growth. The children of such mothers may show permanent physical and mental impairment.

If the baby suffers from birth complications such as lack of oxygen at the time of birth, he may suffer permanent brain damage. He would be born, as a mentally retarded child about whom very little can be done. Due attention to ensuring a healthy prenatal environment is necessary for any child to have a fuller development of his intellectual capabilities.

7. Postnatal Environment

When we speak of environmental determinants of intelligence, we ordinarily mean the environment the child faces after he is born. Environment consists of a heterogeneous array of stimulations ranging from home experiences to the ecology of the natural habitat. Enriched environment accelerates cognitive development, while impoverished environment produces just the opposite effect.

8. Home environment

Home is the first learning institution for the child during his early years of development. Needless to mention that it exercises tremendous influence on child’s understanding of the external world, and his conceptions of success and failure. The home provides an identity for the child, builds his self-concept, and prepares him to face the world.

The home environment consists of all the mental and behavioral transactions taking place among the family members. The environment can be stressful or supportive for the child. A supportive and warm home environment that encourages exploration, curiosity, and self-reliance leads to higher intellectual competence in children (Kelly & Woreil, 1977).

In unsupportive home environments, where the families members punish or reject the child, or parents are extremely authoritarian and impose a set of rigid rules and regulations on the child, child’s intellectual competence becomes low. A restrictive home atmosphere inhibits early exploration and curiosity in children, and creates high anxiety in the child as a result of which his natural potentials fail to grow to the optimal level.

The home environment also influences child’s emotions, motivations and beliefs, which are closely linked with intellectual competence.

9. Parent-Child interaction

Parents are the first teachers for the child. The nature of mental behavioral transactions between the parents and the child has a critical influence on his intellectual competence. Children’s intellectual develops is faster when parents provide emotional security, make the family environment more supportive, praise the child’s achievement, allow independence, support the intellectual achievement of children. On the contrary, with stressful family conditions and protective parents encouraging dependence in child the child’s intellectual development suffers.

Both cultural and sex differences in intellectual abilities result from pare values and expectations for children’s achievement. A study by Ha Stevenson revealed that Chinese and Japanese children in the United States show exceptional ability in mathematics, because their parents emphasize achievement orientation in children during their early formative period development.

Parental expectations for boys and girls differ and are reflected in their achievements. Parents consider mathematics more important for boys than girls. There are many studies to suggest that girls outshine boy language skills, whereas boys show superior performance in mathematical and spatial tests.

The mother is more important than the father in determining the intellectual level of the home environment, because she spends more time with child When the mother is the better-educated parent, the intellectual compete of the college going and university students is higher than those having father as the better-educated parent.

In a study (Kagan & Moss, 1962), it was sir that high achievement in boys was associated with high maternal reinforcement and encouragement during the first three years of life. It should, however kept in mind that although parent-child interaction is an important determinant of achievement in children, other factors such as social class, education social opportunities set important limits on children’s attainment.

10. Social and environmental deprivation

If the environmental opportunities and stimulations are low, children show poor cognitive performance. The most frequently noted study was the one conducted by Skeels (1966) involving 25 children reared up in orphanages with very little stimulation. At about 18 months of age, 13 of these children were transferred to another setting, where older retarded women took care of them. After about 2 years, these children gained 28 IQ points. The other 12 children, who stayed back in the unstimulating orphanage, exhibited a 26- point drop in their IQs.

The two groups of children also showed different patterns of adjustment and personality characteristics as adults. A follow-up study after 20 years revealed that the 13 children removed from the orphanage had normal intellectual functioning and social adjustment. The intellectual condition of those 12 children who were in the orphanages was very poor.

Studies show that longer the children remain in impoverished environment, the more depressed their IQs would be (Asher, 1935). For example, with the introduction of schools, roads and radios in a community in the USA in 1930s, the average IQ of individuals increased by 10 points (Wheeler, 1942).

In another study J. Hunt (1982) investigated the effects of enrichment on 11 children in Iranian orphanage, who were developmentally and emotionally retarded. They were rescued, and put into special enriched environmental program by Hunt who arranged specially trained caretakers for them.

They provided these infants special attention, played verbal games with them, and responded to their difficulties, problems, and wants. The effect was striking in that all displayed large improvement in their language skill, and social interactions with people and events, and began behaving intelligently. The Project Head Start in the year 1965 in USA provided compensatory education on social skill, and special intellectual training. The program was initially effective in raising the IQs of children coming from the disadvantaged homes in slum areas.

Studies conducted by Dash and Das (1984, 1989) reveal that an opportunity for education such as schooling significantly influences children’s cognitive capacities.

In a study in rural India, they have shown that schooled children show superior performance on a variety of intellectual tasks compared to their unschooled age-mates. Schooling improves children’s abilities to memorize, reason, and classify using a variety of principles.

Many of the intellectual processes either develop slowly or do not develop at all in children, who do not attend schools. School provides an enriched social environment for children, and allows their thought processes to grow free from the concrete physical and social contexts. Similar findings have been obtained in African countries by Scribner and Cole (1979).

Are the adverse effects of impoverished environments reversible? Can children suffering the ill effects of social deprivation increase their IQs, when raised later in stimulating environmental conditions? Some argue that the adverse effects operate on a relatively permanent basis. But other researchers have shown that early intellectual retardation can be overcome by providing adequate enriched environmental experiences (Kagan, 1972).

11. Socioeconomic status (SES)

Children of the upper socioeconomic strata of the society are exposed to more intellectual stimulation, get better social opportunities, and are nurtured with better nutrition. All these are believed to influence their intellectual development in a positive direction. The index of socioeconomic status (SES) is based on parental education, occupation, and income. The higher is the socioeconomic status of the parents, the higher is the average IQ of children.

The children of low socioeconomic status score approximately 10 to 15 IQ points below the middle-class and higher-class children (Hall and Kaye, 1980). These differences are present by the first grade and are sustained throughout the school years. Parental occupation is closely related to the IQ level of children (Harrell and Harrell, 1945).

The intellective support provided to children at home differs from one SES to another. Moreover, children from varying SES levels bring different attitudes and cognitive styles to the problem-solving situation, which affect their performance (Yando, Seitz and Zigler, 1979).

In Orissa, Jachuck and Mohanty (1974) found that children of high SES performed significantly better than children from low SES on a variety of intellectual tasks. Even older children from low SES performed poorly than the younger children of high SES. For low SES children, they observed progressive retardation in intellectual skills. Rath, Dash and Dash (1975 reported the adverse effects of social class on intellectual reasoning. These finding have been supported by a number of studies conducted in the Indian subcontinent.

12. Race and culture

Many studies have noted racial and cultural differences in performance on lard intelligence tests (Jensen, 1969; Kennedy, 1966). Jensen (1969) observed clear differences in the cognitive competence of whites and blacks,studies conducted by Lesser, Fifer, and Clark (1965) investigated the verbal reasoning, number facility, and space conceptualization of children from : groups: Jewish, Chinese, Puerto Rican and black.

They found that racial membership significantly influenced both the pattern and level of intellectual sentence. In fact, racial differences were more prominent than the SES differences. Culture refers to a system of beliefs, attitudes, and values that assed from one generation to the next. In Indian context, there are prominent subcultures defined by caste groups, and traditional parental occupations.

The socialization practices in these subcultures are different. Studies have been conducted in rural Orissa comparing children of different groups.

The Harijan children scored west among the three groups and the Brahmins scored the highest. The Brahmins have a highly verbal articulate culture compared to the other groups; studies conducted in this region (Das and Singha, 1974; Jachuck and Mohanty, 1974) have reported significant differences in the cognitive level of children differing by caste, culture, and SES.

It may be unfair to compare the performance level of children from different cultures, as the skills required for successful adaptation in different cultures great deal The skills tapped by the standard cognitive tests are those  that are demanded in more technologically advanced cultures and higher SES groups.

As a result the tests are biased in favor of their competence, and it is no wonder that we find inferior performance of lower caste children on these. The appropriateness of the test items has to be considered in any investigation comparing the performance of children from various cultural and groups.

13. Sex differences

The overall IQ scores of boys and girls are very similar. There is some evidence that sex differences exist for particular kinds of cognitive abilities Review of a number of studies has shown that females are superior in language skills, verbal fluency, and reading, while males are superior in mathematical reasoning and spatial abilities (Oetzel, 1966). While neither sex is sup the two sexes show different patterns of intellectual abilities.

These have been supported by several researchers. Some argue that intellectual differences between sexes reflect different child-rearing practice; socio-cultural training. The parents and the society train boys and differently in terms of what to expect from them. It is known that intelligence related to personality characteristics.

Boys are socialized in a way so promote self-reliance and competence, which are positively correlate intelligence. On the other hand, the traits are discouraged in girls so mi that high intelligence is often considered a masculine quality.

The sex differences also partly result from the fact that many items standardized intelligence tests are biased in favor of the male population. Hence sex differences are the products of the test itself. Researchers differ in their convictions regarding sex differences. A group of researchers that sex differences are reflections of constitutional and genetic difference between males and females. The most reasonable conclusion is that differential abilities are the products of some combination of genetic and environmental factors.

15. Personality dispositions

There is some evidence to suggest that changes in IQ are related to general pattern of adjustment and personality. In a longitudinal study (5 Baker, and Nelson, 1958), 140 children were tested at intervals bet and 12 years of age. The 35 children, whose IQs increased remarkable found to have personality traits of assertiveness, independence, self-in and competitiveness.

On the contrary the 35 children who showed a d trend were found to lack these traits. If the personality traits were not acceptable, the advantages would be minimized. For example, child show temper tantrums have been found to display drops in their IQs (Peskin, 1964). Good intellectual functioning requires the ability to harness one’s emotions and utilize them in a constructive manner.

16. Physiological conditions

The physiological conditions such as nutrition, health, drugs, disease, and physical injury affect the cognitive competence of the child. Healthy body gives a healthy mind. The mental development is associated with biochemical processes and hormones within the body. The biological processes within the body provide a necessary but not sufficient condition for intellectual development.

Poor health and susceptibility to diseases would retard the growth of brain cells, and consequently the intellectual skills. Physical injury to the brain during early childhood years is likely to result in minimal brain damage thus seriously restricting the development of intellectual faculty. Similarly, intoxicating drugs and alcohol consumption would adversely affect the biological processes and the development of brain cells.

Thus, the internal physiological conditions are critical for the expression of intelligent behavior.

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