How Does Heredity and Environment Influence Intelligence?

Genetic Influences on Intelligence

Research findings confirming that intelligence is a by-product of our heredity revolves around finding specific genetic markers and associating IQ levels of parents and siblings.

Genetic markers on intelligence are believed to be found in chromosomes 4, 6 and 22. The genetic marker in chromosome 6 appears in 1/3 of children with high IQ, and only 1/6 of children with average IQs.

Identical twins appear to possess more similar IQ levels than fraternal twins. Arthur Jensen in 1969 reviewed research studies about intelligence and found that the IQ levels of identical twins are 32% more similar than the IQ levels of fraternal twins. He also found that the IQ levels of identical twins reared together are just 11% more similar than the IQ levels of identical twins reared apart. Thus, Jensen concludes that the environment plays very minimal role in developing intelligence.

Apart from looking at the intelligence of twins, parents' intelligence also contributes to the genetic debate. Research shows that the IQ of biological parents are more predictive of the child's IQ than the IQ of the adoptive parents.

Environmental Influences on Intelligence

Environmental influences come in different packages - family, ethnicity and culture, socio-economic status, gender, and education.

The family has a critical role in elevating IQ scores. Communication received during the first 3 years of life is the primary predictor of Stanford-Binet IQ score at age 3. Moreover, simply moving or exposing children to families with better environments increases their IQs by 12 points.

Ethnicity also plays an influential role. On average, African American schoolchildren score 10-15 points lower on standardized tests than White American schoolchildren. Also, only 15-25% of African American schoolchildren scored higher than 50% of White American schoolchildren. Additionally, African Americans are prone to stereotype threat. In one study, 50% of African American and White American students at Stanford University were told that the verbal part of the Graduate Record Examination (GRE) will be used to assess their intellectual abilities, whereas the other 50% were told that the test will be simply administered to establish its reliability and validity. Both groups of White American students scored similarly on the two scenarios. However, African Americans scored better on the second scenario than the first. The first scenario triggered a stereotype threat. The African American students were pressured on the first scenario, that how they'd score in the test might confirm a negative stereotype about their ethnicity. As a result of their anxiety, they performed worse than they'd normally do.

The notion of intelligence also differs across cultures. Europeans place emphasis on thinking and reasoning skills; Kenyans on participation in family and community activities; Ugandas on planning and execution; Iatmul tribe in Papua New Guinea on remembering the names of 10-20,000 clan members; and tribes at the Caroline Islands in the Pacific Ocean near eastern Philippines cherish the ability to navigate land and sea by reading the stars.

Socio-economic status is also very important. Having the means and the resources gives a sense of security and access to stimulating environments. Within a certain ethnicity, those in the low SES scored significantly lower than those in the middle and higher SES.

One's gender can also influence intelligence. On average, males and females have similar IQ levels. However, males' scores are more variable. Males specifically perform better in spatial and abstract reasoning, while females tend to be better at finding synonyms.

Lastly, and perhaps the most important of all, is education. Intervention day care increases IQ score by 17 points at age 3, and 5 points by age 15. South African Indian children lost 5 IQ points per year of schooling delayed due to the unavailability of teachers. The gap between African and White Americans on standardized test scores is narrowing, especially in college, most likely because of having similar educational environments. In 1999, James Flynn also discovered that the human race is becoming more and more intelligent (also called the Flynn Effect). In the ancient times, only adult priests can read; nowadays, 7-year old children must already know how to read.