9.3: Genetic and Environmental Influences on IQ

We shouldn't exaggerate the role of practice in intelligence and IQ testing.

Practice Movie Portrayal of Geniuses isn't enough to achieve excellence.

Practice is important, but unless we have innate talent in our chosen domain, it won't matter much.

Good Will Hunting, a 1997 movie, is not accurate in its portrayal of childhood or adolescent influences on IQ geniuses.

We've talked about intelligence for a long time.

We haven't said much about its causes or nature's role in its development.

The genetic and environmental contributors to IQ have been better understood by psychologists over the past few decades.
Significant flash points of controversy remain.

Scientists can study genetic influences on psychological characteristics in three different ways: family studies, twin studies, and adoption studies.

They've done it for both intelligence and consistency.

With increasing biological distance, the proportion of relatives who achieved intellectual greatness declined.

Intellectually brilliant individuals had many first degree relatives who were also brilliant, but fewer second degree relatives who were also brilliant.
The correlation of IQ for brothers and sisters raised in the same family is about 0.45, whereas for cousins it's about 0.15.

We don't know if the trait is genetic, environmental or a mix of both.

Family studies don't allow investigators to disentangle the effects of nurture and nature, so they've turned to more informative research designs.

The twin design's logic is easy to understand.

We can compare the correlations in IQ between the two types of twins because they share the same genes.
There are a few assumptions we won't bother with, and higher identical twin correlations strongly suggest genetic influence on a trait.
In almost all cases, studies of twins reared together have shown evidence of higher identical twin correlations for IQ.
Twin correlations have ranged from the 0.7- to 0.8-range in typical IQ studies.
In all studies of twins raised together, identical twin correlations have been lower than 1.0.

The findings tell us two things.

The higher identical twin correlations tell us that IQ is influenced by genetics.
The best estimate for the heritability of IQ is between 40 and 70 percent.
The heritability of IQ seems to increase from 20 percent in infancy to 80 percent in adulthood, perhaps because people become less influenced by their environments as they move away.

The twin findings don't tell us which genes are relevant to intelligence, and investi gators have tried with mixed success to find genes specific to intelligence.

Whatever these genes are, they appear to slice across multiple domains of mental ability, including attention, working memory, and perhaps even risk for Alzheimer's disease.
It's clear that intelligence isn't due to just one or even a small number of genes; instead, it appears to be tied to enormous numbers of genes, each probably exerts tiny effects on brain functioning.
It's clear that there isn't a "gene" for intelligence or just a few genes.

There is one exception to the moderate to high heritability of IQ.

Increasing evidence suggests that heritability of IQ may be low in individuals, especially children, at or below the poverty line.

It is possible that the effects of environment on intelligence may swamp the effects of genes at high levels of environmental deprivation.
When environments are optimal, people may be able to actualize their genetic potential toward learning and seeking out new information, thereby boosting their intelligence.
The range of environments in our sample can affect heritability, so it's not a fixed number.
There's less opportunity for people in poor neighborhoods to realize their intelligence because they don't have access to environmental resources like books and computers.

Twin findings provide convincing evidence for environmental influences on IQ, because the correlations for IQ are always less than perfect.

If genetic influences alone were used, identical twins would correlate 1.0 if the IQ tests were reliable.
The studies don't tell us what the environmental influences are, but the fact that they correlate less than 1.0 tells us.

We've only talked about studies of twins raised together.

The studies are Ruling Out Rival Hypotheses, which is nerable to a rival hypothesis.
There are important alternative studies of identical and fraternal twins that can be used to exclude this possibility.

Twins reared apart was conducted by Minnesota.

A sample of more than 40 identical twin pairs reared apart were just as similar on three measures of IQ as were identical twins reared together.
The sample sizes of these studies are relatively low because twins reared apart are extremely rare.

The studies of intact family members are limited because they can't disentangle genetic from environmental influences.

Adoption studies allow us to separate the effects of environment and genes on IQ because they are raised by parents who share an environment but not genes.

The similarity between adoptive children and adoptive parents can be misinterpreted as an environmental effect.

In adoption studies of IQ, researchers try to control for the correlation of IQ between biological and adoptive parents.

Adoption studies show that the environment contributes to IQ.

When adopted into homes with more enriched environments, adopted children from extremely deprived environments show an increase in IQ.
In a study of French children raised in a poor environment, children who were adopted had an average 16-point IQ edge over children who weren't.

Adoption studies show that the IQs of adopted children tend to be the same as those in orphanages, and that they show an increase in their biological parents.

The resemblance of the adoptive parents in IQ to the adoptees in IQ is dissipated once the adoptive environment is more attentive.

Twin and adoption studies show that both genes and environment affect IQ scores.

Although they've made progress in identifying promising candidates, psychologists don't know for sure.
Some of the environmental influences have more compelling evidence than others.

Recent research suggests that how we think about intelligence may affect it.

Carol Dweck showed that people who believe that intelligence is a fixed entity that doesn't change tend to take less academic risks.
They tend to give up after failing on a problem because they think they can't increase their intelligence.
They persist after failing on a problem because they believe that effort can pay off.
They may perform better in the long run on challenging intellectual tasks.

There are claims that intelligence is related to performance on mental tests.

Increasing numbers of children in a family leads to a decline in IQ.

Zajonc was correct that later-born children tend to have slightly lower IQs than earlier-born children.

It's not clear if he understood the correlation correctly.
Parents with lower IQs are more likely to have more children than parents with higher IQs.

The correlation is that children who come from larger families have lower IQs than children who come from smaller families.

Studies show that educated people have more neural connections than less educated people.

The number of years of school is related to IQ scores.
It's possible that the causality arrow is reversed, as some authors have interpreted the correlation as meaning that education leads to higher IQ.

They may be more likely to stay in school and attend college.

Individuals with high IQ scores tend to do better in their classes.

The researchers looked at pairs of children who were almost exactly the same age, but in which one child attended an extra year of school because he or she was born a few days earlier.

Public schools often have cutoff dates for how old children must be to start school.

Children who have attended an extra year of school tend to have higher IQs, even though they are nearly the same age.

Children's IQs tend to go down during the summer.

Students who drop out of school are more likely to have lower IQs.

Arthur Jensen argued in the late 1960s that IQ is highly heritable and therefore difficult to modify by means of environmental intervention.

Jensen's argument was based on the fact that heritability implies that a trait can't be changed.

The hope was that the program would allow them to catch up with their peers.

The federal Head Start program has yielded consistent results, but they have been disappointing.
The programs were launched in the 1960s to give a positive side to the world.
There are studies showing that children from deprived environments.
After the Head Start programs end, they don't typically produce increases in IQ.
The results from other studies fade with time.

Even when short-term increases in IQ are found, they may be due to "teaching to the test" since the increases don't extend to the IQ test items most linked to general intelligence.

Several studies show that Head Start and similar early intervention programs result in lower rates of dropping out of high school and being held back a grade compared with control conditions.

They may give higher levels of early literacy and understanding of others' emotions.

Intelligence and IQ Testing improve children's social and emotional functioning, including their ability to inhibit aggression, gain acceptance from peers, and develop close relationships with their teachers.

Robert Rosenthal and Lenore Jacobson wanted to look at the effects of teacher expectancies on IQ.

We saw it earlier in the text.

Rosenthal and Jacobson looked at the expectancies of teachers instead of researchers.
The fake IQ test was given to students in the first through sixth grades.
The results showed that 20 percent of the students would show remarkable gains in intelligence over the next 8 months.
Rosenthal and Jacobson lied to the teachers.
The 20 percent of students labeled as bloomers scored about four IQ points higher than the other students when they were retested a year later.
Expectations had become reality.

The replicability effect is usually modest.

We don't know how this effect occurs, although there is evidence that teachers smile at, make eye contact with, and nod their heads at students they think are smarter than other students.

They may reinforce these students' learning.

There are limits to the effects of expectancy on IQ and achievement.

The effects are only significant when teachers don't know their students well, and when teachers have worked with students for at least a few weeks.
It's hard to convince teachers that their impressions of their students are incorrect.

A group of families in a poor area of Georgia were studied by Arthur Jensen.

Cumulative declines are differences that increase over time for African American children.
Older siblings had lower IQs than younger siblings, with a decrease of 1.5 IQ points per year.
Jensen's explanation was that siblings in this impoverished region experienced progres Lead in toy sively more intellectual deprivation as they aged, leading them to fall Lead from peeling paint further behind other children.

The former children may have been more likely to suffer from the ill effects of lead in cans.

Poor people often have inadequate diet.

Studies from poor areas in everyday life suggest that there is a link between poor nutrition in childhood and decreased IQ.
The relationship between lead intake and IQ of preschool children from an impoverished region is controversial.

The children who received supplements had higher school-related test scores than the children who didn't.

The intake of high-fat and high-sugar foods in childhood is linked to slightly lower IQ scores several years later, although these data are only correlational and could be due to unmeasured factors.

Can the results be duplicated in lead-contaminated dust or lead paint chips?

It's not clear how much of the association is attributable to the direct effects of lead itself as opposed to poverty or other factors.

Breast-feeding has been the subject of scientific controversy.

On one side of the debate are researchers who claim that infants who are breast-fed end up with higher IQs than children who are bottle-fed.
The link between infant breast-feeding and adult IQ seems to last for at least 30 years, and may even extend to enhanced income and educational achievement.
Milk formula contains about 100 ingredients that aren't found in mothers' milk, including several that speed up the myelinization of neurons, as you'll recall from earlier in the text.

The effect of breast-feeding on IQ could be explained by these confounds.

James Flynn noticed something odd in the 1980s.

The magnitude of the Flynn effect is mind-blowing.

The average IQ of the United States, Europe, and South America has risen at a faster rate than the rest of the world.

Good news for you young readers of this, recent evidence indicates that the Flynn effect may be continuing, despite some initial data suggesting that it is slowing or even reversing.

Test sophistication has been increased.

The research shows that IQ scores have a correlation with exposure to Israel progressive matrices.

The modern world is more complex.

The causes of this effect are not clear.

We're forced to process far more information gains over time because of the discovery of IQ.

Intelligence and IQ Testing 345 did.

One American differs from his especially reasoning that involves geometrical objects, which are a starring role in the Civil War, in that he is placing more emphasis on abstract reasoning.

The explosion of information may be putting pressure on us to become more intelligent.

The upper, tail of the bell curve is thought to be unaffected by the Flynn effect.

There is a chance that this finding is related to diet.
The rates of severe malnutrition in many parts of the world are declining, and people are better fed than ever before.
There is good evidence that nutrition can affect IQ.

In the United States, families have become smaller, allowing parents to spend more time with their children.

More parents have access to intellectual resources.

Children and adolescents spend more time in school than previous generations.

As can be seen in the sizes of these men Journal Prompt and their uniforms, most people are discussing some of the possible environmental influences on IQ scores identified by considerably larger today than they were psychological scientists.

The era of the U.S. Civil War might have had a positive effect on these.

Dramatic differences in nutrition have occurred over the past 150 years.

Enhanced nutrition may account for the Flynn effect.

We would love to be able to achieve enormous acti success in college and beyond with minimal effort.

Many companies have capitalized on our desire for increased intelligence by making sensational claims.

You can develop your brain potential and become smarter by practicing an hour a day on our memory tests.

In multiple studies, Supersynapse's in-house scientists have shown that this remarkable product can increase intel igence.

Scientific skepticism requires us to evaluate claims with an open mind but to insist on compelling evidence before we accept them.

As you evaluate this claim, consider how the six principles of scientific thinking are.

We need to know more about SuperSynapse's in-house studies because they have improved on the tests.

The apparent improvement in IQ scores might be due to the drop-out from the studies.