Intelligence quotient

An intelligence quotient (IQ) is a score from special tests that try to measure human intelligence. These tests check different thinking skills and give a number to show how well someone does compared to others.

In the past, IQ was found by comparing a person's "mental age" to their real age, then multiplying by 100. Today, IQ scores are set so most people score around 100. About two-thirds of people score between 85 and 115.

IQ scores help in many ways. They can help put students in the right school programs or show who might do well in certain jobs. Scientists also study IQ scores to see how intelligence can differ and what may affect it. One interesting fact is that IQ scores for many groups have been going up over time. This is called the Flynn effect.

Even though IQ tests are used a lot, they have limits. Intelligence is a big idea, and no single test can show everything about how smart someone is. Sadly, some people have used IQ ideas to support unfair beliefs about different groups. Today, experts say these old ideas are wrong and not based on real science.

History

Main article: g factor

Main article: Cattell–Horn–Carroll theory

People used to guess how smart others were by watching how they acted. Later, scientists made tests to measure intelligence better.

The first big step came from Francis Galton, who tried to link intelligence to things like reflexes and head size. His ideas helped start the field of psychometrics. In 1905, Alfred Binet and Théodore Simon made a test to find children who might need extra help in school. This test measured a child's "mental age"—how well they did compared to others their age.

In 1912, William Stern introduced the term "IQ," short for Intelligence Quotient. He suggested dividing a person's mental age by their actual age and multiplying by 100 to get an IQ score.

Later, psychologist Charles Spearman noticed that students who did well in one school subject often did well in others too. He thought this showed a general mental ability, which he called g for "general factor."

During World War I, the U.S. military used intelligence tests to assign soldiers to different roles. Many soldiers took these tests, which helped make psychology more respected and led to more research and jobs in the field.

Different scientists developed new ways to understand intelligence. Some, like David Wechsler, created tests that looked at both verbal and nonverbal skills. Others, like Raymond Cattell and John Horn, talked about two types of intelligence: fluid intelligence (solving new problems) and crystallized intelligence (using knowledge from experience). These ideas helped shape modern IQ tests, which often give more than one score to show a person's strengths.

Modern tests

There are several IQ tests used today, especially in English-speaking countries. Two of the most common are the Wechsler Adult Intelligence Scale for adults and the Wechsler Intelligence Scale for Children for school-aged children. Other well-known tests include the Stanford–Binet Intelligence Scales, Woodcock–Johnson Tests of Cognitive Abilities, and the Kaufman Assessment Battery for Children.

Some additional IQ tests you might hear about are Raven's Progressive Matrices, Cattell Culture Fair III, Reynolds Intellectual Assessment Scales, and several others. These tests give scores with an average of 100, and most scores are within about 15 points above or below that average. A score of 50 does not mean a person has only half the ability of someone with a score of 100.

Reliability and validity

IQ tests are thought to be very reliable. This means that if you take the same test again, you will likely get a similar score. But scores can change depending on the test or the day you take it. Things like feeling less motivated or very anxious can make your score lower. IQ tests measure some types of intelligence, but they might not show other important skills like creativity or how well you work with others. Some people think using only IQ scores ignores other valuable mental abilities.

We also need to think about whether IQ tests treat everyone fairly, no matter their cultural background. Scientists use special ways to find and change questions that might help one group more than another. Since the early 1900s, IQ test scores have gone up around the world. This is called the Flynn effect. People wonder if this means everyone is getting smarter. Some research shows that this increase might be slowing down or even going down a bit in some places. IQ can change when we are children and usually goes down a little after we become adults, but we are still learning exactly when and how this happens.

Genetics and environment

Environmental and genetic factors both affect a person's IQ. Scientists study how important each factor is.

Research shows that as people get older, genes play a bigger role in shaping IQ. In children, genes explain about 45% of IQ differences. By late adolescence and adulthood, this rises to around 75%. This may be because people with certain genes choose environments that help their natural abilities. Family environments also matter, especially in childhood, but their effect lessens as people age. Other experiences, like interactions with friends, can also affect IQ. Many genes are involved in brain development and intelligence, but each one has only a small effect on a person's IQ.

Interventions

Educational programs aimed at improving IQ, such as the Head Start Program, have shown short-term benefits. Students may see better scores on tests, but this doesn't always mean their thinking skills get better.

Some smaller studies, like the Abecedarian Project, suggest that training working memory might boost IQ. Research from 2008, involving the Universities of Michigan and Bern, hints that special working memory training could improve certain types of intelligence. More studies are needed to understand these effects better.

Music

Further information: Mozart effect

Some people think learning music when you are a child can help your brain. But studies show this isn’t always true. It might just be that some children who learn music are already clever.

There is also a belief that listening to classical music can boost your IQ. But research shows this only works for a very short time—like 10 to 15 minutes—and doesn’t really make your IQ higher forever.

Brain anatomy

Main article: Neuroscience and intelligence

Scientists have learned that parts of the brain are linked to intelligence. For example, the size and shape of areas like the frontal lobes can influence IQ. Other factors include the amount of gray matter, the thickness of the brain's outer layer, and how active some brain parts are. These findings help us learn how our brains support thinking and learning.

Health

Main articles: Impact of health on intelligence and Cognitive epidemiology

Health is important for doing well on IQ tests. Problems when a baby is growing or early in childhood, like getting sick, can change how the brain grows. But eating good food, having clean water, and going to school can help make these problems smaller.

Countries make rules to help people stay healthy, like putting vitamins in food and keeping bad things out of the environment. Learning about how health and intelligence are connected helps us see why some people might get different scores on IQ tests.

Social correlations

The American Psychological Association says that children with higher IQ scores often learn more in school than those with lower scores. But good grades depend on many things, like how hard a child works and how much they like school. IQ scores can help guess how well a child will do in school, but they are only one part of the story.

Research also shows that IQ can be linked to job performance. People with higher IQs might do better in jobs that need thinking and learning new things. But other things, like physical strength, can also matter for some jobs. IQ can also relate to income, but many other things, like family and education, are important too.

Group differences

IQ scores can be different on average between groups of people, like different ethnic or racial groups, and between males and females. Scientists believe these differences come from things in the environment, not from genetics. For example, studies show that the gap in IQ scores between racial groups has gotten smaller over time. This suggests that better environments and opportunities can help make these differences smaller.

Research also shows that males and females have the same overall intelligence, but they might be better at different specific things. For example, females often do better on verbal tasks, while males might do better on spatial tasks. These differences are small and do not mean one group is smarter than another. Most IQ tests are made so that males and females have the same average scores.

Public policy

Main article: Intelligence and public policy

In the United States, some rules and laws use a person's IQ score to help make choices about school programs and jobs. But in 1971, the U.S. Supreme Court said that IQ tests could not be used to hire people unless they were directly related to the job.

Other countries also make rules to help improve intelligence, like better food and staying away from harmful things.

An IQ test can help doctors understand if someone might have an intellectual disability. In the United Kingdom, a test called the eleven plus exam was used for many years to decide which school children would attend when they were eleven years old.

Classification

Main article: IQ classification

IQ classification is a way that IQ test publishers group IQ scores into different categories, such as "superior" or "average". Before IQ tests, people tried to sort others by their abilities by watching how they behaved. These observations still help check that the IQ test classifications make sense.

High-IQ societies

Main article: High-IQ society

Some special groups allow only people who score very high on intelligence tests. The most famous of these is Mensa International. There is also a smaller group called the Triple Nine Society, which has an even higher score requirement.