Intelligence quotient

An intelligence quotient (IQ) is a score that comes from a set of standardized tests meant to measure human intelligence. These tests look at different thinking skills and give a number that shows how well someone does compared to others. In the past, IQ was calculated by comparing a person's "mental age" to their actual age, then multiplying the result by 100. Today, IQ scores are set so that most people score around 100, with about two-thirds of people scoring between 85 and 115.

IQ scores are used in many ways, such as placing students in the right school programs or helping decide who might do well in certain jobs. Scientists also study IQ scores to learn about how intelligence differs among groups of people and what factors might affect it. One interesting finding is that IQ scores for many groups have been going up over time, a trend known as the Flynn effect.

Even though IQ tests are widely used, they have their limits. Intelligence is a complex idea, and no single test can capture everything about how smart someone is. Also, history shows that some people have misused IQ ideas to support unfair beliefs about different races or groups of people. Today, most experts agree that these old, unfair ideas are wrong and not based on real science.

History

Main article: g factor

Main article: Cattell–Horn–Carroll theory

Historically, people tried to guess how smart others were by watching how they acted in daily life. Later, scientists created tests to measure intelligence more precisely.

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. Then, in 1905, Alfred Binet and Théodore Simon created 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. Over 1.7 million 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 that are organized so that the average score is 100, with most scores falling within a range of about 15 points above or below that average. However, this does not mean that someone with a score of 50 has only half the ability of someone with a score of 100.

Reliability and validity

Psychometricians consider IQ tests to have high statistical reliability, meaning they produce similar scores when taken again. However, scores can vary depending on the test or the occasion. Influences like low motivation or high anxiety can lower a person's score. IQ tests measure certain types of intelligence but may not capture broader aspects like creativity or social skills. Critics argue that relying only on IQ scores overlooks other important mental abilities.

The concept of test bias examines whether different groups, such as those from various cultural backgrounds, receive fair and equal results. Researchers use specific methods to identify and remove questions that may favor one group over another. Since the early 20th century, IQ test scores have generally increased worldwide, a trend known as the Flynn effect. This rise in scores has sparked discussions about whether it reflects a true increase in intelligence. Some studies suggest this trend may have slowed or reversed in certain Western countries in recent decades. IQ can change during childhood and generally declines with age after early adulthood, though the exact timing of these changes remains unclear.

Genetics and environment

Environmental and genetic factors both influence a person's IQ. Scientists have studied and debated how important each of these factors is.

Research shows that as people grow older, the role of genes in shaping IQ becomes more significant. In children, genes account for about 45% of IQ differences, but by late adolescence and adulthood, this rises to around 75%. This may be because people with certain genes tend to choose environments that further enhance their natural abilities. Family environments also play a role, especially in childhood, but their influence decreases as people get older. Other experiences outside the family, such as 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 but often lack long-term results. While students may see better scores on tests, this doesn't always mean their thinking skills, like memory or attention, 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. However, more studies are needed to understand how long these effects last and if they apply to different kinds of intelligence tests and real-world success.

Music

Further information: Mozart effect

Some people think that learning music when you are a child can help your brain work better. But studies have shown that this isn’t always true, and it might just be because some children who learn music are naturally cleverer.

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 in the long run.

Brain anatomy

Main article: Neuroscience and intelligence

Researchers have found that several parts of the brain relate to intelligence. For example, the size and shape of certain brain areas, like the frontal lobes, can affect IQ. Other factors include the amount of gray matter in the brain, the thickness of the brain's outer layer, and how active certain parts of the brain are. These discoveries help scientists understand how our brains help us think and learn.

Health

Main articles: Impact of health on intelligence and Cognitive epidemiology

Health plays a big role in how well someone does on IQ tests. Problems during pregnancy or early childhood, like infections, can affect brain development. But with good nutrition, clean water, and proper schooling, many of these effects can be improved.

Countries have made rules to keep people healthy, like adding vitamins to food and limiting harmful substances in the environment. Studying how health and intelligence are connected helps us understand why some people may score differently on IQ tests.

Social correlations

The American Psychological Association's report states that children with higher IQ scores tend to learn more in school compared to those with lower scores. However, good grades depend on many factors besides IQ, such as persistence and interest in school. Studies show that IQ scores can help predict school performance, but they are just one piece of the puzzle.

Research also suggests that IQ can be related to job performance. Generally, people with higher IQs may perform better in jobs that require reasoning and learning new information. However, other factors like physical strength can also influence performance in certain jobs. Studies indicate that IQ has a moderate relationship with income, but many other factors, such as family background and education, also play important roles.

Group differences

IQ scores can vary on average between different groups of people, such as ethnic or racial groups and males and females. However, scientists agree that these differences are due to environmental factors, not genetics. For example, studies have shown that the gap in IQ scores between different racial groups has shrunk over time, suggesting that improving environments and opportunities can help close these differences.

Research also shows that, while males and females have equal overall intelligence, they may excel in different specific areas. For instance, females often perform better on verbal tasks, while males may do better on spatial tasks. These differences are small and do not mean one group is smarter than another. Overall, most IQ tests are designed to ensure that males and females have the same average scores.

Public policy

Main article: Intelligence and public policy

In the United States, some laws and policies use a person's IQ score when making decisions about things like school programs and jobs. However, in 1971, the U.S. Supreme Court decided that IQ tests could not be used to hire people unless they were directly related to the job being done. Other countries also create policies to help improve intelligence, such as better nutrition and keeping away from harmful substances.

An IQ test can also 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 using IQ tests, people tried to sort others by their abilities using observations of their behaviour. These observations still help check that the IQ test classifications make sense.

High-IQ societies

Main article: High-IQ society

Some special groups let in 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.