Intelligence quotient

An intelligence quotient (IQ) is a total score derived from a set of standardized tests or subtests designed to assess human intelligence.

Previously IQ was a score obtained by dividing a person’s mental age score, obtained by administering an intelligence test, by the person’s chronological age, both expressed in terms of years and months. 

The resulting fraction was multiplied by 100 to obtain the IQ score.

IQ tests today: the raw score is transformed to a normal distribution with mean 100 and standard deviation 15.

Two-thirds of the population scoring between IQ 85 and IQ 115 and about 2.5 percent each above 130 and below 70.

Intelligence tests are estimates of intelligence. 

A concrete measure of intelligence cannot be achieved due to the abstract nature of the concept of intelligence.

IQ scores have been shown to be associated with: nutrition, parental socioeconomic status, morbidity and mortality, parental social status, and perinatal environment.

Debate exists about the significance of heritability estimates and the mechanisms of inheritance.

Brain volume does not correlate to intelligence, or intelligence quotient (IQ).

IQ scores are used for educational placement, assessment of intellectual disability, evaluating job applicants, predictors of job performance and income, to study distributions of psychometric intelligence in populations and the correlations between it and other variables. 

Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect. 

Different kinds of IQ tests exist and include: visual,  verbal, abstract-reasoning problems, arithmetic, vocabulary, or general knowledge.

The Wechsler test is the most popular test in the United States.

Fluid intelligence (Gf) is the ability to solve novel problems by using reasoning, and intelligence and is hypothesized to be a knowledge-based ability that was very dependent on education and experience. 

Fuid intelligence was hypothesized to decline with age.

Crystallized intelligence is largely resistant to the effects of aging, and is reflected in a person’s general knowledge, vocabulary, and reasoning based on acquired information. 

IQ tests give an overall score, but are now also give scores for many of these more restricted abilities, identifying particular strengths and weaknesses of an individual.

There are a variety of individually administered IQ tests in use.

The most commonly used individual IQ test series is the Wechsler Adult Intelligence Scale (WAIS) for adults and the Wechsler Intelligence Scale for Children (WISC) for school-age test-takers. 

Other commonly used individual IQ tests include the current versions of the Stanford–Binet Intelligence Scales, Woodcock–Johnson Tests of Cognitive Abilities, the Kaufman Assessment Battery for Children, the Cognitive Assessment System, and the Differential Ability Scales.

IQ scores can vary to some degree for the same person on different IQ tests.

A person does not always belong to the same IQ score range each time the person is tested.

IQ tests exhibit high reliability, although test-takers may have varying scores when taking the same test on differing occasions, and may have varying scores when taking different IQ tests at the same age. 

Any estimate of IQ has an associated standard error that measures uncertainty about the estimate. 

The confidence interval can be approximately 10 points and reported standard error of measurement can be as low as about three points.

Influences such as low motivation or high anxiety can occasionally lower a person’s IQ test score.

IQ scores much higher than 160 are considered dubious.

IQ tests may fail to serve as an accurate measure of broader definitions of human intelligence, such as creativity and social intelligence. 

IQ tests are valid measures of the kind of intelligence necessary to do well in academic work. 

Many dispute the value of IQ as a measure of intelligence altogether, arguing IQ test scores alone neglects other important aspects of mental ability, and does not fully account for the different skills and knowledge types that produce success in human society.

Differential item functioning (DIF), sometimes occurs when participants from different groups based on gender, race, or disability, with the same latent abilities give different answers to specific questions on the same IQ test.

Since the early 20th century, raw scores on IQ tests have increased in most parts of the world.

IQ test scores have been rising at an average rate of around three IQ points per decade. 

Flynn effect has slowed or reversed course in some Western countries beginning in the late 20th century: the negative Flynn effect.

Norwegian military conscripts’ test records found that IQ scores have been falling for generations born after the year 1975, and that the underlying cause of both initial increasing and subsequent falling trends appears to be environmental rather than genetic.

IQ can change to some degree over the course of childhood.

The current consensus is that fluid intelligence generally declines with age after early adulthood, while crystallized intelligence remains intact.

Fluid intelligence peaks at a relatively young age, often in the early adulthood, while longitudinal intelligence is stable until mid-adulthood or later. 

Subsequently, intelligence seems to decline slowly.

Environmental and genetic factors play a role in determining IQ.

The general figure for the heritability of IQ,is 0.45 for children, and rises to around 0.75 for late adolescents and adults.

The shared family environment accounts for 0.25–0.35 of the variation in IQ in childhood, but by late adolescence, it is quite low.

A very large proportion of the over 30-40,000 human genes are thought to have an effect on the development and functionality of the brain: none have a strong effect. 

The interaction of genetic effects with socioeconomic status, such that the heritability was high in high-socioeconomic families, but much lower in low-socioeconomic families.

These findings have been replicated in infants, children, adolescents, and adults in the US, but studies show no link between heritability and socioeconomic status outside of the US.

Some effects may even reverse sign outside the US.

Genes for high IQ initiate an environment-shaping feedback cycle, with genetic effects causing bright children to seek out more stimulating environments that then further increase their IQ. 

In general, educational interventions, have shown short-term effects on IQ, but long-term follow-up is often missing. 

In the US, very large intervention programs such as the Head Start Program have not produced lasting gains in IQ scores. 

Even when students improve their scores on standardized tests, they do not always improve their cognitive abilities, such as memory, attention and speed.

Training in using one’s working memory may increase IQ. 

Music training does not reliably enhance children and young adolescents’ cognitive or academic skills, and that listening to classical music raises IQ at best with a short-term effect, lasting no longer than 10 to 15 minutes, and is not related to IQ-increase

Factors correlated with intelligence in humans, include the ratio of brain weight to body weight and the size, shape, and activity level of different parts of the brain. 

Factors affecting IQ include:  the size and shape of the frontal lobes, the amount of blood and chemical activity in the frontal lobes, the total amount of gray matter in the brain, the overall thickness of the cortex, and the glucose metabolic rate.

Height gains are strongly correlated with intelligence gains.

Both height and skull size increases are a result of a combination of phenotypic plasticity and genetic selection over this period.

Micronutrient deficiencies change the development of intelligence: 

iodine deficiency causes a fall, on average, of 12 IQ points.

 In the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas.

Infectious diseases-a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks: the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.

It is suggested that improvements in gross domestic product (GDP), education, literacy, and nutrition may have an effect on IQ mainly through reducing the intensity of infectious diseases.

States with a higher prevalence of infectious diseases have  lower average IQ. 

Several health factors can lead to significant cognitive impairment, particularly during pregnancy and childhood when the brain is growing and the blood–brain barrier is less effective. 

Such impairment may sometimes be permanent, or sometimes be partially or wholly compensated for by later growth.[citation needed]

There is a very close and consistent link between IQ scores and infectious diseases, especially in the infant and preschool populations and the mothers of these children: postulating  that fighting infectious diseases strains the child’s metabolism and prevents full brain development. 

Subsequent factors such as good nutrition and regular quality schooling can offset early negative effects to some extent.

Health policies regarding nutrients and toxins known to influence cognitive function have been instituted: fortification of certain food products and laws establishing safe levels of pollutants such as lead, mercury, and organochlorides.

Improvements in nutrition, and public health have been implicated in worldwide IQ increases.

Children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. 

The correlation between IQ scores and school grades is about .50: Achieving good grades depends on factors other than IQ, such as persistence, interest in school, and willingness to study.

Some measures of educational aptitude correlate highly with IQ tests.

The most valid predictor of future job performance is general mental ability.

IQ is more strongly correlated with reasoning and less so with motor function.

IQ-test scores predict performance ratings in all occupations.

For highly qualified activities such as research, and management a low IQ score is more likely to be a barrier to adequate performance.

For minimally-skilled activities: manual strength, speed, stamina, and coordination is more likely to influence performance.

The effects of IQ on job performance have been greatly overestimated, with the current estimates of the correlation between job performance and IQ are about 0.23.

IQ exerts a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.

General cognitive ability, but not specific ability scores, predict academic achievement, except  that processing speed and spatial ability predict performance on the SAT math test beyond the effect of general cognitive ability.

Studies indicate an increase in IQ translates into an increase in performance at all levels of IQ.

The link from IQ to wealth is much less strong than that from IQ to job performance. 

Some studies indicate that IQ is unrelated to net worth.

The American Psychological Association: IQ scores accounted for about a quarter of the social status variance and one-sixth of the income variance. 

An estimated correlation between IQ and income to be about 0.23.

The correlation between IQ and income averages a moderate 0.4, and the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. 

The impact of non-IQ factors on income:individual’s location, inherited wealth, race, and schooling are more important as factors in determining income than IQ.

The correlation between IQ and crime is only 0.2.

People with IQs between 70 and 90 have higher crime rates than people with IQs below or above this range, with the peak range being between 80 and 90.

Around eight IQ points, separate criminals from the general population, especially for persistent serious offenders. 

Children with conduct disorder have lower IQ than their peers.

US county-level crime rate analysis found that higher average IQs were very weakly associated with lower levels of property crime, burglary, larceny rate, motor vehicle theft, violent crime, robbery, and aggravated assault.

It has also been shown that the effect of IQ is heavily dependent on socioeconomic status, and a longitudinal study has shown that this relationship is entirely mediated by school performance.

Multiple studies have found that higher IQs in early life are associated with lower mortality and morbidity rates later in life.

IQ scores vary on average between ethnic and racial groups: these differences have fluctuated and in many cases steadily decreased over time.

These differences stem from environmental rather than genetic causes.

The existence of differences in IQ between the sexes has been debated.

No scientific evidence that the average IQ scores of different population groups can be attributed to genetic differences between those groups: environmental factors, not genetic ones, explain the racial IQ gap.

Female subjects performing better on tasks related to verbal ability,and males performing better on tasks related to rotation of objects in space, often categorized as spatial ability.

Men and women are essentially equal in general intelligence.

Male advantages on some cognitive tests are minimized when controlling for socioeconomic factors.

There is slightly more males than females in the top and bottom of the IQ distribution.

A meta-analysis focusing on average gender differences in math performance found nearly identical performance for boys and girls on intelligence quotient test.

Top 10 personality traits of high IQ people include:

Empathy and compassion

Highly observant and adaptable

Thirst to learn new skills and about the world at large

Maturity and self-control

Good short-term memory

Mentally flexible

Willingness to self-educate

Ability to ask the right questions

Good language, reasoning, and judgement skills

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