What Is IQ?

IQ scores produce measurable levels of intelligence derived from intelligence tests such as the Stanford-Binet- intelligence Scale, Wechsler Intelligence Scale for Children and the Wechsler Adult Intelligence Scale.

Intelligence tests are designed using the median score set to 100; thus an average IQ score is considered average at a numerical value of 100 or an IQ of 100; and by using a standard deviation of 15. A median score is considered the 'middle' score or the numerical value that separates the higher half of a sample from the lower half. The standard deviation of 15 is simply implied as a score that may be correct within 15 IQ points.

intelligence scores are used in many forms such as predictors of educational achievement or by people with special needs, by social scientists who study the distribution of intelligence scores in populations and the relationships between intelligence score and other variables, and as predictors of job performance and income.

The Fluctuation of Intelligence Scores

intelligence can change to some degree over the course of childhood. However, in one longitudinal study, the mean IQ scores of tests at ages 17 and 18 were correlated at r=.86 with the mean scores of tests at ages 5, 6 and 7 and at r=.96 with the mean scores of tests at ages 11, 12 and 13.

intelligence scores for children are relative to children of a similar age. That is, a child of a certain age does not do as well on the tests as an older child or an adult with the same IQ. But relative to persons of a similar age, or other adults in the case of adults, they do equally well if the IQ scores are the same.

For decades, it has been reported in practitioners' handbooks and textbooks on intelligence testing that intelligence declines with age after the beginning of adulthood. However, later researchers pointed out that this phenomenon is related to the Flynn effect and is in part a cohort effect rather than a true aging effect.

There have been a variety of studies of intelligence and aging since the norming of the first Wechsler Intelligence Scale drew attention to IQ differences in different age groups of adults. Current consensus is that fluid intelligence generally declines with age after early adulthood, while crystallized intelligence remains intact. Both cohort effects (the birth year of the test-takers) and practice effects (test-takers taking the same form of intelligence test more than once) must be controlled for to gain accurate data. It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.

The peak of capacity for both fluid intelligence and crystallized intelligence occurs at age 26. This is followed by a slow decline.

Intelligence and Educational Performance

The American Psychological Association's report "Intelligence: Knowns and Unknowns" states that wherever it has been studied, 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 intelligence scores and grades is about .50. This means that the explained variance is 25%. Achieving good grades depends on many factors other than intelligence, such as "persistence, interest in school, and willingness to study".

It has been found that the IQ correlation with school performance depends on the intelligence measurement used. For undergraduate students, the Verbal IQ as measured by WAIS-R has been found to correlate significantly (0.53) with the GPA of the last 60 hours. In contrast, Performance IQ correlation with the same GPA was only 0.22 in the same study.

Intelligence and Real Life Accomplishments

Average adult combined IQs associated with real-life accomplishments by various tests:

• The average intelligence scores of neurosurgeons, research scientists, and university professors 135 plus MD's or PhD's 125
• College graduates average of 112 (KAIT, 2000; K-BIT, 1992), 115 (WAIS-R)
• 1 to 3 years of college yield an average score 104 (KAIT, K-BIT), 105-110 (WAIS-R)
• Clerical and sales workers average an IQ of 100 - 105
• High school graduates, skilled workers (e.g., electricians, cabinetmakers) have an average intelligence score of 100 (KAIT, WAIS-R), and 97 (K-BIT)
• 1–3 years of high school (completed 9–11 years of school) an average of 94 (KAIT), 90 (K-BIT), and 95 (WAIS-R)
• Semi-skilled workers (e.g., truck drivers, factory workers) average around 90 - 95
• Elementary school graduates (completed eighth grade) have an average intelligence score of 90
• Elementary school dropouts (completed 0–7 years of school) who average an IQ of 80 - 85 have 50/50 chance of reaching high school 75

Average IQ of Various Occupational Groups:

• Professional and technical personnel have an average IQ of 112
• Managers and administrators have an average IQ of 104
• Clerical workers; sales workers; skilled workers, craftsmen, and foremen have an average IQ of 101
• Semi-skilled workers (operatives, service workers, including private household; farmers and farm managers) have an average IQ of 92 and Unskilled workers average IQ of 87

Type of Work That Can Be Accomplished With Low IQ Scores

• Adults can harvest vegetables, repair furniture with an intelligence score of 60
• Adults can do domestic work, simple carpentry with an intelligence score of 50
• Adults can mow lawns, do simple laundry with an intelligence score of 40

There is considerable variation within and overlap between these categories. People with high intelligence quotients are found at all levels of education and occupational categories. The biggest difference occurs for low intelligence quotients with only an occasional college graduate or professional scoring below 90.