The Science Of IQ Testing

The science of valid, reliable and standardized IQ tests

Can your intelligence be measured by IQ tests?

Yes. Intelligence can be measured, and a scientific IQ test measures it well. IQ tests are the most well-researched and validated of all psychological tests and assessments. Spearman’s discovery, in 1904, of a general intelligence factor (G) common to many different mental abilities such as reasoning, general knowledge and vocabulary, has always been replicated in data sets involving large numbers of people taking a variety of mental tests.

G can be factored into fluid intelligence (Gf) – which is your  ’on the spot’ reasoning and problem solving ability that does not depend on background knowledge or expertise – and crystallized intelligence (Gc) – which is your ability to apply learned skills, knowledge & expertise. IQ tests can accurately measure both. In fact, in general both Gf and Gc are highly correlated. If you know someone’s fluid IQ level, you can accurately predict their general IQ level (G).

Types of IQ test

There are a number of different  intelligence tests and subtests used in IQ testing. Crystallized intelligence (Gc) subtests  require specific cultural knowledge (e.g. vocabulary, mathematical, or factual knowledge). Fluid intelligence (Gf) subtests, that do not depend on acquired knowledge, may require seeing abstract relationships in series of shapes, or doing logical reasoning.

Our recommended free online IQ tests [link] for measuring your IQ before and after training with High IQ Pro® are fluid intelligence tests. It is fluid intelligence that our IQ software targets.

The IQ bell curve & standardized IQ tests

Individuals differ widely in their levels of intelligence. The IQ bell curve (also known as the “normal curve” in statistical jargon) describes the spread of people along the intelligence continuum, from low to high. Areas under this curve represent percentages in the population. A standardized IQ test is designed so that the average IQ level is 100, and the distribution of scores in the general population follows a bell curve. Most people cluster around the average – between 85 and 115 IQ points. This is classified as a ‘normal’ IQ. A few individuals are either very smart or very dull. An IQ score of 130 is considered the criterion for ‘giftedness’; an IQ score below 70 is considered the threshold for mental retardation. An IQ of 120 or higher is considered to be ’superior’.


IQ bell curve
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What is an IQ standard deviation?
What is an IQ standard deviation?  In basic terms, the standard deviation is a measure of the spread of the IQ bell curve distribution. 1 standard deviation is usually 15 IQ points on a standardized IQ test. As a rule of thumb about 2/3 of the population within 1 standard deviation of either side of the average IQ score (=100). About 95% of the sample will be within 2 standard deviations from 100.
With the standard deviation and the average score, you can calculate percentiles.  Percentiles tell you the percent of people that have a score equal to or lower than a certain score. These percentiles are used by high IQ societies such as Mensa to determine IQ levels for membership. The most demanding high IQ society – The PARS Society – require a proof of intelligence level at or above +5 standard deviations from the average – that is, an IQ of 175 or greater. Only 1 in 3.33 million have an IQ this high!
The online IQ tests we recommend for assessing your pre and post High IQ Pro training have three properties that are important for all IQ and psychometric tests: they are standardized, reliable and valid.
IQ test standardization
Each IQ test is standardized. This means that the IQ test has been designed to have an IQ bell curve distribution of scores in the general population, with an average (mean) score of 100, and a standard deviation (a measure of the ’spread’ of the distribution) of 15 (or in some tests 16).
The IQ bell curve
you can meaningfully compare your IQ score with other individual’s IQ scores in terms of the IQ bell curve which describes the distribution of IQ scores for the whole population. On this curve, a score of 100 is average, and most people score close to 100. Areas under the curves represent percentages in the population. Nearly 70% of the population score between 85 and 115. A score of approximately 130 or greater qualifies you for Mensa membership. As you can see in the graph, only 2% of the population score at or above this level.
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Can your intelligence be measured?Yes. Intelligence can be measured, and a standardized IQ test measures it well. IQ tests are regarded as the most accurate of all psychological tests and assessments. Spearman’s discovery, in 1904, of a general intelligence factor (‘G’) common to many different mental abilities, has always been replicated in data sets involving large numbers of people taking a variety of mental tests. G can also be factored into fluid intelligence (Gf) – ‘on the spot’ reasoning and problem solving ability that does not depend on background knowledge or expertise – and crystallized intelligence (Gc) that is the ability to apply learned skills, knowledge & expertise.Types of intelligence testThere are several different types of intelligence tests and subtests used in IQ testing. Crystallized intelligence (Gc) subtests assess words or numbers and require specific cultural knowledge (e.g. vocabulary, or mathematical or factual knowledge). Fluid intelligence (Gf) subtests may require seeing abstract relationships in series of shapes, or doing logical reasoning. Despite all the differences in these tests, they all measure in part the same general intelligence factor (G)- especially fluid intelligence subtests.What is an IQ score? And what does my IQ mean?The IQ bell curve & standardized IQ test explainedIndividuals differ widely in their levels of intelligence. The IQ bell curve (also known as the “normal curve” in statistical jargon) describes the spread of people along the intelligence continuum, from low to high. Areas under this curve represent percentages in the population. A standardized IQ test is designed so that the average IQ level is 100, and the distribution is a bell curve. Most people cluster around the average – between 85 and 115 IQ points. This is a normal IQ. A few are either very smart or very dull. An IQ score of 130 is considered the criterion for ‘giftedness’; an IQ score below 70 is considered the threshold for mental retardation. An IQ of 120 or higher is considered to be ’superior’. A score over 110 can be called a ‘high normal IQ’. When a test is standardized like this, it is called a ‘psychometric test’.The IQ bell curve for IQ test scoresWhat is an IQ standard deviation?What is an IQ standard deviation?  In basic terms, the standard deviation is a measure of the spread of the IQ bell curve distribution. 1 standard deviation is usually 15 IQ points on a standardized IQ test. As a rule of thumb about 2/3 of the population within 1 standard deviation of either side of the average IQ score (=100). About 95% of the sample will be within 2 standard deviations from 100.With the standard deviation and the average score, you can calculate percentiles.  Percentiles tell you the percent of people that have a score equal to or lower than a certain score. These percentiles are used by high IQ societies such as Mensa to determine IQ levels for membership. The most demanding high IQ society – The PARS Society – require a proof of intelligence level at or above +5 standard deviations from the average – that is, an IQ of 175 or greater. Only 1 in 3.33 million have an IQ this high!Professional standardized  IQ testsThe tests below are widely used standardized and validated tests for intelligence. They are administered by qualified psychologists and clinicians. High IQ societies will often only accept IQ scores from a selection of these tests, administered by a professional, to ensure that your IQ score is authentic.WAIS IVStanford-Binet Intelligence Scales 5th editionRaven’s Advanced Progressive Matrices (culture free)Kaufman Brief Intelligence TestReynolds Intellectual Assessment ScalesLeiter International Performance Scale (culture free)Free online IQ testsThere are many free IQ tests, intelligence quizzes & brain tests on the web. Most are not scientifically valid and do not measure your true IQ level. By clicking on the link below you can take one of our recommended free IQ tests with better scientific validity to answer the question ‘what is my IQ?’ The results of these tests are confidential.
Professional standardized  IQ tests
The tests below are widely used standardized and validated tests for intelligence. They are administered by qualified psychologists and clinicians. High IQ societies will often only accept IQ scores from a selection of these tests, administered by a professional, to ensure that your IQ score is authentic.
WAIS IV
Stanford-Binet Intelligence Scales 5th edition
Raven’s Advanced Progressive Matrices (culture free)
Kaufman Brief Intelligence Test
Reynolds Intellectual Assessment Scales
Leiter International Performance Scale (culture free)
Free online IQ tests
There are many free IQ tests, intelligence quizzes & brain tests on the web. Most are not scientifically valid and do not measure your true IQ level. By clicking on the link below you can take one of our recommended free IQ tests with better scientific validity to answer the question ‘what is my IQ?’ The results of these tests are confidential.

What is an IQ standard deviation?

In basic terms, the standard deviation is a measure of the spread of the IQ bell curve distribution. In most IQ tests 1 standard deviation is 15 IQ points on a standardized IQ test. Some IQ tests (e.g. the Stanford-Binet Intelligence scales) have a standard deviation of 16. Because of this, you need slightly higher scores to match the same percentiles as tests with sds of 15.

As a rule of thumb about 2/3 of the population scores within 1 standard deviation of either side of the average IQ score (=100). About 95% of the sample will be within 2 standard deviations from 100.

With the standard deviation and the average score, you can calculate percentiles.  Percentiles tell you the percent of people that have a score equal to or lower than a certain score. These percentiles are used by high IQ societies such as Mensa to determine IQ levels for membership. The most demanding high IQ society – The Pars Society – require a proof of intelligence level at or above +5 standard deviations from the average – that is, an IQ of 175 or greater. In theory only 1 in 3.33 million have an IQ this high!

3 scientific criteria for all good IQ and psychometric tests

The online IQ tests we recommend for assessing your pre- and post-High IQ Pro® training have three properties that are important for all IQ and psychometric tests: they are standardized, reliable and valid.

  • Standardization. This means that the test is distributed as a bell curve with a known average and standard deviation in the population, so that comparisons of individual scores are meaningful.
  • Reliability. A reliable test is one that is consistent – either internally (different IQ questions or sub-tests should all measure the same thing), or over time – so that if you re-test someone using the same test, it should give you the same measure. If you score 120 on one day and 140 on the next, this is not a reliable test.
  • Validity. If a test is valid it measures what it is supposed to – our real intelligence (or general cognitive ability) and not ‘IQ test taking’ or ’schooling’. This is partly established in terms of what intelligence-requiring outcomes in real life (e.g. educational achievement and career success) IQ tests can predict. IQ tests are very good at predicting these real world outcomes [link]. For a more extensive list of what IQ scores can predict, see the bottom of this page.

Official IQ tests administered by certified psychologists

The tests listed below are widely used by professional psychologists and test administrators.  High IQ societies will often only accept IQ scores from professionally administered IQ tests such as these to ensure that your IQ score is the real deal!

Free online IQ tests that meet the 3 criteria for scientifically valid IQ tests

There are many free IQ tests, intelligence quizzes & brain tests on the web. Most are not scientifically valid and do not measure your true IQ level. We have selected the online tests that meet the relevant criteria to accurately measure your true IQ. [link]

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APPENDIX: What IQ tests can predict statistically

The following data is taken from Grady Tower’s article ‘Theories of Multiple Intelligence’ from the The Prometheus Society’s Journal, Gift of Fire Issue No. 33, September 1988 [link]:

A score obtained from a conventional IQ test can be used to predict performance on many activities outside the classroom. As evidence for this, here is a list of factors that are positively or negatively linked with IQ.

POSITIVE CORRELATES:

  • Achievement motivation
  • Altruism
  • Artistic preferences and abilities
  • Craftwork
  • Creativity
  • Dietary preferences (low-sugar, low-fat)
  • Educational attainment
  • Eminence, genius
  • Emotional sensitivity
  • Extra-curricular attainments
  • Health, fitness, longevity
  • Humor, sense of
  • Income
  • Interests, breadth and depth
  • Involvement in school activities
  • Leadership
  • Learning ability
  • Linguistic abilities (including spelling)
  • Logical abilities
  • Marital partner, choice of
  • Media preferences (newspapers, TV channels)
  • Memory
  • Migration (voluntary)
  • Military rank
  • Moral reasoning and development
  • Motor skills
  • Musical preferences and abilities
  • Occupational status
  • Occupational success
  • Perceptual abilities (for briefly-presented material)
  • Practical knowledge
  • Reading ability
  • Social skills
  • Socio-economic status of origin (parental)
  • Socio-economic status (achieved)
  • Sports participation
  • Supermarket shopping ability
  • Values, attitudes

NEGATIVE CORRELATES:

  • Accident-proneness
  • Acquiescence/obedience
  • Alcoholism
  • Authoritarianism
  • Crime
  • Delinquency
  • Dogmatism
  • Hysteria vs other neuroses
  • Impulsivity
  • Infant mortality
  • Psychoticism
  • Racial prejudice
  • Smoking
  • Truancy
  • Weight height ratio, obesity

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