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“”I have no idea. People who boast about their I.Q. are losers.
|—Stephen Hawking, when asked about his IQ|
IQ (intelligence quotient) is
something which flat earthers lack a psychometric method considered to measure general intelligence (or g) and derived from the fact that various tests of cognitive ability tend to have highly correlated scores for an individual. The correlations are due to a hypothesised general factor of intelligence or g. IQ correlates with educational achievement and job performance (with a coefficient of 0.65-0.75 for educational achievement  and about 0.5 for job performance), but is only predictive of these traits within certain values of IQ. Since IQ is simply a score, it is inaccurate to claim that one "has" a high IQ or a low IQ, when it more or less means "scored high" and "scored low".
IQ has been criticised for only measuring some sorts of intelligence and for being inadequate in that it unfairly emphasises certain types of abstract problem solving (ones which are not exactly common pastimes in poorer parts of the world). This perception has probably been reinforced by common tropes in movies and books of high IQ characters who have little "street smarts", or who possess little "common sense". Some go even further and argue that IQ tests measure social class, not intelligence.
By contrast, obsession with IQ tends to minimize the varied nature of crystallized and semi-fluid intelligence factors as described in Cattell–Horn–Carroll theory. IQ (as a proxy for g) has become an oversimplified tool for approaching intelligence in academia, and particularly among lay people with superiority complexes. In fact, one of the key markers of good science, an underlying causative mechanism, is very poorly established for g, and thus IQ.
Another issue is that it is a normalised measure, calibrated to have 100 as the average and 68%  of people in the 85-115 range under Wechsler Adult Intelligence Scale. So an IQ of 140 measured now is not the same as an IQ of 140 measured 10 years ago. There are by this measure people are getting less intelligent. It was never designed to be an absolute measure. Groups that boast about their high IQ generally don't retest their members periodically, presumably to avoid the embarrassment of having to cashier one of their members every now and again.
Testing outside of a formal environment for bragging rights should probably be looked on as something of a scam. Fun for bragging on social media, but little else. Paying for IQ tests means that, by definition, you have an IQ below 100. Don't do it, but if you must, never own up to it.
Importantly, most adult skills heavily depend on what is called crystallized intelligence, the ability to tune one's intellectual ability towards solving particular kinds of problems, rather than fluid intelligence, the kind of broad analytical ability that IQ is intended to measure. This is not a failing of IQ as a measure of intelligence, but certainly something that people who place a lot of stock in the concept tend to gloss over.
Though numerous familial and twin studies have found high heritability estimates for IQ, all attempts to find direct genetic relationships that drive IQ scores have failed to deliver on the expectations of that heritability . This "heritability paradox" has some explanations with serious academic weight, such as by epigenetic characteristics, such as imprinting, which can be passed from parent to child without any permanent multi-generational genes attached, and common developmental factors such as nutrition and prenatal hormone exposure.
- The Science of Second-Guessing The New York Times Magazine 12 December 2004
- IQ tests hurt kids, schools -- and don't measure intelligence Salon 7 July 2013
- Ken Richardson & Sarah Norgate. Does IQ Really Predict Job Performance? Applied Developmental Science (Volume 19, Issue 3), Taylor & Francis. 3 July 2015
- Adam Hampshire, et al. Fractionating Human Intelligence Neuron (Volume 76, Issue 6), Cell Press. 20 December 2012
- What Does IQ Really Measure? Science Magazine 25 April 2011
- Ken Richardson. What IQ Tests Test Theory & Psychology (Volume 12, Issue 3), SAGE Publications. 1 June 2002
- Lazar Stankov. Overemphasized “g” Journal of Intelligence (Volume 5, issue 4), MDPI. 1 October 2017
- IQ to Percentile Conversion IQ Test for Kids
- Bernt Bratsberg & Ole Rogeberg. Flynn effect and its reversal are both environmentally caused Proceedings of the National Academy of Sciences of the United States of America (Volume 115, Number 26), National Academy of Sciences. 26 June 2018
- Human Intelligence and Polymorphisms in the DNA Methyltransferase Genes Involved in Epigenetic Marking
- Mendelian and polygenic inheritance of intelligence: A common set of causal genes? Using next-generation sequencing to examine the effects of 168 intellectual disability genes on normal-range intelligence