Where our facts come from

Every study, textbook and official page cited on LifeScore, in one place. 50 are in our checked list, 33 of them peer-reviewed papers, each with what we use it for and which pages cite it. A further 41 come from the reference lists of our newest guides and are listed at the end until they are checked.

By the LifeScore Editorial Team · Updated 14 October 2026

How this list works

When a page states a figure or a finding, a small number next to it links to the source in that page's reference list. This page gathers all of them. We confirmed that each source exists and says what we use it for before adding it, and the date shows when we last checked. Sources we could not confirm were left out, and so were the claims that relied on them.

Some entries are encyclopaedia or publisher pages rather than studies. We use those for definitions and for facts a publisher states about its own test, and we say so. The rules are in our editorial policy. A broken link or a source that does not say what we claim is a correction: tell us.

The IQ scale and norms

American Mensa (2026). Qualifying test scores. American Mensa. us.mensa.org
The scores Mensa accepts on each test (for example 130 on the Wechsler scales, 132 on the Stanford-Binet).
Cited on How the IQ test is scored, What is a good IQ?, Standard deviation, High-IQ societies. Checked 14 October 2026.
Pearson Assessments (2024). WAIS-5 overview brochure. Pearson. pearsonassessments.com
How a professional IQ test is normed (2023 to 2024 census-matched sample) and how long it takes.
Cited on How the IQ test is scored, Average IQ, Standard deviation, How IQ is calculated, WAIS index scores. Checked 14 October 2026.
Wicherts, J. M., Dolan, C. V., & van der Maas, H. L. J. (2010). A systematic literature review of the average IQ of sub-Saharan Africans. Intelligence, 38(1), 1-20. doi.org/10.1016/j.intell.2009.05.002
Why cross-country IQ averages need care: tests, norms and the Flynn effect all move them.
Cited on Average IQ. Checked 14 October 2026.
Wikipedia contributors (2026). IQ classification. Wikipedia. en.wikipedia.org
The descriptive bands test publishers attach to score ranges, gathered in one table.
Cited on Average IQ, What is a good IQ?, Standard deviation, WAIS index scores, Levels of giftedness. Checked 14 October 2026.
Wikipedia contributors (2026). Intelligence quotient. Wikipedia. en.wikipedia.org
History of the deviation IQ and the mean-100 scale.
Cited on Standard deviation. Checked 14 October 2026.
Wikipedia contributors (2026). Stanford-Binet Intelligence Scales. Wikipedia. en.wikipedia.org
The Stanford-Binet scale used a standard deviation of 16 before the fifth edition moved to 15.
Cited on Standard deviation, How IQ is calculated. Checked 14 October 2026.
Wikipedia contributors (2026). Cattell Culture Fair Intelligence Test. Wikipedia. en.wikipedia.org
The Cattell scale uses a standard deviation of 24.
Cited on Standard deviation. Checked 14 October 2026.

Measurement, reliability and retesting

American Educational Research Association, American Psychological Association, & National Council on Measurement in Education (2014). Standards for Educational and Psychological Testing. AERA (open access edition, 2021). testingstandards.net
What reliability, validity and norms mean, and what a test publisher should report.
Cited on How the IQ test is scored, How the Reasoning practice test is scored. Checked 14 October 2026.
Lord, F. M., & Novick, M. R. (1968). Statistical Theories of Mental Test Scores. Addison-Wesley. ets.org
The binomial error model behind our estimate of how much a short test score can wobble.
Cited on How the IQ test is scored. Checked 14 October 2026.
Scharfen, J., Peters, J. M., & Holling, H. (2018). Retest effects in cognitive ability tests: A meta-analysis. Intelligence, 67, 44-66. doi.org/10.1016/j.intell.2018.01.003
Why second attempts score higher, and why gains level off after the third attempt.
Cited on How the IQ test is scored. Checked 14 October 2026.
Wikipedia contributors (2026). Normal distribution. Wikipedia. en.wikipedia.org
The maths of the bell curve that turns a score into a percentile.
Cited on How the IQ test is scored, What is a good IQ?. Checked 14 October 2026.

Age, development and education

Hartshorne, J. K., & Germine, L. T. (2015). When does cognitive functioning peak? The asynchronous rise and fall of different cognitive abilities across the life span. Psychological Science, 26(4), 433-443. doi.org/10.1177/0956797614567339 · Free full text
Different abilities peak at different ages, from processing speed near 18 to vocabulary in the late 60s.
Cited on Average IQ, The g factor, IQ myths. Checked 14 October 2026.
Ramsden, S., Richardson, F. M., Josse, G., Thomas, M. S. C., Ellis, C., Shakeshaft, C., Seghier, M. L., & Price, C. J. (2011). Verbal and non-verbal intelligence changes in the teenage brain. Nature, 479, 113-116. doi.org/10.1038/nature10514 · Free full text
Teen IQ scores can move by up to 20 points over about four years, in step with brain changes.
Cited on WISC-V scores, IQ myths, IQ test for teenagers. Checked 14 October 2026.
Ritchie, S. J., & Tucker-Drob, E. M. (2018). How much does education improve intelligence? A meta-analysis. Psychological Science, 29(8), 1358-1369. doi.org/10.1177/0956797618774253 · Free full text
Each extra year of education adds about 1 to 5 IQ points.
Cited on Average IQ, What is a good IQ?, Heritability of IQ, IQ and income, What affects IQ, IQ myths. Checked 14 October 2026.

The Flynn effect

Bratsberg, B., & Rogeberg, O. (2018). Flynn effect and its reversal are both environmentally caused. PNAS, 115(26), 6674-6678. doi.org/10.1073/pnas.1718793115
Norwegian brothers show the rise and the fall happen within families, so the cause is environmental.
Cited on The Flynn effect. Checked 14 October 2026.
Dworak, E. M., Revelle, W., & Condon, D. M. (2023). Looking for Flynn effects in a recent online U.S. adult sample: Examining shifts within the SAPA Project. Intelligence, 98, 101734. doi.org/10.1016/j.intell.2023.101734
Declines in several abilities among almost 400,000 US adults tested online, 2006 to 2018.
Cited on The Flynn effect. Checked 14 October 2026.
Flynn, J. R. (1984). The mean IQ of Americans: Massive gains 1932 to 1978. Psychological Bulletin, 95(1), 29-51. doi.org/10.1037/0033-2909.95.1.29 · semanticscholar.org
The first systematic evidence that scores rose about 0.3 points a year in the United States.
Cited on The Flynn effect. Checked 14 October 2026.
Flynn, J. R. (1987). Massive IQ gains in 14 nations: What IQ tests really measure. Psychological Bulletin, 101(2), 171-191. iapsych.com
Gains of 5 to 25 points in a generation, largest on fluid reasoning tests.
Cited on The Flynn effect. Checked 14 October 2026.
Pietschnig, J., & Voracek, M. (2015). One century of global IQ gains: A formal meta-analysis of the Flynn effect (1909-2013). Perspectives on Psychological Science, 10(3), 282-306. doi.org/10.1177/1745691615577701
About 3 points per decade across 271 samples and 4 million people, slowing in recent decades.
Cited on The Flynn effect, Heritability of IQ. Checked 14 October 2026.
te Nijenhuis, J., & van der Flier, H. (2013). Is the Flynn effect on g? A meta-analysis. Intelligence, 41(6), 802-807. research.vu.nl
Score gains are smallest on the tests most tied to general ability.
Cited on The Flynn effect. Checked 14 October 2026.
Teasdale, T. W., & Owen, D. R. (2005). A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse. Personality and Individual Differences, 39(4), 837-843. iapsych.com
Danish conscript scores peaked in the late 1990s and then fell.
Cited on The Flynn effect. Checked 14 October 2026.
Winter, E. L., Trudel, S. M., & Kaufman, A. S. (2024). Wait, where's the Flynn effect on the WAIS-5? Journal of Intelligence, 12(11), 118. pmc.ncbi.nlm.nih.gov
The WAIS-IV to WAIS-5 comparison suggests about 1.2 points per decade, not 3.
Cited on The Flynn effect, WAIS index scores. Checked 14 October 2026.

General intelligence (g)

Carroll, J. B. (1993). Human Cognitive Abilities: A Survey of Factor-Analytic Studies. Cambridge University Press. cambridge.org
The three-stratum model of abilities, from more than 460 datasets.
Cited on The g factor. Checked 14 October 2026.
Deary, I. J. (2012). Intelligence. Annual Review of Psychology, 63, 453-482. doi.org/10.1146/annurev-psych-120710-100353
A field overview that treats the positive manifold as the starting point.
Cited on The g factor. Checked 14 October 2026.
Johnson, W., Bouchard, T. J., Krueger, R. F., McGue, M., & Gottesman, I. I. (2004). Just one g: Consistent results from three test batteries. Intelligence, 32(1), 95-107. doi.org/10.1016/S0160-2896(03)00062-X · experts.umn.edu
The general factors from three different batteries correlated .99 to 1.00.
Cited on The g factor. Checked 14 October 2026.
Kovacs, K., & Conway, A. R. A. (2016). Process overlap theory: A unified account of the general factor of intelligence. Psychological Inquiry, 27(3), 151-177. doi.org/10.1080/1047840X.2016.1182000
Tests may correlate because they draw on overlapping executive processes.
Cited on The g factor. Checked 14 October 2026.
Spearman, C. (1904). "General intelligence," objectively determined and measured. American Journal of Psychology, 15, 201-293. psychclassics.yorku.ca
The paper that introduced the general factor of intelligence.
Cited on The g factor. Checked 14 October 2026.
van der Maas, H. L. J., Dolan, C. V., Grasman, R. P. P. P., Wicherts, J. M., Huizenga, H. M., & Raijmakers, M. E. J. (2006). A dynamical model of general intelligence: The positive manifold of intelligence by mutualism. Psychological Review, 113(4), 842-861. doi.org/10.1037/0033-295X.113.4.842 · pubmed.ncbi.nlm.nih.gov
The positive manifold can arise without a single general ability.
Cited on The g factor. Checked 14 October 2026.
Warne, R. T., & Burningham, C. (2019). Spearman's g found in 31 non-Western nations: Strong evidence that g is a universal phenomenon. Psychological Bulletin, 145(3), 237-272. psycnet.apa.org
A general factor appears in 97 datasets from 31 non-Western countries.
Cited on The g factor. Checked 14 October 2026.

What IQ predicts

Calvin, C. M., Deary, I. J., Fenton, C., Roberts, B. A., Der, G., Leckenby, N., & Batty, G. D. (2011). Intelligence in youth and all-cause-mortality: Systematic review with meta-analysis. International Journal of Epidemiology, 40(3), 626-644. doi.org/10.1093/ije/dyq190 · Free full text
Higher youth test scores went with lower mortality over decades of follow-up.
Cited on The g factor. Checked 14 October 2026.
Deary, I. J., Strand, S., Smith, P., & Fernandes, C. (2007). Intelligence and educational achievement. Intelligence, 35(1), 13-21. doi.org/10.1016/j.intell.2006.02.001
General ability at 11 correlated .81 with exam results at 16 in 70,000 English children.
Cited on The g factor. Checked 14 October 2026.
Heckman, J. J., Stixrud, J., & Urzua, S. (2006). The effects of cognitive and noncognitive abilities on labor market outcomes and social behavior. Journal of Labor Economics, 24(3), 411-482. doi.org/10.1086/504455
Persistence and self-control matter for wages and schooling alongside cognitive ability.
Cited on IQ and income. Checked 14 October 2026.
Ritchie, S. J., & Bates, T. C. (2013). Enduring links from childhood mathematics and reading achievement to adult socioeconomic status. Psychological Science, 24(7), 1301-1308. doi.org/10.1177/0956797612466268 · pubmed.ncbi.nlm.nih.gov
Reading and maths at 7 predict status at 42, beyond family background and IQ.
Cited on IQ and income. Checked 14 October 2026.
Sackett, P. R., Zhang, C., Berry, C. M., & Lievens, F. (2022). Revisiting meta-analytic estimates of validity in personnel selection: Addressing systematic overcorrection for restriction of range. Journal of Applied Psychology, 107(11), 2040-2068. doi.org/10.1037/apl0000994
Earlier selection validities were overcorrected; cognitive tests still predict, but less than once claimed.
Cited on The g factor. Checked 14 October 2026.
Schmidt, F. L., & Hunter, J. E. (1998). The validity and utility of selection methods in personnel psychology: Practical and theoretical implications of 85 years of research findings. Psychological Bulletin, 124(2), 262-274. doi.org/10.1037/0033-2909.124.2.262
The classic estimate of how well general mental ability predicts job performance.
Cited on The g factor. Checked 14 October 2026.
Strenze, T. (2007). Intelligence and socioeconomic success: A meta-analytic review of longitudinal research. Intelligence, 35(5), 401-426. doi.org/10.1016/j.intell.2006.09.004
IQ correlates about .56 with education, .43 with job level and .20 with income.
Cited on What is a good IQ?, IQ and income, IQ myths. Checked 14 October 2026.
Zagorsky, J. L. (2007). Do you have to be smart to be rich? The impact of IQ on wealth, income and financial distress. Intelligence, 35(5), 489-501. eric.ed.gov
Each IQ point went with higher income, but not with higher wealth.
Cited on IQ and income. Checked 14 October 2026.

Genetics and heritability

Briley, D. A., & Tucker-Drob, E. M. (2013). Explaining the increasing heritability of cognitive ability across development: A meta-analysis of longitudinal twin and adoption studies. Psychological Science, 24(9), 1704-1713. doi.org/10.1177/0956797613478618 · Free full text
Existing genetic influences are amplified with age, from about age 8.
Cited on Heritability of IQ. Checked 14 October 2026.
Haworth, C. M. A., et al. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15(11), 1112-1120. doi.org/10.1038/mp.2009.55 · Free full text
Heritability of 41% at 9, 55% at 12 and 66% at 17 in 11,000 twin pairs.
Cited on Heritability of IQ. Checked 14 October 2026.
Plomin, R., & Deary, I. J. (2015). Genetics and intelligence differences: Five special findings. Molecular Psychiatry, 20(1), 98-108. doi.org/10.1038/mp.2014.105 · Free full text
Heritability of intelligence rises from about 20% in infancy to about 80% in later adulthood.
Cited on Heritability of IQ. Checked 14 October 2026.
Savage, J. E., et al. (2018). Genome-wide association meta-analysis in 269,867 individuals identifies new genetic and functional links to intelligence. Nature Genetics, 50, 912-919. doi.org/10.1038/s41588-018-0152-6 · pubmed.ncbi.nlm.nih.gov
205 genomic regions linked to intelligence; each variant has a tiny effect.
Cited on Heritability of IQ. Checked 14 October 2026.
Tucker-Drob, E. M., & Bates, T. C. (2016). Large cross-national differences in gene x socioeconomic status interaction on intelligence. Psychological Science, 27(2), 138-149. doi.org/10.1177/0956797615612727 · Free full text
The gene-by-income effect shows up in the US but not in Western Europe or Australia.
Cited on Heritability of IQ. Checked 14 October 2026.
Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14(6), 623-628. doi.org/10.1046/j.0956-7976.2003.psci_1475.x · pubmed.ncbi.nlm.nih.gov
Among poor families, shared environment explained most IQ variation in 7-year-olds.
Cited on Heritability of IQ. Checked 14 October 2026.

Test publishers and official test information

Criteria Corp (2024). CCAT score report guide. Criteria Corp. criteriacorp.com
On the real CCAT a raw score of 24 is the 50th percentile; reports show a raw score and a percentile.
Cited on How the CCAT practice test is scored. Checked 14 October 2026.
Criteria Corp (2024). CCAT information brief. Criteria Corp. criteriacorp.com
The CCAT has 50 questions and a 15-minute limit, and most people do not finish.
Cited on How the CCAT practice test is scored. Checked 14 October 2026.
Pearson Assessments (2008). Wechsler Adult Intelligence Scale, Fourth Edition (WAIS-IV): product information, ages 16:0 to 90:11. Pearson. pearsonassessments.com
The adult test that applies from age 16.
Cited on IQ test for teenagers. Checked 14 October 2026.
Pearson Assessments (2014). Wechsler Intelligence Scale for Children, Fifth Edition (WISC-V): product information, ages 6:0 to 16:11. Pearson. pearsonassessments.com
Which test a psychologist uses for children and young teens, and its age range.
Cited on IQ test for teenagers. Checked 14 October 2026.
U.S. Department of Defense, Defense Testing and Assessment Center (2026). Appropriate use of ASVAB scores (executive note). officialasvab.com. officialasvab.com
The AFQT is a composite of four subtests, reported as a percentile from 1 to 99.
Cited on How the ASVAB practice test is scored. Checked 14 October 2026.
Valencia College (2025). About the ATI TEAS. Valencia College. valenciacollege.edu
The TEAS 7 format as a nursing school publishes it: 170 questions, 150 scored, 209 minutes.
Cited on How the TEAS practice test is scored. Checked 14 October 2026.
Wikipedia contributors (2026). Wonderlic test. Wikipedia. en.wikipedia.org
The classic Wonderlic format: 50 questions in 12 minutes, with 20 set as average.
Cited on How the Wonderlic practice test is scored. Checked 14 October 2026.

From our newest guides, not yet checked for this list

These sources appear in the reference lists of the guides named next to them. They will move into the checked list above, with a note on what we use each one for, once we have confirmed them.