Factors That Affect IQ Test Results: What Moves a Score

Many things move an IQ test result, but by very different amounts. On the day, anxiety costs about three points, retaking the test adds about four, and a single measurement carries about 5 points of error either side. Over years, each extra year of school adds roughly one to five points, and a test normed decades ago flatters scores by about three points a decade. Genes matter too, and their influence grows with age. This guide ranks the factors by size and timescale, so you can tell what is noise on the day, what is a feature of the test and what builds up over childhood.

• On the day: anxiety, tiredness, illness and a recent head injury can lower a score for that sitting without changing the ability behind it.
• In the test: the age of the norms, the scale, the language of the questions and whether the test was properly standardised change what a number means.
• Over childhood: schooling, nutrition, lead, air pollution and severe deprivation shift scores for years.
• Across life: the heritability of general cognitive ability rises from 41% at age 9 to 66% at age 17 (Haworth and colleagues, 2010).
• Size check: most day-to-day factors are worth a few points, not a whole classification band.

Which factors affect an IQ test result, and by how much?

The table puts the effects found in our guides side by side. They come from different studies and populations, so read them as sizes of effect, not as numbers to add together.

The sources are the sections below: test anxiety and IQ scores, Hausknecht and colleagues, 2007 for practice, can a concussion lower your IQ, Pietschnig and Voracek, 2015 for norms, Ritchie and Tucker-Drob, 2018 for schooling, ADHD and IQ, toxic stress and IQ and Haworth and colleagues, 2010.

What changes an IQ score on the day of the test?

Anxiety. Studies that measure test anxiety alongside cognitive performance find a relationship of roughly r = -.20, which translates to something on the order of three points on the usual scale: a few points between an average test taker and a highly anxious one, not fifteen (test anxiety and IQ scores).

Sleep, illness and injury. A single bad night blunts attention, working memory and processing speed for that sitting without touching underlying ability (sleep and IQ covers the longer-term picture). A meta-analysis of 39 studies of mild traumatic brain injury found a moderate effect on cognitive tests that had disappeared by 3 months in unselected or prospective samples (can a concussion lower your IQ). Low mood matters as well: depression lowers performance on attention, memory and processing speed while an episode lasts.

Practice and familiarity. In a meta-analysis of 107 samples and 134,436 people, scores rose on retesting by about .26 standard deviations, roughly 4 IQ points, more with coaching or when the same form was reused (Hausknecht and colleagues, 2007). Knowing the question formats helps for the same reason; see how to prepare for an IQ test.

Effort and conditions. A hurried or distracted sitting is worth less than a calm one, and a noisy room or a broken time limit can lower a score without lowering ability. This is one reason a one-to-one test under standard conditions is not comparable with a casual online one.

Does the test itself change the result?

Yes, in four ways. Norms: raw performance has risen by about 0.28 IQ points a year, roughly 3 points a decade, though the gain has slowed (Pietschnig and Voracek, 2015), so a test normed decades ago flatters today’s test taker (why IQ norms expire). Scale: the same performance prints as a different number on a standard deviation 15, 16 or 24 scale (standard deviation 15 versus 16). Language and culture: bias in the narrow statistical sense is usually small, but familiarity with the content can still help (cultural bias in IQ tests). Standardisation: an individually given test with a recent norm sample is a different instrument from an online quiz (our guide to good online IQ tests).

Stereotype threat, the idea that a reminder of a negative stereotype lowers performance, is often listed among the factors, but it has had a rougher time in replication than almost any finding of comparable fame (stereotype threat and IQ tests).

Which slower factors shape scores through childhood?

• Schooling: a 2018 meta-analysis pooled 142 effect sizes from over 600,000 participants and found that each additional year of education is associated with a gain of roughly one to five IQ points, with a central estimate of a little over three (Ritchie and Tucker-Drob, 2018; does school raise IQ).
• Nutrition: it affects scores substantially when a real deficiency is present and corrected, and barely at all when it is not (nutrition and IQ).
• Lead: childhood lead exposure lowers IQ scores, and neither the WHO nor the US CDC recognises a blood lead level below which no effect is seen (lead exposure and IQ).
• Air pollution: a 2024 meta-analysis of six studies and 4,860 children found a small but significant drop in IQ per increase in fine particles (air pollution and IQ).
• Severe deprivation: in a randomised trial of Romanian children, the first major follow-up found an average IQ of 81 in the foster-care group, 73 in children who stayed in institutions and 109 in children never institutionalised (toxic stress and IQ).
• Family income: children from wealthier families score higher on average, but the best-known claim about how income changes heritability has been partly replicated and partly not (socioeconomic status and IQ).
• Chronic sleep loss: sustained short sleep across development may matter more than one bad night (sleep and IQ).

How much of an IQ score is genetic?

A great deal, and more as people get older. In a sample of 11,000 pairs of twins from four countries, the heritability of general cognitive ability rose linearly from 41% in childhood (age 9) to 55% in adolescence (12) and to 66% in young adulthood (17) (Haworth and colleagues, 2010). The authors suggest that genotype-environment correlation explains the rise: as children grow up, they increasingly select, modify and even create their own experiences, partly based on their genetic propensities.

Heritability describes how much of the variation in a population goes with genetic differences, not how fixed one person’s score is, and it says nothing about differences between groups. Our guides to whether IQ is genetic and whether siblings have similar IQs cover the evidence and its limits.

Does your IQ change over time?

Yes, by a few points either way, and more in childhood than in adulthood. A meta-analysis of 205 longitudinal studies found a rank-order stability of about .76 for adults at age 20 across a five-year gap (Breit and colleagues, 2024), so most people keep their place in the distribution but individual scores move. Part of the movement is measurement error, and part is regression toward the average when a score was unusually high or low the first time (regression to the mean). Our guides to the IQ scale by age, whether you can improve your IQ and the most accurate age to be tested cover the timing.

How should you read a score, given all this?

• Treat a single score as a band of about 5 points either side, not a point (an IQ score is a range, not a number).
• Ask which test, which edition and which norms produced it, and whether it was given under standard conditions.
• Look for a reason on the day: illness, a poor night, anxiety or a recent head injury. If there is one, a retest after recovery is more informative than the first score.
• Compare bands and percentiles rather than single points (how to read an IQ test report).

Quick answers about what affects IQ test results

• What factors affect IQ test results? On the day: anxiety, sleep, illness and effort. In the test: norms, scale, language and standardisation. Over childhood: schooling, nutrition, lead, deprivation and genes.
• Can anxiety lower your IQ score? Yes, by about three points on average (r = -.20), not fifteen.
• Does sleep affect an IQ test? One bad night blunts attention, working memory and speed for that sitting; chronic short sleep in childhood may matter more.
• Can you raise your score by practising? A retest adds about 4 points, mostly through familiarity with the format.
• Does your IQ change? By a few points, yes; the rank order of adults is quite stable (about .76 over five years).
• How much of IQ is genetic? Heritability rises from 41% at age 9 to 66% at age 17 in a large twin sample.

Sources

• Haworth, C.M.A., Wright, M.J., Luciano, M., Martin, N.G., de Geus, E.J.C., van Beijsterveldt, C.E.M. et al. (2010): The heritability of general cognitive ability increases linearly from childhood to young adulthood, Molecular Psychiatry, 15(11), 1112-1120.
• Hausknecht, J.P., Halpert, J.A., Di Paolo, N.T. and Moriarty Gerrard, M.O. (2007): Retesting in selection: a meta-analysis of coaching and practice effects for tests of cognitive ability, Journal of Applied Psychology, 92(2), 373-385.
• Pietschnig, J. and 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.
• Ritchie, S.J. and Tucker-Drob, E.M. (2018): How much does education improve intelligence? A meta-analysis, Psychological Science, 29(8), 1358-1369.
• Breit, M., Scherrer, V., Tucker-Drob, E.M. and Preckel, F. (2024): The stability of cognitive abilities: a meta-analytic review of longitudinal studies, Psychological Bulletin, 150(4), 399-439.
• The anxiety, concussion, ADHD, lead, air pollution, nutrition, sleep and early-deprivation figures are those reported in our own guides linked above, each of which cites its primary sources.