Two siblings grow up in the same house, eat the same food, go to the same school. One is calm, tidy and quiet; the other is loud, late and fearless. A teacher can usually predict who will volunteer for the school play. A parent, asked how much of that difference was the house and how much was something else, often finds the answer is neither of the two things they expected.
Most of psychology looks for what people share: how memory works, how vision works. Differential psychology, the study of individual differences, asks what varies between people and how far. It has the most useful measurements in psychology, and some of the most abused. This piece covers how traits are measured, how stable they are over a life, what an IQ score is, what "heritable" does and does not mean, and where the field's popular claims fall apart.
If personality is real, it should be in the dictionary. That is the lexical hypothesis: the differences that matter to people get words. In 1936 Gordon Allport and Henry Odbert combed an English dictionary for about 18,000 terms describing people. Decades of work with factor analysis, which finds the few dimensions behind many correlated ratings, converged on five broad ones: openness, conscientiousness, extraversion, agreeableness and neuroticism (emotional instability), the "Big Five" or OCEAN. A "trait" here is a tendency measured on a continuum, with most people in the middle, not a type.
A test is a handful of questions. One question is a poor measure: "I am the life of the party" depends on the party. Add questions about the same trait and the random noise averages out. The formula is the Spearman–Brown prophecy: if a single item has reliability r, a test of k items has reliability k·r ÷ (1 + (k − 1)·r).
The formula is exact; the single-item values are illustrative (real items vary). A common rule of thumb asks for about 0.7 or more for research and higher for decisions about individuals.
Without JavaScript: with a single-item reliability of 0.3, ten items give about 0.81 and two items give about 0.46.
Now try ten items yourself. These come from the International Personality Item Pool (IPIP), a public-domain bank of items that Lewis Goldberg started so that researchers would not have to pay for tests. Answer how well each describes you. This is for illustration, not a measurement: two items per trait is far too few, and no score here means anything about you.
Nothing is stored or sent. Answers: 1 = very inaccurate, 5 = very accurate; reversed items are flipped before averaging.
Notice what the output looks like: five dimensions, each with a position on a scale. A good adult measure has about 10 to 20 items per trait, and the trait scores correlate with real outcomes in the way you would expect, but only modestly. Conscientiousness predicts job performance and health to a small-to-moderate degree; extraversion predicts who is sociable at parties.
Compare the Myers–Briggs Type Indicator, the best-known workplace "test". It sorts people into 16 types by cutting four continuous scales at the middle, so two people very close to the cut get different types; studies of retesting find that many people get a different type on a second try after some weeks. Its dimensions overlap with four of the Big Five, but the type labels throw information away, and it is not generally used in research. HEXACO, a competing model, adds a sixth factor, honesty–humility.
There are two senses of "does personality change?" Rank-order stability asks whether someone who is more extraverted than most at 20 is still more extraverted than most at 40. Mean-level change asks whether people on average become more extraverted. They can differ completely: everyone can grow calmer and keep their ranking.
The classic meta-analysis (Brent Roberts and Wendy DelVecchio, 2000) pooled 152 longitudinal studies and found test–retest correlations of about 0.31 in childhood, 0.54 in college, 0.64 at 30, and then up to about 0.74 for those aged 50 to 70. Later work (Bleidorn and colleagues, 2022) questions the late plateau and finds little further rise after about 25; the late-life data were thin. The direction of the finding is sturdy: personality is far from fixed in childhood, and gets more settled but never frozen.
Simulated people: 400 draws with the given correlation (Roberts and DelVecchio 2000). The scatter is computed here, not real data. Intervals are several years, and the correlation is a summary, not a rule for any one person.
Without JavaScript: with a correlation of 0.31 about 42% of people stay in the same third of the group over time; at 0.74 about 61% do.
On average, people become more conscientious, agreeable and emotionally stable from their twenties into middle age: the maturity principle (Roberts, Walton and Viechtbauer, 2006). It is an average, of course, and some people move the other way. Major life changes, such as a first job or a long relationship, seem to matter more than time alone, but the causal story is not settled.
In 1904 Charles Spearman noticed that children who did well on one school subject tended to do well on the others and named the shared factor g. Today the strongest cognitive-ability models (the Cattell–Horn–Carroll model) arrange abilities in layers, with g at the top and narrower skills below it, such as verbal reasoning, spatial ability and processing speed. Alfred Binet built the first practical test in 1905 to find French schoolchildren who needed extra help; it was not meant to rank people as fixed.
Modern IQ scores are not "points of cleverness". They are a place on a bell curve, scaled so that the population average is 100 and the standard deviation is 15 (some older tests used 16). A score of 130 is two standard deviations up, about the 98th percentile. And every score has error: even good tests have a standard error of measurement of a few points.
Percentiles use the normal curve; the 95% range is score ± 1.96 × the standard error. Real error varies by test and by score.
Without JavaScript: a score of 100 is the 50th percentile; with a standard error of 3 the 95% range is about 94 to 106; a score of 130 is about the 98th percentile on a scale with an SD of 15.
IQ scores predict school grades and, less strongly, job performance and some health outcomes, which is why they are among the most useful numbers in psychology and also why misuse is dangerous. Early twentieth-century IQ testing was tied to eugenics and used to justify restrictions on immigration and forced sterilisation in the United States, and group differences in scores have been misread as proof of innate differences. Howard Gardner's "multiple intelligences" are popular in classrooms, but they are mostly not supported by tests, which find that his abilities correlate positively, and the "learning styles" idea has not held up in experiments.
Scores have also been rising. The Flynn effect is the rise in average test scores of about three IQ points per decade through the twentieth century in many countries. In Norway, Denmark and elsewhere it has levelled off or reversed since the 1990s, and a 2018 study by Bratsberg and Rogeberg found the reversal appears within families, which points to the environment (schooling, media, testing habits) and not to a change in genes.
"Intelligence is about 50% heritable." What does that mean? Heritability is the share of the variation in a population, in one setting, that is associated with genetic differences. It is not the share of one person's trait that comes from genes, it says nothing about how changeable the trait is, and it depends on how much the environment varies. The classic picture (Richard Lewontin's seeds): plant genetically varied seeds in uniform, good soil and nearly all the difference in height is genetic; plant the same seeds in patchy soil and much of the difference is environmental. The genes did not change, so the heritability did.
Twin studies estimate it by comparing identical twins (who share all their genes) with fraternal twins (about half). Falconer's formula: heritability h² = 2(rMZ − rDZ), where the r's are the twin correlations. The shared-family environment is c² = 2rDZ − rMZ, and the rest is non-shared environment (plus measurement error), e² = 1 − rMZ.
Presets are illustrative, not measured values. Falconer's estimate is a rough tool: it assumes no gene–environment interaction and equal environments for both kinds of twin. In the soil part, genetic variance is fixed at 100 and h² = 100 ÷ (100 + environmental variance).
Without JavaScript: identical twins correlate 0.8 and fraternal 0.5 give h² = 0.6, c² = 0.2, e² = 0.2. With genetic variance 100 and environmental variance 100, h² = 0.5; with 400, h² = 0.2.
What do real studies find? In twin studies of IQ, heritability tends to be low in childhood and to rise toward adulthood, in the region of 40% in childhood to well over half in adults, a pattern that surprises people who expect genes to fade with age. Eric Turkheimer's "three laws" of behaviour genetics (2000) summarise a vast literature: all human traits are partly heritable; family environment shared by siblings matters less than expected; a large share of variation is not explained by genes or by shared family at all. Turkheimer's own 2003 finding that IQ heritability was lower in poor US families is a good example of how contested the details are: a 2016 meta-analysis (Tucker-Drob and Bates) found it in US studies but not in the western Europe or Australian data.
Molecular genetics adds a puzzle. When researchers look directly at DNA, the traits that twin studies call highly heritable are linked to thousands of tiny genetic differences, and the total they capture is smaller than the twin estimate (the "missing heritability"). Polygenic scores built from these differences predict less within a family, between siblings, than across unrelated people, partly because they pick up environment and family background too.
Most psychology experiments are done on university students, in a few rich countries. A 2008 estimate by Jeffrey Arnett found that about 96% of participants in leading psychology journals came from countries with about 12% of the world's population, and in 2010 Joseph Henrich, Steven Heine and Ara Norenzayan named the problem: WEIRD samples (Western, Educated, Industrialised, Rich, Democratic), which are, on several measures, unusual people.
The Big Five does appear in many languages and countries, which is a strength. But the picture is less tidy at the edges: a study of the Tsimane forager-farmers in Bolivia (Gurven and colleagues, 2013) did not recover the five clean factors; some lexical studies find slightly different structures; and the sex differences in personality scores are, oddly, larger in some richer and more gender-egalitarian countries (Schmitt and colleagues, 2008), a result researchers explain in competing ways.
Culture shapes even perception. The Müller-Lyer illusion, two equal lines that look unequal when one ends in outward-pointing fins and the other in inward-pointing ones, was tested in many societies by Marshall Segall and colleagues in the 1960s. Henrich and colleagues used it as an example of a basic effect that varies: people raised in carpentered, right-angled environments tend to be more fooled than some people from small-scale societies. Try it. A one-person demo tells you nothing about a culture, but it shows the illusion is real.
The upper line is always 200 units long. Most people who set the lower line to look equal choose a length under 100%, because the outward fins make the line look longer. One person on one screen is a demo, not a study.
Without JavaScript: the two lines are exactly equal, but the one with the outward fins looks longer to most viewers.
Several popular ideas about individual differences are weaker than their reputation. Left-brain and right-brain "types" are not supported; a study of over 1,000 brain scans (Nielsen and colleagues, 2013) found no evidence that people use one hemisphere more. "We use only 10% of our brains" is false. The "Mozart effect", a brief boost in spatial reasoning after listening to Mozart reported with 36 students in 1993, did not hold up beyond a small arousal effect in later meta-analyses. Learning styles, the idea that people learn better in the style they prefer, has failed in experiments (a 2008 review by Pashler and colleagues found no adequate evidence).
The Dunning–Kruger effect is a good case. The popular version says "the less you know, the more confident you are". The 1999 paper by David Dunning and Justin Kruger found that people in the lowest quartile on tests overestimated their rank, while people in the top quartile slightly underestimated it, but nobody in the bottom group rated themselves above the top group. Critics (for example Gignac and Zajenkowski, 2020) argue that much of the pattern is a statistical artefact: if self-estimates are noisy at all, they will pull toward the middle, so low scorers seem to overestimate and high scorers to underestimate. Dunning and others reply that real miscalibration remains. The tool below shows how much the pattern arises from noise alone.
1,000 simulated people with a made-up level of insight; no "blind spot" is built in. Bars show the average actual percentile (top bar of each pair) and the average self-estimated percentile (second) for each quartile.
Without JavaScript: with an insight correlation of 0.5, the lowest quartile averages about 13 in reality and about 31 in self-estimate; the top quartile about 88 and about 69.
People differ in stable, measurable ways, mostly along five broad dimensions, and measurement works when many items are averaged. Traits settle with age and never freeze; IQ is a range on a curve; heritability is a fact about a population in one environment, not a verdict on any one person. Be most sceptical of the tidy, surprising claims.
Try this: next time you read "X% heritable" or "scientists found a gene for Y", ask three questions: in what population, in what environment, and does it say anything about changing it?
Sources. Allport and Odbert, Trait-Names: A Psycho-Lexical Study (1936); Goldberg's IPIP (ipip.ori.org; the items used are from the IPIP 50-item Big-Five markers); Spearman–Brown formula (Spearman 1910, Brown 1910); Roberts and DelVecchio, Psychological Bulletin 2000 (152 longitudinal studies; .31, .54, .64, about .74); Bleidorn and colleagues, 2022, on the late-life plateau; Roberts, Walton and Viechtbauer, Psychological Bulletin 2006; Spearman 1904; Binet and Simon 1905; Carroll 1993; Flynn effect of about 3 points per decade (Trahan et al. 2014 give 2.93) and the Norwegian reversal (Bratsberg and Rogeberg, PNAS 2018); Falconer's formula; Lewontin 1970s on heritability; Turkheimer, Current Directions in Psychological Science 2000 and Psychological Science 2003; Tucker-Drob and Bates, Psychological Science 2016 (14 studies, 24,926 twin and sibling pairs; the effect appeared in US studies, not in Western Europe or Australia); Arnett, American Psychologist 2008; Henrich, Heine and Norenzayan 2010; Segall, Campbell and Herskovits (1966); Gurven et al. 2013; Schmitt et al. 2008; Nielsen et al. 2013; Pashler et al. 2008; Rauscher et al. 1993; Dunning and Kruger 1999; Gignac and Zajenkowski 2020. Computed in node from the code on this page: reliability at r = 0.3 for k = 10 is 0.811 and for k = 2 is 0.462; a score of 130 is the 97.7th percentile at SD 15. The Myers–Briggs claim is stated generally; this piece does not give a retest percentage. This piece describes research and is not a personal assessment.