Two children grow up at the same table and go to the same school. One reads at five but can't sit still for a minute; the other struggles with letters but can take apart and reassemble any toy. Parents often wonder whether something is wrong. The branch of psychology that studies such differences has a reassuring answer: difference is the rule, not the exception.
What differential psychology studies
Experimental psychology looks for general laws of the mind and often treats differences between people as a nuisance, mere random 'noise' in the data. Differential psychology makes these differences its main subject: how people differ, by how much and why.
The idea is old. Plato and Aristotle wanted education and work to take people's differences into account, and Arab scholars described people on any trait as upper, middle or lower. It became a science only at the end of the 19th century, when traits began to be measured; Francis Galton, among other things, showed that many human traits follow a normal distribution. Each person is as unique as a fingerprint, yet uniqueness does not rule out science: fingerprints, too, can be classified and studied.
Between children and within one child
Psychologists distinguish two kinds of difference. Differences between people show how one child differs from another. Differences within a person show how unevenly one child's own abilities have developed: a girl may be excellent at languages, average at maths and weak at drawing. Such an uneven profile is normal rather than a warning sign, and it changes with age: an area that was weak at seven may later become a strength.
Another key idea: traits differ from one another in kind (height is one thing, musicality another), but people differ on any one trait only in degree. We are all spread along a continuous scale, and an individual difference simply shows how far a person is from the group average. That is why 'either/or' labels such as 'good at maths' and 'no good at maths', or 'clever' and 'not clever', are unscientific: real children don't fall into two boxes.
The bell curve: why most children are in the middle
Measure any trait in a large number of children, whether height, reading speed or memory, and plot the results: you get a bell-shaped curve. Most results gather near the average, and they become rarer and rarer towards the edges. This is the normal distribution. About 68% of people fall within one standard deviation of the average, and about 95% within two. On intelligence tests, where the average is 100 and the standard deviation is 15, this means that about 68% of people score between 85 and 115, and about 95% between 70 and 130.
So on any single trait most children are 'in the middle', and that is not mediocrity but the norm. Yet because each child has many traits, almost every child is above average at something and below average at something else. (The bell shape partly reflects how tests are built, too: test makers adjust their items until scores take this form.)
From general to specific abilities
Abilities are organised in a hierarchy. At the top is a general ability that shows up in almost any mental task. Below it are broad groups, such as verbal and educational abilities on one side and practical and mechanical ones on the other; then narrower abilities; and at the base very specific skills. Today's mainstream view, based on John Carroll's 1993 re-analysis of more than 460 data sets, is similar: a general factor g sits above several broad abilities. This explains both why abilities tend to go together and why a child can still be strong in one area and weak in another.
Two more distinctions help. A test shows what a child can do at their best, while everyday life shows what they usually do; the psychologist Lee Cronbach called this maximum and typical performance. Motivation, interest, tiredness and anxiety make the difference between the two. And, as the Soviet psychologist Boris Teplov showed, abilities can compensate for one another: different combinations of strengths can lead to the same success.
What this means for parents and teachers
- Compare your child with their own earlier self, not with siblings or classmates: 'Last month you read one page; now you read two.'
- Look at the whole profile, not at one mark or one number.
- Avoid 'either/or' labels: a child who keeps hearing 'you're just not made for maths' stops trying.
- Use strengths to support weaker areas: a child who loves drawing can illustrate new words, and a football fan can practise sums with match scores.
- The profile changes with age, so don't make early, final judgements.
- Give your child the chance to show their best: rested, calm and interested.
This site draws on the theory of multiple intelligences, which is still debated as a scientific theory, in the same spirit: as a way of noticing strengths, not as a test or a label.