'He's clever, just like his grandfather.' 'She would read more if we'd bought more books.' Families often argue about where abilities come from as if one answer had to win. Psychology spent decades on the same quarrel and arrived at an answer that is more interesting and more hopeful: heredity and environment work together.
An old quarrel and a wrong question
For a long time there were two camps. Environmentalists believed that education makes the person: everyone is born with roughly equal potential, and failure is the fault of the school system. Hereditarians replied that education only helps inborn potential to unfold.
Today this either/or framing is seen as the wrong question. Almost every trait arises from heredity and environment acting together, so the useful question is a different one: how far can a trait change, and under what conditions? Two common mistakes follow from the old quarrel: 'inherited' does not mean 'fixed', and 'acquired' does not mean 'easy to change'. A lack of conversation and weak schooling in the early years, for example, can leave long-lasting gaps.
In psychology, environment means everything a person actually experiences, starting even before birth. That is why even two children from one family who share a room do not share the same environment: each has their own experiences.
What twins and adopted children showed
Identical twins share all their genes, while fraternal twins are no more alike genetically than ordinary brothers and sisters. In 1937 Newman, Freeman and Holzinger compared twins on intelligence tests, including 19 pairs of identical twins raised apart. A correlation of 1 means perfect agreement, and 0 means no link at all. For identical twins raised together the correlation was about 0.88, for identical twins raised apart about 0.77, and for fraternal twins about 0.63.
Heredity clearly matters: identical twins brought up in different homes were still more alike than fraternal twins brought up together. But environment matters too: separated twins were less alike than those who grew up together, and some pairs differed considerably.
In 1928 Freeman and colleagues studied about 400 adopted children in Chicago and found that children placed in families with a richer, more stimulating environment scored higher. These early studies had few participants, and children were often placed with families similar to their birth families, so today they are seen as a page of history rather than final proof.
Early experience, school and changing scores
Goldfarb's studies in the 1940s showed that children who spent their first three years in an institution, rather than with a foster family, later lagged behind in intelligence, language, memory and planning, and were more anxious and aggressive. School matters too. In 1951 Husén published results for 722 Swedish children tested in the third grade and again ten years later: the scores of those who completed secondary school rose by about 11 points, and those of early leavers by about 2. A modern meta-analysis by Ritchie and Tucker-Drob (2018) found that each extra year of education raises measured intelligence by roughly 1 to 5 IQ points.
Scores also change in childhood. In a long-term study in Berkeley, California, analysed by Honzik, Macfarlane and Allen in 1948, tests given before about 18 months predicted nothing about later results, and during the school years many children's scores shifted a lot: for more than half by around 15 points, and for about one child in ten by 30 points or more. Programmes for young children have reported gains of 10–15 points, but without continued support the gains fade: in one programme, children were 14 points ahead of a comparison group at age 3, but only 7 points ahead at age 5.
What we know today
During the 20th century, average scores on IQ tests rose by about 3 points per decade in many countries; James Flynn described this in 1984, and it is known as the Flynn effect. Genes cannot change that fast, so schooling, nutrition, health and a more complex way of life must have played a part. At the same time, the heritability of measured intelligence, meaning the share of the differences between people in a group that is linked to differences in their genes, is substantial and even rises from childhood to adulthood. Both facts are true at once: heritable does not mean fixed.
What families can do
- Talk and read together every day. If your child dislikes reading, read aloud to them, talk about the stories and let them choose the books.
- Make time for play: building sets, chess, pretend play (playing shop or doctor) and outdoor games all exercise the mind.
- Look after sleep and nutrition, the basic conditions for a growing brain.
- Keep schooling steady: regular attendance and real interest in what your child is learning. Every year counts.
- Combine warmth with independence. In a long-term study at the Fels Institute in Ohio, active and independent children tended to gain on intelligence tests, while passive and dependent ones fell behind. Give love, and leave room to try, decide and make mistakes.
- Don't give up on a child because of one score or family history, and don't take a gift for granted either.