An old argument, and why it still runs

Whether a child is shaped more by inheritance or by upbringing is one of the oldest disputes in developmental psychology, and it has never quite been settled — partly because underneath it sits a harder question about what we owe children.

The nature position holds that genetics does most of the work: that traits such as temperament and intelligence follow largely from what a child inherits. The nurture position holds that experience does it — parenting, schooling, relationships, language, culture, and the resources a family can draw on.

The debate is easiest to examine through intelligence, because it has been measured for longer, and more contentiously, than anything else in the field. By intelligence we mean something fairly ordinary: the ability to learn from experience, understand the world, reason logically, and make good use of what is available when facing a difficulty.

The case for nature

The evidence for a genetic contribution is substantial, and it comes mainly from three designs — twin studies, family studies and adoption studies — each of which prises genes and environment apart in a different way.

The best known is the Minnesota Study of Twins Reared Apart (Bouchard et al., 1990), a longitudinal study of identical twins separated early and raised in different homes. Their IQ scores correlated at roughly .70. Two people who shared their entire genome and almost none of their upbringing still scored remarkably alike.

Bouchard and McGue’s earlier meta-analysis (1981) pooled more than a hundred family, twin and adoption studies using standardised measures. Identical twins raised together correlated at .86 — well above fraternal twins or ordinary siblings raised in the same house. Across that literature, around 60% of the variance in IQ scores was attributable to genetic influence. Identical twins share all of their genetic material and ordinary siblings about half, and the scores track that difference.

More recent genome-wide association studies add an important qualification. Intelligence is polygenic: influenced by hundreds of variants, each contributing a very small effect. There is no gene for it, and no test that reads it off.

The case for nurture

The environmental evidence is just as hard to dismiss, and it is clearest in early childhood.

Hart and Risley (1995) followed children from families across a range of socioeconomic circumstances and recorded the language surrounding them. Children in higher-income households were exposed to dramatically more words in their earliest years, and those differences were strongly associated with later vocabulary and IQ scores. What a child hears before they can speak turns out to matter a great deal.

The starkest evidence comes from Rutter and colleagues (2007), who followed children adopted out of severely deprived institutional care. Those adopted before six months showed a remarkable cognitive catch-up. Those adopted later carried persistent deficits. The finding cuts both ways: early deprivation leaves durable marks, and a sufficiently different environment, applied early enough, can change the trajectory.

Where the evidence actually lands

Set side by side, the two bodies of work look like a stalemate. They are not. What the research increasingly shows is that genes and environment are not two forces competing for the same territory — they are entangled, and each changes what the other can do. This is the interactionist position.

Its clearest demonstration is Turkheimer et al. (2003), who found that the heritability of IQ is not a fixed number. In wealthier families, genetic factors accounted for most of the variance in children’s IQ. In low-income families, environmental factors accounted for most of it.

That result is worth sitting with, because it inverts how heritability is usually deployed in argument. Where children already have adequate nutrition, schooling and attention, most of what separates them is genetic — the environment has stopped being the limiting factor. Where those things are scarce, the environment is doing the separating. Heritability runs high precisely when circumstances are equal, and it collapses when they are not.

Genes, on this reading, describe a range. Nutrition, early education, stability and parental engagement decide how much of that range a child ever occupies.

What this means for parents

The practical implications are less dramatic than the debate, and considerably more reassuring.

  • A child’s temperament is real and largely not your doing. Working with it goes further than trying to replace it.
  • The early years carry disproportionate weight, particularly for language. Talking, reading and responding to a small child is not enrichment; it is the mechanism.
  • Heritability describes populations, never individuals. It cannot tell you what any particular child is capable of.
  • Where circumstances are hard, the environment is doing more of the work, not less. That is an argument for support, not for resignation.

Children are not passive recipients of either inheritance or upbringing. What a child brings shapes the responses they draw from the people around them, and those responses shape what develops next. Understanding development means giving up the either/or: neither nature nor nurture acts alone, and what we observe is always the two of them running at once.