Nature versus nurture

What it says

The old question asks which matters more, what a person is born with or what happens to them. The modern answer is that the question is badly put. Every trait that has been measured is shaped by both, and the interesting work is in how they combine.

The main tool has been the twin study. Identical twins share all their genes and fraternal twins share half, on average, and both kinds usually share a home. If identical twins are more alike on a trait than fraternal twins, genes are part of the reason. The size of that gap gives a number called heritability.

Heritability is the share of the variation in a trait, across a population, that goes with genetic differences. It is a statement about differences between people, not about one person. A heritability of fifty percent does not mean half of your trait came from your genes. It means that, in that group at that time, about half of the spread between people lines up with their genes.

Four things the evidence says

What fifty years of twin and family studies agree on: Eric Turkheimer's three laws, and the finding that complicates them.

1. Every trait is heritable

   Polderman and colleagues pooled fifty years of twin studies in 2015. The average heritability of a human trait came out near fifty percent, and no measured trait came out at zero.

2. Nothing is wholly heritable

   No trait came out at a hundred percent either. Identical twins raised together differ on every measure, and what is left after genes and the shared home is a large share of the spread.

3. The shared home matters less than expected

   Being raised in the same family makes siblings less alike as adults than most people assume. For personality, the effect of the shared home is small once genes are counted.

4. Genes and surroundings act on each other

   Turkheimer and colleagues found in 2003 that the heritability of IQ in young children was near zero in the poorest families and high in well off ones. The same genes count for more where circumstances let them.

Where it came from

Galton coined the pairing. He had argued in Hereditary Genius in 1869 that ability runs in families. In 1874 he set nature and nurture side by side as the two things a person is made of. The phrase has outlived most of what he believed, including the program of selective breeding he gave a name to.

The twin method came later. Comparing identical with fraternal twins, and twins raised apart with twins raised together, became the standard design through the twentieth century. The Minnesota study of twins reared apart, reported by Thomas Bouchard and colleagues in 1990, is the one most people have heard of.

What holds up, and what does not

The twin findings have held up very well. Polderman and colleagues pooled nearly all of them in 2015: close to eighteen thousand traits from more than fourteen million twin pairs. The pattern was the same across the lot. Robert Plomin and colleagues listed the ten findings of behavior genetics that have replicated most often in 2016, and the first is that every trait is heritable. Turkheimer's 2000 summary is still the standard short version: every trait is heritable, the shared home matters less than genes, and much of the spread is neither.

The weaknesses are in what the numbers are taken to mean. Heritability moves with circumstances: the IQ result in poor families shows that a trait can be highly heritable in one group and barely heritable in another. A heritable trait is not a fixed one. Eyesight is strongly heritable and glasses fix it. And the newer work that reads genes directly has found the genetic side spread across thousands of small effects. None of them decides anything on its own.

What it does not mean

It does not mean anybody's traits are fixed at birth, or that a high heritability puts a trait beyond reach. Heritability describes a population, not a person, and it says nothing about what would happen if circumstances changed. It also does not take sides. Nature against nurture is a false choice, and the research that settled it did so by measuring both at once.

Check yourself

Three questions on what is above. Each answer says why.

1. A trait has a heritability of fifty percent. Pick the statement that says what that means.

   1. Half of one person's trait came from their genes and half from their upbringing
   2. About half of the spread between people in that population goes with genetic differences
   3. The trait cannot be changed by circumstances
   Show the answer

   Answer: B. Heritability is a share of the variation across a group, never a split inside one person. It also says nothing about whether the trait can change.

2. Which of these is one of Turkheimer's three laws of behavior genetics?

   1. The effect of being raised in the same family is smaller than the effect of genes
   2. Every trait is either inherited or learned, never both
   3. Twins raised apart grow up with nothing in common
   Show the answer

   Answer: A. The second law is the one most people find surprising: the shared home makes siblings less alike as adults than expected. The other two laws say every trait is heritable and that much of the spread is neither genes nor family.

3. What did Turkheimer and colleagues find in 2003 about the heritability of IQ in young children?

   1. It was the same in every kind of family
   2. It was near zero in the poorest families and high in well off ones
   3. It was high in the poorest families and near zero in well off ones
   Show the answer

   Answer: B. Heritability rose with family income. Where circumstances were hardest, genetic differences counted for least, which is why a heritability figure belongs to a group in a setting rather than to a trait.

Sources

1. Galton, F. (1874). English Men of Science: Their Nature and Nurture. Macmillan.
2. Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., and Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978).
3. Turkheimer, E. (2000). Three laws of behavior genetics and what they mean. Current Directions in Psychological Science, 9(5).
4. Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., and Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14(6).
5. Polderman, T. J. C., Benyamin, B., de Leeuw, C. A., Sullivan, P. F., van Bochoven, A., Visscher, P. M., and Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7).
6. Plomin, R., DeFries, J. C., Knopik, V. S., and Neiderhiser, J. M. (2016). Top 10 replicated findings from behavioral genetics. Perspectives on Psychological Science, 11(1).

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