The study of twins provided much of the ammunition for the hereditarian counter-attack.

ECONOMIST: WHAT WE LEARN FROM TWINS

Some of it emerged from the first large-scale post-Burt study of separated identical twins, run by Thomas Bouchard, a professor at the University of Minnesota.

ECONOMIST: WHAT WE LEARN FROM TWINS

If the first six months of a child's life matter as much as most people think they do, then spending even that short time together could influence the result of a twins study.

ECONOMIST: WHAT WE LEARN FROM TWINS

Mr Bouchard saw an article on a pair of reunited identical twins, and decided to make a study of them.

ECONOMIST: WHAT WE LEARN FROM TWINS

The study, which looked at nearly 600 sets of Swedish twins, some identical, some fraternal, who were either raised together or separated early and reared apart, found that being raised in the same environment had little effect on stress and stress-related health problems.

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The twins in the Minnesota study had an average of five months together before they were separated, and nearly two years together after their reunion before Mr Bouchard got hold of them.

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Mr Plomin has also set up a study working with 10, 000 pairs of identical and non-identical twins in Britain.

ECONOMIST: WHAT WE LEARN FROM TWINS

The study excluded twins and triplets as they are known to be at increased risk of premature birth and low birth weight.

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In the US study, researchers from the University of Southern California, Los Angeles, and colleagues identified 392 pairs of twins where one or both had Alzheimer's from the Swedish Twin Registry.

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The only reason she could afford to do that while supporting her twins was because the state of Maine allowed her to keep her unemployment benefits and study with the help from a Pell Grant.

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