And it is only a predisposition towards having non-identical twins that is passed down through families.

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Another American scientist, Robert Plomin, has been working with 25, 000 pairs of identical and non-identical twins in Sweden.

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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

They also looked at the same patterns in non-identical twins.

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Non-identical twins will agree less frequently.

ECONOMIST: Twin research

The twin rule of pathology states that any heritable disease will be more concordant (that is, more likely to be jointly present or absent) in identical twins than in non-identical twins and, in turn, will be more concordant in non-identical twins than in non-siblings.

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Maybe Galton's classic twins study was invalid: perhaps you cannot look at the similarities between identical twins, and those between non-identical ones, and conclude that the difference must necessarily be due to genes.

ECONOMIST: WHAT WE LEARN FROM TWINS: The mirror of your soul | The

It was more common for identical twins to both have Alzheimer's compared to non-identical, and a statistical analysis of risk rates in the groups gave the researchers their estimate of how significant genetic factors are.

BBC: NEWS | Health | Alzheimer's risk 'is 80% genetic'

If you are a twin (identical or non-identical) aged over 15 who would like to volunteer to join the 10, 000 twins on the register of the Twin Research Unit at St Thomas' Hospital, call 020 7188 5555.

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