Conversely, those traits with a large environmental component will be shared by identical and fraternal twins in equal measure.
This means that behavioural traits with a large genetic component are more likely to be shared by identical twins than fraternal twins.
Among fraternal twins, the likelihood that a second twin will have autism is 35% nearly twice the risk other siblings face, the study found.
Like fraternal twins, they were born at the same time, about four and a half billion years ago, and took roughly the same shape.
No such correlations were seen in the behaviour of fraternal twins.
Traditional methods of estimating the influence of genes and the environment on intelligence have largely been limited to comparisons between people who are related, such as identical or fraternal twins.
They used the classic trick of neutralising the effect of upbringing and exposing that of genetics by comparing identical twins (who share all their genes) with fraternal twins (who share half).
Statistically, that would not matter too much if the amount of epigenesis were the same in identical and fraternal twins, but research published last year by Art Petronis of the Centre for Addiction and Mental Health in Toronto and his colleagues, suggests it is not.
For their research, published in the journal Alcoholism: Clinical and Experimental Research, researchers led by Alexis Edwards of Virginia Commonwealth University studied more than 7, 000 fraternal and identical twins.
Where both twins had the disease, there was an average of 3.66 years difference in age at onset between 25 sets of identical twins, and 8.12 years difference between 20 sets of fraternal (dizygotic) twins, suggesting genes had a role in determining when Alzheimer's was seen.
If parents exhibit a tendency to treat identical twins more similarly than fraternal ones, for instance, then what researchers see as genetic factors could turn out to be environmental ones.
As is well known, twins come in two varieties: fraternal, in which the individuals have half their genes in common, just like ordinary siblings, and identical, in which the individuals have all their genes in common.
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.
Instead, identical twins are epigenetically closer to each other than the fraternal sort.
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