[dcolkitt]

Most twin studies simply compare identical and fraternal twins in non-adopted households.

Since identical twins share twice the genetic covariance of fraternal twins, the impact of family environment can be backed out from the respective intra-pair correlation coefficients. In the limit case if genetics played no factor, then fraternal and identical twins should have identical pairwise correlations.

For example suppose pairs of identical twins have 45% pairwise correlation for adult IQ. And say fraternal twins have 25% correlation for the same measure. That would tell us that the population level variance of adult IQ is 40% attributable to genetic heritability, 5% to environmental heritability , and 55% to non-heritable factors (i.e. not genetics and not family environment).

[MaximumYComb]

In case you're interested, IQ is much more heritable than that for adults. The following quote is from Wikipedia:

"Twin studies of adult individuals have found a heritability of IQ between 57% and 73%[6] with the most recent studies showing heritability for IQ as high as 80%[7] and 86%.[8]. IQ goes from being weakly correlated with genetics, for children, to being strongly correlated with genetics for late teens and adults. "

[aidenn0]

Right, those studies can tell us about heritability, but cannot distinguish between common environment and genetics.

[objektif]

Why would parents of twins pay much more attention to one than the other?

[taneq]

Maybe because, for whatever reason, one cries a lot more than the other?

[objektif]

But that would be an edge case. I would have assumed that identical twins behave similarly. How do they find enough neglected twins to make a reliable conclusion.

[dhimes]

I’ve known three sets of identical twins. In each case the personalities were very different. Outgoing vs reserved, aggressive vs laid back, etc. I could easily see one wanting to be held more than the other.

[scruple]

Anecdotal, but... My (fraternal) twins have very distinct and different personalities. One requires an order of magnitude more attention than the other. We try to make up the difference, but it's hard.