[PeterisP]

That's exactly what is getting ruled out by using fraternal twins and identical twins - you measure the difference of this effect between these groups.

If the outcome was determined by how they were raised (or where they lived, or who their parents were), the link would be equally strong between fraternal twins and identical twins; but if the outcome depends a lot on genetics, then the effect will be much stronger in identical twins than in fraternal twins. This does rule out pretty much all other factors except genetics.

[Real_S]

Thank you for clearly sharing how and why scientists can accurately measure genetic effects in twin studies. You are correct in your assertion that mono vs di - zygotic twin studies are usually good measures of heritability.

I would add a slight technical caveat. Twin studies are excellent at measuring additive genetic effects, i.e. when different genes add together nicely to contribute to a trait (height is a good example). Twin studies typically assume that genes do not interact in funny ways (e.g. epistasis). Thankfully, for many, but not all, traits examined the additive genetic effect catches most of the heritability.

If epistatic interactions drive a trait, however, the statistical models typically used in twin studies would actually under-estimate the influence of genetics.

Further reading: http://www.apa.org/monitor/apr04/second.aspx