[PeterisP]

could it be be that such studies underestimate these effects because of this obvious assumption that fraternal twins share 50% of their genes - IMHO they share more than that because for all genes that "matter" their parents genes would be quite correlated.

Contrary to the popular saying, like attracts like. Parents are exceedingly likely to share visible traits and also mental traits - so the correlation between siblings wouldn't be halfway between random member of population and an identical twin, but much closer to the identical twin.

[klodolph]

"Like attracts like" is an interesting assumption, there is some evidence that people are attracted to others that have genetic differences rather than similarities. A common misconception, for example, is that the genetic differences between ethnic groups are larger than genetic differences within an ethnic group, in fact, the reverse is true, if we speak of averages. (Think of two overlapping bell curves with means that are very close to each other, if you want to visualize what I'm trying to say.) Or in more concrete terms, if you select a random person with European ancestors and a random person with African ancestors, the expected genetic similarity is quite close to the expected genetic similarity of two random persons with the same ancestry.

Furthermore it's not actually that critical whether fraternal twins share 50% or 75% or 90%, the important part is that identical twins share nearly 100%, and the number for fraternal twins is significantly less, so if you see differences for fraternal twins that you don't see for identical twins, you can make a strong case that genetic differences explain the observation.

And finally, the idea that fraternal twins share ~50% of their genes is a bit of a simplification. On average, two randomly selected humans will have genetic similarity of 99.5% or so. So that "50%" quote is just a ballpark figure scaled to the relative genetic diversity of the human population to begin with.

Or it depends on how you measure "genetic similarity" anyway.