[beavus]

What you are forgetting is that the famous Minnesota twin database is useful precisely because it contains data on twins separated at birth, sometimes even raised in different countries. Otherwise there would be the confounding factor you speak of.

It would be impossible to examine the "nature versus nurture" issue if all the study subjects shared both.

[longtermmemmory]

the minesota twin studies is one of the most discredited studies because it did not take into account that the twins were often placed in similar environments by design. There were mack truck size holes in the methodology.