|MadSci Network: Genetics|
The main problem with twin studies is that it is rare to have a case where monozygotic (identical) twins are separated. Without separation it is not possible to compare nature to nurture and some of the only people to do twin separations on purpose were the Nazis and I think the Americans also did some on multiple births. It is difficult to measure differences in personality because people don't really like qualitative tests. You can't really say twin A is a 50% more angry person than twin B. This doesn't mean that twin studies should be ignored. Where twins have been separated there are often incredible similarities between the choices the twins make.
Try the links in the MadSci Library for more information on Genetics.