What would you like to research in/about identical twins?

Last sunday we watched an interesting program on identical twins.
Lots of twins came together in, I believe, America, where a lot of tests and research was done on them, and by them some of the twins were doctors themselves and did research too.
Fields of research were: raising of twins also on twins who grew up separate from each other, sexual preference, physical- and mental health, etc.
Obviously twins are a dream for scientists from all different expertise.
What would you like to research/find out about twins?