In the study conducted by Bouchard of the Univ.of Minn. on twins separated at birth, they studied both identical & fraternal twins. However, there is another worldwide phenomenon which they failed to address, & which definitely ought to be related to the study:
The issue of doppelgangers (look-alikes) who perhaps though not “officially” blood-related, nevertheless may be GENE related.
There’s an interesting website, http://www.ireadfaces.com
In the same vein, it would be intriguing if some compu-whiz could program a website called lookalikes.com (or some such) wherein people who register could scan thumbnail photos of themselves, then drag the photos to an area within an array of faces most closely resembling his/her phenotype, and then complete a form re: all sorts of tendencies, habits, tastes, voice-pitch (such as those addressed in the MN study) as part of the registration process. Eventually, if enough people scan in their photos, there’s bound to be some lookalikes or near look-alikes, who could then reference each others’ profiles to check for similar tendencies, & perhaps email each other or post on forums. It might be fun, though of course far from scientific, or controlled. The idea probably wouldn’t get off the ground though, because of privacy & security issues.