Twin studies have somewhat fallen out of favor in genetics-- instead of estimating parameters like the heritability of a trait, it's now common to simply skip to studies designed to locate genes that play a role in them. If the heritability turns out to be zero, then, well, not a whole lot is going to turn up in a genome scan, but it's generally cost-effective to take that risk.

A new article in Trends in Genetics highlights another use for twin studies-- research in epigenetics. From the abstract:

Analyses of epigenetic twin differences and similarities might yet challenge the fundamental principles of complex biology, primarily the dogma that complex phenotypes result from DNA sequence variants interacting with the environment

One could argue that epigenetics might turn out to be the mechanism by which the environment interacts with genetics, rather than an entirely new paradigm, but this is definitely an area to watch.

Related: Epigenetic information passed through the generations, Nature epigenetics update.