So this is a complicated answer. One aspect that makes it difficult to quantify is the various types of intellect that factor into “highly intelligent.” Many recorded and verified cases of DID are of highly intelligent individuals, yes, but if you think about it deeper than those before have answered--would it not require a highly intelligent and capable brain to create various constructs and sections of memory, skills, capabilities, identity, etc. Even in the not so exceptionally intelligent ones, you have to admit that alone merits more than your average brain capabilities. Also, we must factor in the misdiagnosed (as identity and dissociative orders are plenty) and those who attempt to fake DID. And finally, I'd like to point out that “a study" means near to nothing scientifically. If data can be replicated to find the same results in various settings and circumstances, independent of said effects, then it is accepted as scholarly fact. Anyone can do a study. Anyone can also mess up the data and calculation of the sample error. As well as have an unnoticed bias or common uncontrolled factor that influences results--like the perception of intelligence and tests administered ~ Davy Coker, Psychology & Philosophy at University of Southern Mississippi