Do people with DID have higher intellect?
Myths regarding people who have DID having higher “intelligence” have been debunked. DID is a brilliant mechanism for encoding information in the brain differently to hide it from ourselves.
So the different brain configurations between headmates are like thumbprints. It proves that we are all using the same body differently. It accounts for the conversion disorder portion of our experiences (the physiological differences and reactions between headmates).
The differences between “healthy” and DID brain scans are mainly C-PTSD (complex trauma) differences. Not intellectual differences.
Our brains are spending extra power on survival, hypervigilance and threat assessments in addition to all the normal language processing, sensory processing, thinking, etc. activities.
Individuals with DID all fall across the usual spectrum of IQ etc. It doesn't affect intelligence. The reasons that therapists thought that people with DID were “smarter” than average are mainly accounted for in sample selection bias.
Most studies of people with DID are skewed, unfortunately. Because the studies always rely on hand-picked participants chosen by a hand-picked pool of DID specialists, most of whom take private pay and thus have middle class clientele who can afford them (or who have investments, savings, home equity to deplete to pay them), and there are a lot of biases in who can get a DID diagnosis (and it's expensive to run an fMRI machine!!), the vast majority of folk in most studies are white cis women. These hand-picked clients are selected for not having comorbid diagnoses, usually have been in therapy for over 10 years including time misdiagnosed, etc. For many reasons they are more likely to have had access to education, may have completed a degree or two, etc.
There's a lot of bias in diagnosing people with developmental disabilities with DID…because of the intellect myth, some therapists refuse to diagnose people with autism with DID, etc. (they're wrong twice there, because people with autism don't have a lower IQ, and people with DID don’t have a higher IQ). If a therapist has this myth bias, they will not diagnose the client with DID, so the samples end us skewed by professional biases that perpetuate the myth. To add to this bias, (too) many people with DID who are POC or AMAB end up in prison (because too many people, especially POC and cis men, with serious mental illness end up in prison).
So any “proof” in studies around IQ of people with DID are due to a ton of sample selection biases. Not due to DID being a disorder of smarter people.
A DID brain is exceptionally busy, and intelligence such as IQ is highly overrated. We are all beautiful, valid, worthy, group beings, no more or less intelligent for having this brilliant survival mechanism in place, and everyone in our system uses our considerable brain-body resources towards group survival.