I think it has a lot to do with how important smart is to both the people in the relationship. Smart people, on the average find smart to be extremely important, and it is a fairly rare occurrence for a person thirty IQ points higher than another to both be aware of all the other aspects of interpersonal relationships unrelated to intellect, and fortunate enough to have a relationship with a person of much lesser IQ who is equally aware that their worth as a human being is not limited by intellect.
I am not sure of the mathematical differences, but I have known two couples of obviously divergent intellectual abilities who have deeply committed relationships of long standing. However, in both cases the “smart” one had profoundly unhappy experiences with the academic social world, and pretty much found being smart a matter of limited applicability in the realm of happiness. They were both smart enough, and both lucky enough to find someone who did not have that terrible sense that intelligence and human worth were synonymous.
By they way, one of the dumb ones describes himself as Rock Stupid, Usually with a grin. His wife generally simply smiles, and says nothing when he says that. I once asked her if he was really all that dumb, and she said, “He can figure out right from wrong, and he knows more about nice than anyone else I know.” She is some sort of techie designer, he is a tire mechanic.
The other couple are a brilliant executive, some sort of financial company, and his gorgeous dumb blond wife. It’s a caricature relationship, and “everyone knows” all the things everyone always knows about that sort of marriage. But, they have been married about thirty years now, their children grown, and moved on. They still seem completely happy to me.
So, while my experience says that statistically both couples are phenomenally lucky, and probably uncommonly wise, I think that pride of intellect is a more likely source of the statistics than some inherent barrier to communication.
Not to mention all the “both of them smart” marriages that seem to have the same, or even greater rate of separation and divorce that I have known, and no doubt you all have known. I think happiness in marriage is not really about being smart, or dumb, or the same. I speak not as an expert, but as just another divorced guy, but I don’t think intellect had a lot do with it. Selfish seemed to be a bigger part. But that’s just me.
Tris