Again, this is only for primates, not for other species like birds or lizards.
Then perhaps we have to look at causation. Is it the bigger brains that make them more social, or is it how their brains are organized? Would a smaller overall brain reduced this effect if the original connections stayed in tact? Cofty mentioned that it is trimming connections that make us smart---in whatever way. These connections are trimmed by our experience---unused portions get nipped, so to speak (hugely simplified). this is why a human that is not exposed to language by around age 9 or so, will forever lose the ability to use language. Those connections have been chopped.
So, with the Baboons, is there something in their environment that makes stronger social connections in the brain at a young age, and something in other primate environments that cause these connections to be trimmed earlier? And of course, this cultural pressure can also come into play with evolution if it affects their fitness.
That's why I'm saying I think we are over-simplifying a very complex issue. There are SO MANY factors, that we don't get all the answers by just looking at one. It's a growing field for sure.
I'm also saying that we don't read an article and then look for ways to confirm the data. Oh look, kids can't read a sun dial. That confirms it. We are getter stupider. Instead we continue to ask questions, we try to falsify the results we find, and we get closer to the answer.