The article says, "Thus, from the size of an animal’s neocortex, the frontal lobe in particular, you could theoretically predict the group size for that animal". So I did a bit googling and found another article, here

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1626378/

This one has another interesting point: "These analyses show that while relative neocortex size is positively correlated with female group size, it is negatively, or not at all correlated with male group size. This indicates that the social intelligence hypothesis only applies to female sociality."

Perhaps after a man gets married with a woman, he could shift much of his social function of the neocrotex to his female partener, and free up that part of the brain for some other interesting stuff...