At least before industrialization hit, people probably experienced relatively high levels of cultural outbreeding depression. Even in our time, businesses that reward high levels of trust show clear tribal advantage:
A handful of Indian Jain families have largely taken over the Antwerp diamond business, taking the Jews’ centuries-old and seemingly unassailable position. The dethroning has taken less than 30 years. A large factor, outside of just working hard, was being more tribal than the Tribe:
A business largely controlled by your own family is always far superior to competitors, Jahwery says. The other diamond dealers from Gujarat would probably agree. They rely on their worldwide family networks to build and maintain headquarters on every continent. […] Ashwin Jahwery has branches in Taiwan, Thailand, China, Australia, Great Britain and Spain, all of them run by his nephews.
“We always have the possibility of global distribution because a cousin or nephew who can blindly be trusted can always be sent to any country to set up operations” […] That the Jews lacked similar extended families was a major disadvantage for them, in Mehta’s opinion.
Moreover, the Jews’ former dominant position in the trust-intensive diamond business was itself a good demonstration of tribal advantage, in this case working to the Hasids’ advantage compared to their more outbred gentile competitors.
Meanwhile, EvolutionistX indirectly illuminates the emotional side of tribal advantage by shining a light on its inverse:
The inverse of clannishness is atomization, and atomization is lonely and stressful. In the atomized society, you are stuck on your own, with no one to catch you if you fall. You might be a single mother or an only child, or a hikikomori. Either way, you’re alone–and most people don’t seem to cope well with loneliness.