Small groups are a fundamental unit of human social organization

Multilevel selection theory tells us that something similar to team-level selection took place in our species for thousands of generations, resulting in adaptations for teamwork that are baked into the genetic architecture of our minds. Absorbing this fact leads to the conclusion that small groups are a fundamental unit of human social organization. Individuals cannot be understood except in the context of small groups, and large-scale societies need to be seen as a kind of multicellular organism comprising small groups.

— David Sloan Wilson, 2019

From the interview This View of Life: Completing the Darwinian Revolution