“150 is the cognitive limit to the number of people a human brain can maintain a coherent social relationship with. ... This number appears regularly in human society; it's the estimated size of a Neolithic farming village, the size at which Hittite settlements split, and the basic unit in professional armies from Roman times to the present day. Larger group sizes aren't as stable because their members don't know each other well enough.
“...More generally, there are several layers of natural human group size that increase with a ratio of approximately three: 5, 15, 50, 150, 500, and 1500 -- although, really, the numbers aren't as precise as all that, and groups that are less focused on survival tend to be smaller. The layers relate to both the intensity and intimacy of relationship and the frequency of contact.”
