Dunbar's Number: 132 to 165 People

Dunbar's number was first suggested by an English anthropologist, Robin Dunbar, in the 1990's, in a study correlating brain size with the size of social groups in primates. It is a supposed likely cognitive limit to the number of people with whom one can maintain stable social relations. These are relationships in which the individual knows who each person is, and what their role is in society, and is somewhat familiar with their behavior in the group. It refers to how easily each person relates to every other person, and how well they are known by their associates. Dunbar tries to count the number of people whom one would include as being familiar enough to approach in a public space, as known "from back home" perhaps, or at least more comfortably than those whom one simply remembers having met on a few occasions.

Indeed, the human mind may be wired for several concentric circles of familiarity and inclusion:
A. You may have 5 or 6 close friends, with whom you prefer to share regular time and activities.
B. You probably have up to 15 to 18 friends from whom you would readily accept an invitation to a party.
C. You may have about 3 times that many with whom you regularly interact on a daily to weekly basis, some 44 to 55 habituated and friendly acquaintances.
D. There may be about 3 times that many whom you recognize as being "one of us" and consider as part of "your village circle" and essentially not strangers. This circle is approximately 132 to 165 people. That is Dunbar's number.
E. And likely, there are about 3 times that many whom you would recognize and have some idea of where and when you last saw them, roughly 400 to 500 people. This is the equivalent of meeting some members of another village while on a trading trip, perhaps identified as peaceful by a sign of shared affiliation or trust.
F. The greater tribe may share a similar style and culture, but be recognized only in a political sense as "allies" and "not our enemies."

Lacking any shared marks of culture, or purposely displaying some contrary signs, persons who don't conform will be viewed as "outsiders" and regarded with antipathy and suspicion. These people are not given any benefits of cooperation, and are not  expected to share "our values," because  they are not trying to assimilate. Indeed, it would not surprise us to learn that they are enemies. They may even be segregated into their own smaller enclaves, or feel exclusive in other ways, often by their own choice. And they may not be welcomed by the larger society because of their desire to be different.

This is a result of our having evolved in a tribal setting. For hundreds of thousands of years, we lived in semi-nomadic groups, following the seasons as hunter-gatherers. The natural abundance of the ecosystems we lived in, and their carrying capacity, thus dictated the size of the groups of which we were a part. Even when we began to live in settled towns, our villages didn't soon grow much bigger. Only the cities that grew up around markets or governments had much reason for populations of many hundreds, depending on the regions they served.

It is only in recent centuries, since the end of the "Dark Ages," that we began regularly living in towns with populations in the thousands. And the local villages still kept their original proportions, with a few big market towns for economic interchange. A city of more than 10,000 was really thought of as a big place, probably the largest within 50 miles.

Later, some national capitals might even have exceeded 100,000 people. But it wasn't until the colonial period that London had a million people. These developments are too recent to have had much effect on the evolution of the human brain, and our modern educational methods are still struggling to adapt.