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Your 150: people you'd greet anywhere
In his book, Dunbar defines the group by a scene: a 3 a.m. stopover in the transit lounge at Hong Kong Airport, and a person you could walk up to and greet as someone you haven't seen in ages, with no embarrassment at all. They would probably be a bit miffed if you didn't.
You wouldn't need to introduce yourself. They know where you stand in their social world, and you know where they stand in yours.
The test has a practical end too: if push came to shove, they would be more likely than not to lend you a fiver, five pounds, if you asked. People who meet all of it make up the group of about 150 that Dunbar reached by comparing primate brains to group size, roughly 148 for a human brain.
How did researchers count a person's 150?
A 2003 study by Russell Hill and Robin Dunbar in the journal Human Nature asked people to list everyone they had sent Christmas cards to. A card signals that someone is worth keeping up with, which makes the list a handy record of real relationships. The cards went to about 68 households on average, and counting the people in those households gave a mean of about 153.
Why does knowing someone take so long?
A 2018 study by communication researcher Jeffrey Hall, published in the Journal of Social and Personal Relationships, estimated how many hours together it takes to build a friendship. Roughly 50 hours moved someone from acquaintance to casual friend, and about 90 hours to friend. It took more than 200 hours to reach close friend. The participants were adults who had recently moved to a new city.