Maria Konnikova looked at the research done by anthropologists on the sizes of social groupings. The research initially studied why non-human primates spent so much time on grooming one another but then got extended to humans.

[I]n the nineteen-eighties, the Machiavellian Intelligence Hypothesis (now known as the Social Brain Hypothesis) had just been introduced into anthropological and primatology discourse. It held that primates have large brains because they live in socially complex societies: the larger the group, the larger the brain. Thus, from the size of an animal’s neocortex, the frontal lobe in particular, you could theoretically predict the group size for that animal.

Looking at his grooming data, [anthropologist and psychologist Robin] Dunbar made the mental leap to humans. “We also had humans in our data set so it occurred to me to look to see what size group that relationship might predict for humans,” he told me recently. Dunbar did the math, using a ratio of neocortical volume to total brain volume and mean group size, and came up with a number. Judging from the size of an average human brain, the number of people the average person could have in her social group was a hundred and fifty. Anything beyond that would be too complicated to handle at optimal processing levels.

So what are the sizes of the groups one finds in human societies?

Judging from the size of an average human brain, the number of people the average person could have in her social group was a hundred and fifty. Anything beyond that would be too complicated to handle at optimal processing levels.

This number is called the Dunbar number but in actuality there are several such numbers.

The best known, a hundred and fifty, is the number of people we call casual friends—the people, say, you’d invite to a large party. (In reality, it’s a range: a hundred at the low end and two hundred for the more social of us.) From there, through qualitative interviews coupled with analysis of experimental and survey data, Dunbar discovered that the number grows and decreases according to a precise formula, roughly a “rule of three.” The next step down, fifty, is the number of people we call close friends—perhaps the people you’d invite to a group dinner. You see them often, but not so much that you consider them to be true intimates. Then there’s the circle of fifteen: the friends that you can turn to for sympathy when you need it, the ones you can confide in about most things. The most intimate Dunbar number, five, is your close support group. These are your best friends (and often family members). On the flipside, groups can extend to five hundred, the acquaintance level, and to fifteen hundred, the absolute limit—the people for whom you can put a name to a face. While the group sizes are relatively stable, their composition can be fluid. Your five today may not be your five next week; people drift among layers and sometimes fall out of them altogether.

I would guess that my personal numbers for each category are way smaller than the ones quoted above, perhaps because I am an introvert who actually shuns large groups of people. I would not dream of inviting 150 people, the so-called ‘casual friends’ group, to a dinner, ever. Also, I am not sure that I can make distinctions between ‘close friends’, ‘ true intimates’ and ‘close support group’. In my life, I have just three categories: acquaintances (which is closest to the casual friends above), close friends (in which I would include intimates and close support group), and those that are somewhere in between.

However, I may be an anomaly because Dunbar finds empirical support for those numbers in various groupings.

The average group size among modern hunter-gatherer societies (where there was accurate census data) was 148.4 individuals. Company size in professional armies, Dunbar found, was also remarkably close to a hundred and fifty, from the Roman Empire to sixteenth-century Spain to the twentieth-century Soviet Union. Companies, in turn, tended to be broken down into smaller units of around fifty then further divided into sections of between ten and fifteen. At the opposite end, the companies formed battalions that ranged from five hundred and fifty to eight hundred, and even larger regiments.

The rise of social media and the ease of making links with strangers does not seem to have changed things that much.

With social media, we can easily keep up with the lives and interests of far more than a hundred and fifty people. But without investing the face-to-face time, we lack deeper connections to them, and the time we invest in superficial relationships comes at the expense of more profound ones.

An interesting feature that researchers find is the importance of physical contact, something that is clearly important to non-human primates as seen in the huge amount of time and effort they spend in grooming one another.

Over the past few years, Dunbar and his colleagues have been looking at the importance of touch in sparking the sort of neurological and physiological responses that, in turn, lead to bonding and friendship. “We underestimate how important touch is in the social world,” he said. With a light brush on the shoulder, a pat, or a squeeze of the arm or hand, we can communicate a deeper bond than through speaking alone. “Words are easy. But the way someone touches you, even casually, tells you more about what they’re thinking of you.”
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The researchers saw the same thing that happened with monkeys, and that had earlier been demonstrated with humans that were viewing positive emotional stimuli: when subjects in the scanner were lightly touched, their bodies released endorphins. “We were nervous we wouldn’t find anything because the touch was so light,” Dunbar said. “Astonishingly, we saw a phenomenal response.” In fact, this makes a great deal of sense and answers a lot of long-standing questions about our sensory receptors, he explained. Our skin has a set of neurons, common to all mammals, that respond to light stroking, but not to any other kind of touch.
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Until social media can replicate that touch, it can’t fully replicate social bonding.

It is possible that as we become more habituated to online interactions, the need for physical contact may diminish. But there is also the worry that with online interactions, we may not adequately learn the coping mechanisms that are part of the process of social bonding. When it comes to interactions with family, friends, neighbors, coworkers, and others in our physical world, there is less chance of avoidance. We have to find ways of dealing with rough patches in the relationships. With online friends, we can simply walk away from those who, for whatever reason, have annoyed us. We can ‘ghost’ them. And that may result in us forming fewer social bonds, even as we feel that we are in touch with so many people.