In early 2020, just before the start of the pandemic, I met a woman who said she practiced “aggressive friendship.” It takes a lot of her time, but she’s the person who regularly invites friends over to her house, who organizes events and outings with her friends. What a fantastic way to live.

I thought of her while reading Robin Dunbar’s recent book, “Friends.” If the author’s name means something to you, it’s probably because of Dunbar’s number. This is his finding that the maximum number of meaningful relationships most people can have is somewhere around 150. How many people are invited to the average American wedding? About 150. How many people are on an average British Christmas card list? About 150. How many people were there in early human hunter-gatherer communities? About 150.

Dunbar argues that it’s a matter of cognitive capacity. The average human mind can maintain about 150 stable relationships at any given moment. These 150 friends are the people you invite to your big events — the people you feel comfortably altruistic toward.

He also argues that most people have a circle of roughly 15 closer friends. These are your everyday social companions — the people you go to dinner and the movies with. Within that group there’s your most intimate circle, with roughly five friends. These are the people who are willing to give you unstinting emotional, physical and financial help in your time of need.

Dunbar argues that the closeness of a friendship is influenced by how many things you have in common. “You are twice as likely to share genes with a friend as you are with any random person from your local neighborhood,” he writes. People tend to befriend those who have similar musical tastes, political opinions, professions, worldviews and senses of humor. You meet a new person. You invest time in getting to know this person, and you figure out which friendship circle you are going to slot him or her into.

Time is one crucial element in friendship. Jeffrey Hall, an expert in the psychology of friendship, studied 112 University of Kansas first years and found that it took about 45 hours of presence in another person’s company to move from acquaintance to friend. To move from casual friend to meaningful friend took another 50 hours over a three-month period, and to move into the inner close friend circle took another 100 hours.

People generally devote a lot more time to their inner circles than to their outer circles. Dunbar found that over the course of a month, people devote about eight and a half hours to each of their five closest friends, and they devote a bit more than two hours a month (basically a dinner or a lunch) to the next 10 who complete their 15-person circle. They devote, on average, less than 20 minutes a month to the other 135 people in their larger friend circle.

These are averages. We each have our own friendship style. Extroverts spend their social energy across more people and have more but weaker close friendships. Introverts invest in fewer people but have stronger ties to them.

The other crucial factor in friendship is social skill, and this is something that, as a society, we don’t take seriously enough. This has become a passionate conviction for me over the past decade. Social life is fast, complex and incredibly demanding cognitively. Americans have only recently begun to teach social and emotional skills in schools, and there are plenty of reasons to believe that online life erodes those skills.

But our happiness in life, as well as our health and fulfillment, is hugely dependent on our ability to be skillfully understanding of and considerate toward others. A lot of the bitterness and alienation in our country flows from the fact that our social skills are inadequate to the complex society we now live in.

The psychologists Michael Argyle and Monika Henderson identified some of the social actions on which friendships are based: standing up for friends when they are not around, sharing important news with them, confiding vulnerabilities with them, providing emotional support when it’s needed.

A lot of the important skills are day-to-day communications skills: throwing the conversation back and forth without interrupting, adding something meaningful to what the other person just said, telling jokes, reminiscing about the past, anticipating how the other person might react to your comment so you can frame it in a way that’s most helpful.

Dunbar and his colleagues Neil Duncan and Anna Marriott sampled conversations other people were having in coffee shops and other venues and found that two-thirds of the conversation time was spent talking about social topics. Dunbar’s research also suggests that the average person can expect to have a close relationship break down about every 2.3 years. That’s roughly 30 relationship breakdowns over an adulthood — usually over things like lack of care and poor communication.

I find Dunbar’s work fascinating, though like so much of the social sciences, it focuses on what can be quantified across populations, so it misses what is particular and unique about each friendship.

Most of this research was done many years ago. Reading it in the context of Covid, I often had a sense that I was glimpsing a lost world. Everything seems so fragile. As we gradually slog back to normal life, this might be the moment to take a friendship inventory, and to be aggressively friendly.