“Neocortex size as a constraint on group size in primates”

Carol and I have finally been reading The Tipping Point by Malcolm Gladwell, the 2000 bestseller that gave a popular account of some scientific research in epidemiology, psychology, and sociology. Like Dr. Johnson, neither one of us has wanted to read the whole book all the way through*, so it lives in the bathroom, and we read bits of it when we’re not looking through the catalogs and magazines that also live there.

But while I haven’t actually read the book, I have been reading the end notes, which are really more informative than the book. In these endnotes I finally came across a reference I have wanted for some time: a reference to the scientific work that helps explain why human organizations with less than about 150 members are qualitatively different than human organizations with more than 150 members. The reference is: Robin I. M. Dunbar, “Neocortex size as a constraint on group size in primates,” Journal of Human Evolution, 1992, vol. 20, pp. 469-493.

And why should we care about the size of primate neocortexes? The neocortex is the part of the brain through which primates keep track of relationships; the larger the neocortex, the more relationships a given species of primate can keep track of; thus the large size of the Homo sapiens neocortex allows us humans to keep track of all the relationships in a group of up to about 150 members. When, however, human organizations are larger than 150 members, individuals can no longer keep track of all the relationships, and the group therefore feels qualitatively different.

This helps explain why congregations often stop growing when their active membership (measured as the average weekly attendance of adults and children) becomes larger than 150. My guess is that because our neocortex can’t handle any more relationships within that group, we literally cannot relate to any newcomers who may arrive. And if the newcomers can’t make connections with the other primates in the congregation, they’re not going to stick around — we primates are social critters who want to make connections with others of our species. This also helps explain why something like three-quarters of all U.S. congregations have an average attendance of fewer than 200 adults and children — we’re just more comfortable in groups with 150 or fewer humans. 

Whether a congregation is growing or not may thus have less to do with the attractiveness of the congregation’s theology than with the neocortex size of the primates who make up that congregation.

* “Johnson, offended at being thus pressed, and so obliged to own his cursory mode of reading, answered tartly, ‘No, Sir, do you read books through?’” — Boswell, Life of Johnson, Monday 19 April 1773.

4 thoughts on ““Neocortex size as a constraint on group size in primates””

  1. I have just been thinking about this same question, because I dipped into A Man Without a Country, by Kurt Vonnegut, on the read-half-the-book-standing-in-the-bookstore plan. He mentioned traveling in Nigeria and meeting an Ibo man who was bringing his newborn baby to meet his relatives. The man had “600 relatives he knew quite well”–that was the phrase, more or less. I don’t know what “quite well” means, nor what the study’s authors mean by “keep track of all the relationships,” but it does make me wonder how much is biological and how much is cultural.

    I’ve even heard the 150-person rule invoked to explain that we can’t learn the names of 150 people, which is ridiculous. Teachers memorize the names of that many students every year, without forgetting the names of all their friends and relatives. And at our church twice that size, I know the names of every regular attendee, plus lots of members and friends who don’t attend regularly. Of course, I’ve just jinxed myself: next Sunday I will forget the name of a long-time member and embarrass us both.

    There’s no question that 150 or so is a common plateau for congregations, but even acknowledging that that there are biological constraints on group size, it seems clear to me that those constraints are quite flexible.

  2. Amy @ 1 — Yes, I would expect the constraints to vary from individual to individual, as is true with any talent. But I didn’t explain fully enough: the constraints are supposedly knowing not only one’s relationships with other people, but also the relationships that others in the group have with each other. Thus the man from Nigeria might know 600 people very well, but he would not necessarily know how the relationships each of those 600 people has with each other.

    As far as how this constrains congregational growth, there’s at least one obvious way past it: break the congregation into smaller units of less than 150 each. This is the advice given in every handbook on church growth that I’ve read.

  3. Dan,

    As a religious movement, is our goal one of increasing our numbers or increasing the size of individual congregations? Are we looking at growth of individual congregations or denominational growth as our goal?

    These may sound like nonsensical questions so let me elaborate.

    One way of increasing the number of individual Unitarian Universalists is to increase the membership size of of congregations. As you noted in your blog post, this may be difficult given a natural tendency for congregations and other groups to top out at 150 members.

    Another approach would be to increase the number of Unitarian Universalist congregations. More congregations could lead to denominational growth while acknowledging the tendency for congregations and other groups to top out at 150 members.

    For this to work well, it would require these smaller congregations to cooperate with each other for specialized demographic ministries that would be harder to sustain in smaller congregations (e.g. campus ministry support, youth ministry, young adult ministry, Our Whole Lives classes for children and youth, special events for children and youth, etc).

    Having more congregations would allow for greater experimentation in worship style and theological focus as well.

    Having more geographic locations would reduce commute times for people attending worship and would reduce overall carbon footprint for attending church.

  4. Steve @ 3 — Those are exactly the questions I’ve been asking myself for some years now. I don’t have any final answers.

    Except that I know that lots of our congregations get lots of visitors each year (over a hundred a year here in Palo Alto), and we retain almost none of them. This says to me that there are people trying to get in to our congregations, and find that access is restricted. If you are trying to make room for those people (as I am), one of the things you have to ask yourself is whether you’d like to start a whole bunch of new congregations (in Palo Alto, maybe a new congregation every two or three years), or try to grow existing congregations. Again, I don’t have a final answer to that.

    All this boils down to one basic question for me: How do we make room for the people who want to get into a liberal religious congregation right now, but who are unable to do so because there’s not enough room?

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