What is the rule of 150 or the Dunbar Number?

Answer 1

There are several key indicators - numbers we take seriously - to indicate "how we're doing." Cholesterol levels, salary, and IQ score, for example. And now the concept of a "Dunbar number" is being tossed around. Briefly, the term is used to suggest that we can only manage, cognitively, a limited number of relationships. The average, suggested in anthropologist Robin Dunbar's research, is just under 150. That's the average, so there are folks who could handle more than that, or fewer than that.

As a neuropsychologist, what's really interesting to me about Dunbar's research is his group's finding of a relationship between the size of a region of the human frontal lobe and subjects' performance on a measure of "intentionality."

In "Theory of Mind," intentionality refers to our ability to understand the intentions of other people. That is, as we grow up we become increasingly aware that there are people outside of us whose brains are thinking different thoughts, whose hearts are beating with different passions. Just like "working memory" requires the mental juggling of bits of data, "intentionality" requires the ability to recognize our own knowledge and motivation but to also mentally represent the knowledge and motivation of other people.

If you're a committed vegan but can see how other people might ethically arrive at a meat-based diet, that flexibility takes cognitive activity that we believe takes place in certain regions of the prefrontal cortex. If you're a strong supporter of the Republican candidate but can imagine or visualize how someone else might arrive at a choice to support the other guy, that takes some crafty and mature manipulation of information. What Dunbar's group calls a firm grasp of "intention."

And Dunbar's research suggests that the variability of intentionality is related to volumetric difference in the orbitofrontal prefrontal cortex. So what's interesting about Dunbar's number may not be the superficial head-count of Facebook friends, but the cognitive abilities which underlie social functioning. One way of measuring cognitive abilities such as "intentionality" is the Imposing Memory Task. (Specific training and background is required to interpret and make sense of tests like these).

And cognitive skills like "intentionality" are directly related to other executive functions which make it easier for us to manage academic, vocational, and self-management tasks. Functions like:

• delaying gratification
• planning ahead
• thinking before acting
• thinking before speaking
• inhibiting off-task behavior
• sticking with important tasks that aren't fun or interesting or easy

In his hilarious response to the Tiger Mother brouhaha, David Brooks observes that "Practicing a piece of music for four hours requires focused attention, but it is nowhere near as cognitively demanding as a sleepover with 14-year-old girls." Sounding like a developmental neuropsychologist (or a really observant parent), he suggests that "managing status rivalries, negotiating group dynamics, understanding social norms, (and) navigating the distinction between self and group..." are at least as demanding as academics, and perhaps more crucial to long term adjustment.

As teachers or therapists think about their students or clients Dunbar's research sheds important light on the inter-relatedness of executive functions. About the role of clinicians and parents who work with autistic spectrum or ADHD students whose social and executive skills are less adequate than their same-age peers.

At very least, these new data give neurodevelopmental context, and heft, to the observation that "She plays well with others"!