Glossary
Dunbar's number
About 150, extrapolated from a relationship between neocortex size and group size across primates. A 2021 reanalysis concluded that specifying any one number is futile: active networks often land in the low hundreds, but whether the brain fixes a limit at 150 is disputed.
Published Updated
Where this comes from
In the reference topic that owns this term
Reference topic 3, The Expensive Organ, states the mismatch as a card: what the system evolved to expect, what it gets instead, and what happens in the gap.
Mismatch card
- What it expects
Cooked, varied food; sleep; a band of about 30 people inside a wider network; daily physical and mental challenge.
- What it gets
Long hours seated (Topic 6), isolation for some, and prices, deadlines and risks its defaults never met.
- What happens in the gap
Defaults misfire as worry, the planning fallacy and reluctance to part with what you own; isolation and inactivity are associated with smaller brain structures and higher dementia risk.
What that chapter recommends
3 of the 7 graded recommendations in Reference topic 3, strongest evidence first. How the grades work.
Sleep 7 or more hours a night on a regular basis
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.the consensus of American sleep medicine. ●●○ Loud snoring, gasping or daytime sleepiness warrant a sleep assessment. See Chapters 23 and 41.
Keep people in your week
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Social isolation is one of 14 modifiable risk factors; addressing all of them might prevent about 45% of dementia. ●●○ (observational) See Chapters 29 and 42.
Walk briskly, and build up gradually
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.In a trial of 120 older adults, walking 40 minutes three times a week for a year, after building up from 10 minutes over seven weeks, enlarged the anterior (front) hippocampus by 2%.
Related terms
The argument behind this term, its studies and its notes are in Reference topic 3 of the reference library. The book gives the practical version. Get the book.