Definition
The generalization instinct is the urge to compress a diverse group into a single category and treat every member as a copy of the prototype that first came to mind.
Categories are necessary — thought without them is impossible — but the instinct tips into error when the within-group variance is larger than the between-group difference, which it often is. "Africa" contains 54 countries spanning four income levels; "the developing world" is now home to most of humanity at multiple stages of development; "millennials" includes the full range of incomes, attitudes, and life situations.
Why it matters
How it works
The instinct shows up in three patterns. First, treating one example as representative of a whole group — one news story stands in for an entire country. Second, treating a category as homogeneous — "Asia" or "Latin America" assumed to behave the way the loudest example does. Third, treating "us" as exceptional and "them" as uniform — the same instinct from a different angle.
Rosling's corrective is to interrogate the category before reasoning from it. Ask what range of cases sits inside; ask how the cases differ; ask whether the difference between two groups is actually larger than the spread within either. If the answer is no, the category is masking the variance you should be working with.
The instinct overlaps with the gap instinct (which splits a continuum into two camps) and the size instinct (which over-weights any single example presented in isolation). All three rely on the brain's preference for clean, named groups over messy distributions. The fix in each case is to recover the distribution before drawing the conclusion.