Definition
An ideology is a unified worldview that explains a wide range of phenomena through a single primary lens — markets, class struggle, evolutionary psychology, identity, faith, or another organizing principle.
Rosling treats ideologies with mixed respect. Each one supplies a powerful analytical tool: a way of cutting through complexity quickly and consistently. Each one also installs a single-perspective trap: once the lens becomes the only lens, every problem starts to look like the kind the ideology was built to solve.
Why it matters
How it works
Ideologies start as theories of the most important cause. A market liberal believes incentives explain most behavior; a Marxist believes material class relations explain most behavior; an evolutionary psychologist believes ancestral selection explains most behavior. Each theory has real explanatory power inside its target domain. The trouble is the word "most" — it grows over time into "all," and the theory becomes a worldview that fits every question by construction.
Two forces lock the lens in. First is intellectual: once you have absorbed an ideology, its concepts shape the questions you ask, so you tend to encounter the kinds of cases it explains well. Second is social: ideologies come with communities, and disagreement carries a cost. Together they make a single-perspective worldview easy to enter and hard to leave.
Rosling is even-handed. He does not call for the abandonment of ideologies — they remain useful — but for awareness of which questions yours handles well and which it handles badly. The factfulness move is to ask, on any given question, whether the answer your ideology suggests is also the answer a different lens would suggest. When two ideologies that usually disagree both converge on the same finding, that finding is unusually robust.