Definition
The straight line instinct is the assumption that whatever trend a graph shows over the recent past will continue as a straight line into the future.
Real-world trends rarely behave that way. Populations, adoption curves, infections, mortality rates, and economic indicators tend to follow S-curves, doubling curves, humps, or slides. A line that has been straight for ten years can bend sharply in the eleventh because something — a saturation, a feedback, a policy — kicks in.
Why it matters
How it works
The instinct is partly a perceptual habit and partly a tool limitation. The default chart shows a few recent data points, the eye traces a ruler between them, and the mind extends the ruler. The same instinct that fits a line through two stars also fits a line through two decades of GDP figures, even though almost nothing in nature grows or shrinks at a constant rate forever.
Rosling's leading example is global population. The fear of unstoppable population growth assumes a continued straight upward line; the actual curve has already flattened markedly, because birth rates fall as child survival, education, and income rise. The same applies to fertility, vaccination coverage, urbanization, and many disease curves — each one has an inflection point that linear thinking hides.
The corrective is a small mental library of curve shapes — S-curve, doubling, hump, slide — and the habit of asking which one this series resembles before extrapolating. When the answer is unclear, the safer move is short-range estimation with explicit uncertainty rather than confident long-range prediction.