Definition
The butterfly effect is the chaos-theory observation that tiny perturbations in a complex system can produce wildly different outcomes far downstream. A butterfly flapping its wings in Brazil, in the metaphor's original form, can set off a chain of atmospheric perturbations that decides whether a tornado forms in Texas weeks later. The same logic, Harari argues, applies to history.
In the Sapiens framing, history is a "level two" chaotic system: it reacts to predictions about itself, so any forecast strong enough to be useful is also strong enough to change the future it was predicting. Tiny accidents — a king's mood, a sailor's wrong turn, a plague reaching one port before another — are not noise to be averaged away. They are the actual content of historical change.
Why it matters
How it works
A butterfly-effect lens on history asks, at every major junction, what else could plausibly have happened. Why did Christianity, a minor Jewish sect, end up the official religion of Rome rather than the cult of Mithras or Isis? Why did Europeans colonise the Americas rather than the reverse? Harari's answer is rarely "because they had to" — it is "because of contingent accidents that, replayed, would frequently give a different result."
The implication is that the present cannot be read backward as the goal of the past. A society's dominant religion, its preferred form of money, its borders, its language map — all of these are frozen snapshots of contingent chain reactions, not inevitabilities.