The Six Types of Simple Rules¶
Definition¶
Across every domain the book examines — from bail hearings to burglary to butterfly mating — simple rules cluster into six recurring types split across two families. Decision rules structure choices: boundary rules sort options into two mutually exclusive categories (do/don't, include/exclude), prioritizing rules rank options when resources can't cover everything, and stopping rules dictate when to reverse or exit a decision already made. Process rules govern execution: how-to rules guide the basic steps of a task, coordination rules synchronize the actions of multiple independent actors, and timing rules govern rhythm, sequence, and deadlines.
In the Book¶
Chapter 2 grounds boundary rules in judges deciding bail (three simple rules explained roughly 95% of actual bail decisions, regardless of the elaborate risk-assessment statutes on the books), in Texas burglars who ignored security systems and landscaping and used one rule two-thirds of the time — "avoid houses with a vehicle parked outside" — and in DARPA, which funds its wildly diverse portfolio (gecko-inspired climbing gear, brain-controlled prosthetics, a robotic pack horse) using just two boundary rules: a project must advance fundamental scientific understanding, and it must have a practical use. Chapter 3 grounds the process-rule side in Don Sull's own rules for bouncing at a rowdy Cambridge bar near Harvard and MIT ("Don't let trouble in the door," "Keep the bikers on your side") and in BBC commentary pioneer Seymour "Lobby" de Lotbiniere's six how-to rules for sports announcers (set the scene, describe the action, give the score regularly, and so on), which trained multiskilled broadcasters who could cover any sport on short notice. Coordination rules are illustrated by schooling fish, Zipcar's shared-vehicle protocols, and Wikipedia's editing norms; timing rules by dragonfly migration and Wag Dodge's split-second decisions during the Mann Gulch forest-fire tragedy.
Why It Matters¶
The taxonomy turns "use a simple rule" from a vague slogan into a design choice: the right type of rule depends on what kind of bottleneck you're facing. If the problem is choosing between mutually exclusive options, you need a boundary rule; if you have more good options than resources, you need a prioritizing rule; if the risk is not knowing when to quit, you need a stopping rule; if the problem is doing a repeatable task well, a how-to rule; if it's synchronizing independent actors, a coordination rule; if it's managing rhythm or sequence, a timing rule. Misdiagnosing which type of bottleneck you actually face — reaching for a how-to rule when what you need is a stopping rule — is a common and correctable failure mode in any domain that relies on heuristics instead of full optimization.