Why do duelists walk 20 paces before firing at their opponents?

When an 18th Century gentleman was subject to an insult to his honor, he would often respond by challenging the offender to a duel. A duel is a formalized contest of armed combat between two individuals (with pistols since the 18th Century), in which individuals are placed back to back with loaded weapons in hand, walk a set number of paces (like 20), turn to face the opponent and fire their pistol (sometimes simultaneously and sometimes in sequence). The contest is described as "formalized" because it was typically governed by a large set of rules about the number of paces one could take, the kind of pistols and bullets that one could use, the determination of the duel's end, and mechanisms for avoiding the duel all together (e.g., "deloping"). Why all of these formal rules? Was it simply because 18th Century gentlemen were fancy fops who delighted in stupid rule making instead of engaging in more primitive, masculine armed combat?

Consider the rule determining the number of steps one must take before shooting. Whether one is 20 paces or 1 pace away, completing the duel allows both parties to retain their honor and dying from the duel is dying from the duel. Yet if both duelists are 20 paces away, the probability of either dying is lower than in the case when both are only 1 step away. However, if you turn quickly and shoot while your opponent is walking his 20 paces, you are likely to win the duel, thus maintaining your honor and retaining your life.

The dominant strategy is for both to walk one step and fire the pistol (an incentive for a first strike). Yet the honor achieved in both cases is the same but the probability of dying is higher for both in that case than if each had walked 20 paces. So duelists made rules to ensure the best outcome would be achieved (honor retained, neither party grievously wounded). To prevent duelists from making little tiny steps (a dominant strategy when having to make 20 of them), a pre-agreed length of 20 paces would be measured out by the duelists' assistants and marked on the ground. Rules were also devised to prevent an arms race on the dueling field (smooth-barrelled, single-shot revolvers were preferred over grooved barreled, multi-shot revolvers. Late 19th century and 20th century warfare exhibits similar issues and responses (e.g., rules to outlaw chemical and biological weapons, rules to restrict access to nuclear weapons).