Christopher Boehm has discovered that, among the tribal and hunter-gatherer human societies he studies, the development of projectile weapons is a key step in the growth and maintenance of equality: it puts the strong at greater risk from the weak. Such weaponry is one reason that human societies are more equalized than those of other primates. But weapons aren’t enough to make equality last. Boehm finds that, to really maintain the new social order, the dominated need to trust one another. They must have stable social bonds and anticipate a long future together. Most important, they must be able to communicate effectively. To Boehm, therefore, there is something distinctively human about sustained equality: only human beings communicate well enough to keep it going for long periods.