Once upon a time, back during the Age of Exploration, there was a marvellous practice called the “silent trade”. It was a solution to a serious coordination problem between groups who had no languages in common, or distrusted each other so much that they refused to come within range of each others’ weapons.

What makes it marvellous is that it constituted experimental proof of the existence of universal, objective ethical principles sufficient to build cooperation among hostile parties.

Here’s how it worked. One party is, say, the captain of a Portuguese merchant ship on the Gold Coast. He wants some of that gold, but nobody speaks the local language. Furthermore, there are disquieting reports from the few survivors that Europeans who venture out of sight of a ship’s guns tend to get eaten.

The other party is a local chief onshore. He has the same problem; he wants cloth and beads and metal knives, but he doesn’t speak the traders’ language. Furthermore Europeans have magical bangsticks that kill at a distance, and there are disquieting reports that tribes who were too welcoming got massacred by gold-seeking adventurers.

So, the captain brings his ship inshore and a heavily armed away party makes several piles of different kinds of trade goods on the beach. When they’re back on the ship, the vessel fires a cannon and retreats far enough offshore to not be a prompt threat.

The cannonshot attracts, as is meant to, the attention of the natives. They come out of the jungle, eye the various piles, and bring out their trade goods; gold, ivory, and whatnot. A pile of goods goes next to each of the traders’ piles. The natives withdraw from the beach.

Now the ship comes back inshore. Traders eye the piles and decide which exchanges they’ll take. Carrying away the native stuff nearest a pile of trade goods signals consent for the natives to take that pile and that pile only. Leaving a pile in place, or splitting it, signals wanting a better offer. Withdrawing a pile says that the native stuff nearest it is uninteresting.

Adjustments made, the ship withdraws again. Now it is the natives’ turn to evaluate the new state of the trade and re-adjust their own piles. The same rules apply; carrying a pile of trade goods away tells the Europeans they can have what the natives had offered for it. leaving a pile in place invites the Europeans to bid up, and withdrawing a pile says the trade goods offered for it are not interesting.

Sometimes, one side might split up one of the other side’s piles, putting goods near one part but not the other; this is a way to say “some of these goods are interesting, but not the others”. Various other elaborations are recorded.

The process would continue until a cycle during which neither side altered its piles or one side withdrew them all. This signaled the close of trade.

The scenario I’ve described is unusual in that the traders and natives might actually get in visual range of each other. In some important instances, such as the longrunning salt-for-gold trade between the North African coast and sub-Saharan West Africa, that never happened; coordination was entirely by drum signal. Silent trade was also reported in the 6th century CE on the East Coast of Africa between Indians and Arabs on one side and interior tribes on the other.

The silent trade flourished in Africa from classical times (Herodotus reported Carthaginians engaging in silent barter with West Africans) to about 1500CE. It fell into disuse only when enough cultural contact developed with the interior tribes for mutual language acquisition.

The most interesting observations about the silent trade begin with the fact that, as far as the historical record can see, nobody ever cheated – or, if they did, it was an unusual and sporadic phenomenon that failed to disrupt the exchanges.

Neither the traders nor the natives nor any third party had the ability to enforce honest dealing. No force could be used, and nobody punished except by the termination of the trade. Yet self-interest policed the process quite effecively.

The silent trade, when historians think of it at all, is usually considered a trivial bit of exotica, a footnote to the sweep of history. But it is much more than that; it is a demonstration of objective universal ethics. To explain this, I need to introduce, or remind you of, a couple of related concepts.

Those of you familiar with Robert Axelrod’s studies of the iterated Prisoner’s Dilemma will recognize a theme here; parties in the silent trade faced an iterated cooperate-or-defect choice (where defecting would have been to simply run off with the other sides’ goods) and settled into a stable tit-for-tat exchange.

In that exchange, both parties achieve what game theorists call “positive-sum” interactions – both are better off than if the exchange had never taken place. (This is distinct from zero-sum interactions, in which one party gains but the other loses, and negative-sum interactions in which both parties lose.)

Now I want to introduce the notion of a Schelling point. This, due to the economist Thomas Schelling, is “a solution that people will tend to use in the absence of communication, because it seems natural, special, or relevant to them.” My favorite example is the effect rivers have on political geography. Two hostile, non-communicating tribes separated by usable land is a recipe for a frontier war, but if a river runs between them both are likely to accept it as a natural boundary and confine their warfare to punishing violations.

Finally I want to exhibit the idea of Lorenzian incomplete aggression. The naturalist Konrad Lorenz famously observed that animals who cannot communicate with language nevertheless express “I could hurt you, but I choose not to” with aggressive behavior that is deliberately interrupted or misdirected short of actual damage. Anyone who has ever seen children roughhousing, or experienced the kind of solid friendship that martial artists can develop after a hard but clean bout of sparring, knows this works in humans too.

Several Schelling points are clear on examination of the silent trade. One is that it takes place at boundaries. By putting goods on the beach with an armed party and then withdrawing, rather than pushing into the jungle to hunt game or find trading partners, the traders did not merely reduce their chances of being eaten, they combined use of a Schelling point with uncompleted aggression.

The rules of the silent trade embody at least two important ethical principles; nonaggression and voluntary reciprocal exchange. If we are asking whether these principles are universal and objective, what better evidence could we ask for than to have seen them mutually agreed on by different groups of humans without the ability to even speak to each other (let alone shared cultural assumptions) and then sustained down the generations for over a thousand years?

The silent trade gives us grounds for a very strong claim: there is a universal objective ethics, and its building blocks include (a) nonaggression, (b) Schelling points, (c) honesty, and (d) voluntary reciprocal exchange. Or to put it more simply, “Do as you would be done by.” – the Golden Rule.

I have no doubt that the behavior of everyone in the Gold Coast story I told above seems natural to the reader. This is because we are actually neurologically wired to participate in universal ethics. Some equivalent of the Golden Rule is live in every human culture, and one of the first results to emerge from evolutionary psychology in the 1990s is that humans seem to come equipped with a cheater-detection module – our performance on logic problems improves when they are framed as questions about whether someone is violating reciprocity.

It would not be not stretching a point very far to say that the silent trade is in our DNA; its building blocks, such as the ability to recognize Schelling points and uncompleted aggression, certainly are. Through the lens of the silent trade, we can begin to see universal ethics as a culture-independent evolved behavior that solves a universal problem – how to achieve and maintain positive-sum cooperation.

This realization challenges several common beliefs, including cultural relativism and Hume’s guillotine – the notion that you can never derive an “ought” (normative moral or ethical statement) from an “is” (a fact about the world). How these falsehoods became so entrenched would be a topic for several more essays, but my point here is the silent trade helps us see past them – not just with a lot of argument and theory but in a practical, concrete, empirical way.

While I will not try to develop the argument here, the reader should consider this proposition: that the only ethical claims we should accept are universal (that is, agreements that could be reached by parties that cannot coerce or even at the limit communicate with each other), and that all other ethical claims are invalid, actually damage the prospects for sustained positive-sum cooperation, and should be discarded.