In World of Warcraft, some characters are, among other things, crafters, can produce useful objects such as pieces of armor that they or others can wear. To do so requires some level of skill, obtained by in-game activities, mostly crafting, and ingredients ("mats" for "materials"). At a high level, the required mats are very expensive in in-game money and the product is better—provides a greater improvement to the player wearing it—than most of what players can get in other ways.

A common arrangement is for the player who wants something made to provide the craftsman with the required mats plus a tip, an additional payment in in-game currency. The crafter makes the item and gives it back to his customer. The mechanics of exchanging goods involve each player showing what he is giving the other; the exchange happens when both approve. For ordinary trade, that provides automatic enforcement; if you don't put up what you offered in exchange for what I am offering, I don't click the relevant button and the trade does not happen. If, however, I hand over the necessary materials to a craftsman in order that he can make me something, there is nothing to keep him from walking off with them without giving me anything in exchange.

What started me thinking about this problem was my wife's account of a recent transaction . Her character is, among other things, a high level blacksmith with the recipe for a very high level piece of armor, one whose materials cost about 6000 gold. At current exchange rates—there is an active market trading WoW gold and other virtual goods for money, although Blizzard, the company that runs the game, tries to discourage the practice—that comes to forty or fifty dollars.

Someone looking for a crafter to make that item posted a query. I noticed it and referred him to my wife's character. He gave her the necessary materials plus some additional gold, she crafted his armor and gave it back to him. A satisfactory transaction for both parties.

So far as we know, she was a stranger to him. Why did he trust her? How did he know she wouldn't simply walk off with his materials, perhaps use them to make the same piece of armor for herself?

Part of the answer is that in order to be a high level crafter one must be a high level character, which normally means having spent hundreds or thousands of hours fighting monsters, going on quests, practicing one's craft. My wife's blacksmith is level 80, the highest level currently available in the game. If she cheated a customer, she risked a permanent black mark on her reputation. A single WoW server has a population in (I think) the tens of thousands, large enough to dilute but not eliminate reputational problems. I suspect that if the same transaction could be done by someone who had just created a new character and played him for a few hours, the level of trust would be much lower. The mechanism is the same one that explains why banks favor expensive buildings faced with marble and provides one explanation of why companies engage in expensive advertising campaigns—in each case, paying a sunk cost to make it clear to customers that it would be expensive for the firm to take their money and run.

There are possible substitutes for trust, but they are costly. The customer could, for example, offer to trade 6000 gold worth of materials for 6000 gold in one exchange, then give the gold back, along with the additional payment, in exchange for the armor. Doing it that way, however, would require the blacksmith to have that amount of in-game cash free for the purpose. In this case, at least, she didn't; she has not yet made the armor for herself because she cannot yet afford to do so. A customer who sets that requirement substantially reduces the number of potential transactions. That would be a serious problem for very high end crafting, where only a few crafters will have the necessary recipe. The customer also risks offending the person he wants to trade with by making his distrust obvious.

On one occasion, my wife noticed someone posting in search of a crafter with an additional requirement: "must be a member of a reputable guild." That is a way of improving the reputational mechanism, since guilds, in-game groups of characters who do things together, also have reputations. I am reminded of Adam Smith's argument in favor of a diversity of small religious groups—that they have an incentive to make sure their members behave well in order to keep up the group's reputation. But in World of Warcraft the practice is uncommon; my wife only remembers seeing one such post, and other people reacted negatively to it.

Trust mechanisms are not perfect—people do sometimes cheat. But I found it interesting that they work well enough so that the sort of transaction my wife's character engaged in, for an amount very large in in-game terms and significant even in dollar terms, is not uncommon.