# Trading in two-player games

In a two-player game, why would one player ever trade with another?

(I'm working on a new game for my publisher at the moment, and this question is coming up at work.)

Let's say you have a three player game, with Alice, Bob, and Charlie. Alice and Bob are considering making a deal. This deal would help Alice a lot, and it would help Bob some, but not as much. Alice is certainly getting more out of it than Bob, but Bob still might agree to it, as it still helps Bob get ahead of Charlie.

Now imagine the same game with only two players -- Charlie isn't in the picture. Alice proposes the same deal to Bob, one that helps Alice a lot and Bob only a little. Bob has no reason to agree to this deal now, since it doesn't help him get ahead of anyone.

How can a two player game have trade deals between players?

Trading in a 2 player game is a difficult problem. You can have some form of forced trading. For example player 1 (after getting granted a trade action somehow) can name a resource, which player 2, has to give them if they have it, while player 2 gets to name a different resource which player 1 has to give them if they have it. If you name a resource the other player doesn't have, they can give you whatever they want.

Alternatively you can have trades via a third-party, such as a bank. The bank can start with some resources, and if you want something from the bank, you have to give the bank a resource it currently doesn't have. You then deplete the bank of the one it previously had, and the other player can then trade that resource for the one you gave the bank, if they want to. In this way the bank acts as a avenue for trades between players, but each trade with the bank is assessed on it's merits, plus the worry the other player might later benefit, or they might trade for the resource you want later, meaning you should trade now!

One simple way to do this is to make sure that each player has information that the other does not have. For example, each can have a hand of cards that only they can see. This way, for some trades, each player can think that they are getting the better end of the deal.

Another option is to have different goals for each player, possibly role-based or randomly chosen. That way, it can become harder to determine or even define who is getting the better end of a trade.

Mutual equally beneficial actions for all parties involved in a game does not provide anything to the contest aspect of games, which are often the entire point of 2 player games. Imagine a chess game where both players decided to remove all pieces except the king and queen and 2 bishops. What's the point? It simply moves the game towards the end faster with no added complexity.

As suggested, it could have some potential in games where each player has limited knowledge. However - this can very easily make the games so much more random.

If there's a 50/50 chance that the game is in a state where the trade will benefit my opponent more than it will benefit me, is it worth taking that risk? If the odds that the trade will benefit me is against me, why would I ever agree to the trade? Yes if I'm behind it could potentially give me a leg up, but then why would my opponent agree to the trade if he's already ahead?

Personally I appreciate games where RNG is a very small factor. But some people enjoy guessing, bluffing and taking risks.

If it stood to me, trading should never be a part of 2 player games. If you are working on a game with trading that allows different number of players, I would personally recommend making special rule exceptions to 2-player games disallowing trading. Possibly allowing bank-trading with a slight disadvantage in trade value. Really depends on the game, though.

• What do you mean by "RNG"?
– Joe
Dec 18, 2014 at 19:19
• @Joe Sorry, old habit "Random Number Generator". What I meant is "Chance" / "Probability". Basically "Let's roll a die and see who gets the highest number" isn't my favorite game... Dec 18, 2014 at 20:25

Castle Panic is an excellent example of a game that does exactly what you describe. The game has two goals. The first, is the players must cooperate to defend the castle from the oncoming hoards, if they don't, they'll lose. The second goal is to be the winner by killing the most monsters. As others have pointed out, there needs to be something forcing the trading, and castle panic handles this issue perfectly.

One key aspect of this trading system, is that the invading hoards are changing position all the time, the game state is difficult to predict more than a turn or two in the future. So there is little point in hoarding cards. Additionally some cards are clearly completely worthless to one player in the short term, while potentially very useful to another, so even though both player's hands are hidden (a point suggested by murgatroid), revealing that you have a useless card doesn't have much impact, and facilitates the trades. (What's worthless to me, can be incredibly value to you based upon the context of the game.)

As you've assessed, in a two-player game, a trading mechanism seems pointless. If the trade benefits one player, then there is no reason for the other player to agree to the trade, and if the trade benefits both players equally, then the trade has no effect on the game.

Well, such a statement makes certain assumptions about the game. If you relax some of these assumptions, it turns out that it can be reasonable to have an inter-player trading mechanism in a 2-player game:

• Assumption: Players can accurately assess the result of trades. If players don't know for sure which side benefits on a trade, it's possible that both sides might agree to a trade. The Settlers of Catan Card Game is a good example of this. If your game has hidden information, this makes it even more likely. You can also take advantage of this by deliberately making it hard for players to assess accuracy. For example, imagine a mechanism where you flip over a timer, and the two players have one minute to either negotiate a trade or both lose one resource to the bank. Sure, it's probably in one player's best interest to stonewall and both lose one, but is one minute enough time for a player to figure out which one of them gains from that?

• Assumption: Players can reject trades freely. Without this assumption, you create forced-trade mechanisms: player X offers a trade that player Y is not allowed to refuse. This can be interesting. Starship Catan is a good example of this. You can even tweak it a little -- player Y can refuse, but then is forced to make a counter-offer that is higher. (The tweaks the trade mechanism into an auction mechanism.) Basari is a good example of this.

• Assumption: The game is zero-sum. Exactly one-player can win. Without this assumption, you can create scenarios where both players lose or both players win, and then a trade mechanism can be interesting -- the players might decide to trade to avoid a double-loss scenario, especially early on in the game where it's not clear which side will win. The 2-team game in Pandemic: In the Lab is a good example of this.

• Assumption: Players are playing only to win. Not all players have winning as their primary goal. They might play games because they want to tell a story, or because they want to socialize with their friends, or because they want to experience some economic growth. In the first case, you might imagine a role-playing game where opposing factions will trade with each other because that's what their characters would do. In the second case, you might imagine some party trivia game where players might trade categories so that they can answer questions more in their field of expertise. In the third case, you might imagine players who are building a large empire and care more about reaching the end of the tech tree than whether the other player is further ahead.

There might be more ways to have a meaningful inter-player trading mechanism in a game, but these should cover most cases.

Catan Junior uses a third-party to assist in trading. There are five types of resources you can gather. The marketplace starts with one of each resource. On your turn, you can trade one time with the marketplace. Once the marketplace ends up with all 5 of the same resource, it resets to its initial state.

From a game theory perspective, an optional mutually beneficial trade between the opponents in a two-player game is unlikely to exist, at least if you only consider games among good players. If you have a game of perfect information, a player would not agree to a trade that worsens his position relative to the other player. Therefore, no trades would ever be made in those games. In a game of imperfect information, a good player will still have a general sense of the expected value of a specific trade of move. A trade would require that each player believes that the trade will improve his position relative to the opponent, while also knowing that the opponent believes the same thing. I don't think it's possible to have such a mechanic (though I would be interested to hear of a game that has one)

Here is another way to look at it. Consider that there are frequently conceptual "resources" of tempo, material, position, and score. When you make a move that affects your opponent, you are basically choosing to trade some of your resources for some of the opponents resources. However, you are dictating all the terms and conditions of the trade and the opponent is forced to accept the trade. (In a game where the opponent can make a defensive or reactionary move, that is essentially a separate trade that the opponent is setting the terms of and which you are forced to accept). If you needed your opponent's permission before you could attack, most games would be completely non-playable.

You're right that there should be no reason for players to openly trade in a 2-player game. One option you could have is a compulsory trade at the end of each round in which each player chooses one card that they want to get rid of.

However, there is a 2-player trading game called Jaipur, in which both players trade with a central board of five cards, but not with each other.

Another way to make players trade with each other is diversified resource system. For example if 1 player will use his starting resources to be able to produce 1 type of resources he won't be able to produce other. For example if player 1 spent his starting gold to build a quarry he will be able to produce stone but he may need other resource like wood to make progress in game. Therefore he will have to trade with other players who also will need resources they can't produce.

• But if there are only two players, and trading helps them both, why would they trade?
– Joe
Dec 16, 2014 at 9:59
• Even when there are only 2 players,trade still may be crucial for faster progres. The good way to do it may be promoting the trade by makeing aquireing resources cheeper by trade than by production. It brings more specialised building/production paths and promotes tradeing with other players. To make it more intresting u can alwyas add mechanics modyfiyng trade deals, for example buildings allowing better offers. Dec 16, 2014 at 10:10
• @Pimpekk Actions providing equally faster progress for all parties in a game provides no interesting element in my opinion. If you want the game to speed up so you can get it overwith, it's probably not a very fun game to begin with? Dec 16, 2014 at 13:10