This large section deals with the very rare case that an overpopulation is created by a player who doesn't want to trigger a catastrophe. I'm talking about when the overpopulation is going to exist at the end of the player's turn, not when a ship happens to briefly create an overpopulation as it passes through a system while it moves twice with a yellow sacrifice move.
Overpopulations are almost always created intentionally so that triggering the catastrophe will destroy the opponent's resources. When I first teach people to play the game, I usually don't bother to tell them that catastrophes are technically optional. I sometimes use the words "overpopulation" and "catastrophe" interchangeably. However, several people have asked me about whether it could ever be a good idea to leave an overpopulation on the board at the end of their turn, so I want to thoroughly explore the topic in this section.
For some idea of why it's not normal to create an overpopulation where you don't want the catastrophe, take a look at this Homeworlds Theater episode. The overpopulating player didn't call catastrophe and simply hoped that their opponent wouldn't either. Fat chance. If you don't want there to be a catastrophe, you shouldn't set up an overpopulation. That move was so poor that the next HW Theater episode was all about replaying the game from that point with a different move.
I'm going to start by arguing that, if your turn is going to end with an overpopulation on the board for any reason, you should always trigger the catastrophe.
Human calculations on Homeworlds positions aren't perfect, so you may not be sure whether a catastrophe is good, bad, or neutral. But as I've just argued, you should trigger all kinds of catastrophes, so it doesn't even matter if you are completely sure which kind it is. Trigger it anyway.
Consider the extensive form game diagram above. If Alice doesn't trigger the catastrophe, then Bob gets to choose between States A, B, and C. Bob should choose whichever of these states is best for him (and therefore worst for Alice). If Alice does trigger the catastrophe, then she is limiting Bob's options because State A is guaranteed. If State A is the worst of the three states for Alice, Bob would choose it, and the outcome is the same whether Alice triggers it or leaves it for Bob. Therefore, Alice can't improve on the strategy of always triggering catastrophes at the end of her turn.
So theoretically speaking, you should never leave an overpopulation on the board after your turn. Strict game theoretical logic dictates that every overpopulation should become a catastrophe, But there are several justifications for not triggering catastrophes that are based on a belief in an opponent's imperfect mental state (nobody thinks as perfectly as the game theory argument requires). You may choose not to trigger a bad-for-you catastrophe as a gamble that your opponent will also choose not to trigger it for some reason. In that case, they will have gifted you another turn to try to resolve the overpopulation advantageously.
Take a look at the diagram above. Let's say that Alice thinks that State C is the best for her (and therefore the worst for Bob) but for some reason, Alice suspects that Bob believes that State C is actually the best for Bob. Alice would like the game to arrive in State C, and she expects Bob to also prefer State C. Alice rationally ignores the overpopulation and hopes that she is right that Bob will choose to ignore it as well.
On Bob's turn, he believes (incorrectly, in Alice's opinion) that State C is best for him. He doesn't want to trigger the catastrophe, and if he is unable to remove the overpopulation some other way, then Bob is in the position that Alice was just in; he is wondering whether to trigger the bad catastrophe. Having just turned down the opportunity to trigger the catastrophe, Alice has given Bob a clue: Bob should know that Alice believes the catastrophe is bad for her and that she thinks that Bob thinks the catastrophe is bad for him (since, as I've just argued, that's the only time Alice would choose not to trigger it). So, if Bob hasn't changed his opinion that State C is better than the alternative (State B), then he may decline to trigger the catastrophe using the same reasoning as Alice. This could carry on until a player finds a way to trade, move, or sacrifice one of the ships involved in the overpopulation.
Notice that Alice's decision to ignore the overpopulation doesn't actually rise from the players' disagreement about which state benefits which player. Alice's decision is based on Alice's belief that Bob's mental calculations are different from her own. If Bob's calculations are different but Alice doesn't know it, Alice should trigger the catastrophe.
Homeworlds is zero-sum game, so Alice and Bob can't both be right: if State C is better for Alice than A or B, then State C is worse for Bob than A or B. But there are various reasons why they might disagree. Most likely it's because both players make mistakes. No one can perfectly calculate how a catastrophe will affect the rest of the game (unless it's very nearly over). If Alice knows Bob well enough, she may know what kinds of miscalculation he tends to make and how that will affect his evaluation of a catastrophe.
For example, suppose that (in Alice's opinion), Bob overestimates the strength of large ships. Say that the overpopulation involves three of Alice's medium ships and one of Bob's large ships. Alice might think that her three mediums are worth more than Bob's large, but Alice also knows that Bob thinks that their large is better than Alice's three mediums. Alice is happy to not trigger the catastrophe, and (assuming that Alice is correct about how Bob thinks) Bob won't trigger it either.
Besides miscalculation, other things can cause players to have different beliefs. For example, if you're playing with time control, each player may think that they are more likely than their opponent to make mistakes under time pressure. Prolonging the game will put both players under time pressure, so each player thinks that they will be at a disadvantage in a long game. If there's a catastrophe that is expected to prolong the game (but that otherwise doesn't give an advantage to either player), then each player will think that triggering the catastrophe is bad for them. If you are aware of your opponent's belief that they are more vulnerable to time pressure than you are (the opposite of your belief), then it is reasonable to not trigger the game-prolonging catastrophe.
Similarly, energy for concentration is a limited resource that needs to be managed, even in a game that isn't under formal time control. If you believe that you are more tired than your opponent but you believe that your opponent thinks that they are more tired than you are, then you can reasonably not trigger a game-prolonging catastrophe.
The examples above take as a premise that a catastrophe that destroys similar amounts of material for both players tends to make the game last longer. It's been my experience that catastrophes (ones in colonies at any rate) are often followed by several turns of rebuilding for both players, which slows the game down. This principle definitely doesn't apply to catastrophes that involve a homestar, which tend to accelerate the game dramatically. Maybe you believe that the catastrophe in your game will make the game end sooner than if the catastrophe didn't happen. In that case, just reverse the beliefs from the earlier arguments. If each player believes themself to be less vulnerable to time pressure than their opponent (and if they are aware of their opponent's opposing belief), then they may reasonably choose not to trigger a game-shortening catastrophe.
To summarize this subsection, it is reasonable for you to not trigger a catastrophe if
A catastrophe is "bad" for a player if it decreases their chances of winning. A player's belief about whether a catastrophe is bad depends on their position-evaluation heuristics and on less-tangible factors such as their ability to calculate under time pressure (combined with how the catastrophe is expected to affect game length).
It feels bad to lose important ships to a catastrophe (unless it brings you closer to victory). So if a catastrophe destroys ships for both players, neither of you may want to trigger it because of the bad feeling of losing ships. So consider the case where there's an overpopulation that would destroy lots of material for both you and your opponent. Even if you think that the catastrophe is slightly better for your opponent's position, you might reasonably not trigger it. You should do this not for strictly logical reasons, but in the hope that your opponent won't feel like losing that much material for only a small advantage.
Finally, you might want to neglect triggering a catastrophe against a very inexperienced opponent. New players might not notice the overpopulation; they might not even know what a catastrophe is. So you might be able to get away with an un-triggered catastrophe due to ignorance, but, I absolutely do not condone exploiting players who don't know all of the rules. You should make it a teaching moment by pointing out what they ought to do. (And by the way, how did such an inexperienced player manage to tangle you into an unwanted overpopulation anyway?)
I can think of one reason to not trigger a catastrophe that doesn't have to do with your opponent's head. It has to do with time control instead.
Suppose that
You might want to leave the overpopulation on the board so that your opponent has to trigger the catastrophe themself, which will cost them some time. If you are playing with real life pieces, carrying out the physical act of clearing the catastrophe pieces from the board and sorting them back in the bank can easily take 10 seconds or more. If you're playing on a digital system, it will probably still take them some time to tell the computer that they want to trigger the catastrophe. All else being equal, it's better if that time comes off of your opponent's clock rather than yours. (Just don't spend so long thinking about it that you wipe out the time savings).
This whole section on when to trigger catastrophes goes out the window if you're not playing Binary Homeworlds. When three or more players are involved, you can engage in diplomacy, which is a totally different beast from what I'm analyzing on this page.
For an easy example of when not to trigger a catastrophe in a three-player game, suppose that Alice is winning against Bob and Carol. Bob and Carol have agreed to cooperate with each other for a while until Alice is less of a threat. Suppose Bob moves a large ship into one of Carol's systems in order to help defend against Alice, but Bob's ship is the same color as three of Carol's ships. Bob may choose not to trigger the catastrophe that would harm himself and his ally. Instead, Bob can end his turn with the overpopulation on the board, trusting that Carol will move, trade, or sacrifice a ship to remove the overpopulation. Of course, Alice would trigger that catastrophe if the overpopulation were still on the board when her turn came.
Arthur O'Dwyer whipped up the example below.
It's Lee's turn. The correct play is
Notice that Lee must refrain from triggering the yellow catastrophe in their own home because this catastrophe would turn the win into a tie. My game-theory based argument does not apply to this situation because Ray will not get a turn in which to trigger the catastrophe that Lee declined.
When playing Homeworlds online, you might not interact with your opponent at all except through the game itself. Computers take care of any timers and record-keeping. But when playing with physical pieces at the same table, there is more room for mistakes and irritation. Be a good sport by having good manners.
Many generic rules of game etiquette are applicable to Homeworlds: stay calm, don't show off, don't complain, etc. In the English-speaking world, chess seems to be the game with the best-established etiquette rules, and that seems like a good place to start. Homeworlds isn't (yet) played much in highly formal settings, but besides my ideas below, you might get a feel for how to show respect to your opponent from this nice article on chess tournament etiquette. But unlike chess, some turns of Homeworlds have multiple complex steps, so there is even more room for misunderstanding. Here's my politeness advice that's specific to Homeworlds.
The way I see it, there are two types of ways to play Homeworlds with more than two players. I'll call these "non-binary" and "super-binary."
Andy Looney posted many thoughts about playing with more than two people here. I have tried to include all of his ideas here.
If you'd like to spice things up by having more than two players on the same board, you need the few extra rules below just to keep things sensible and fair.
In Binary Homeworlds, the game ends as soon as at least one player loses control of their homeworld. Things are more complicated with more players, so you will need to choose another way to decide when the game is over and who won.
The rules of "sinister" and "good vs evil" emphasize the relationship between an eliminated player and their eliminator. It's a little unclear what to do if a poor-sport player wants to resign or self-eliminate just before someone else eliminates them. My advice is to not play with that type of person.
I don't seem to excel at diplomacy in games. I blame it on being an annoying know-it-all that people like to see beaten ;-)
One of the biggest differences between Binary and Non-Binary Homeworlds is that, with three or more players, it makes sense for them to negotiate. When two people (or teams) have perfectly opposite objectives, there can be no negotiation or collaboration between them because each player only succeeds to the extent that the other fails. Every outcome that one player wants the other player needs to avoid. There are no win-win moves. They are playing a "zero-sum" game.
When three or more players each have their own objective, they can use diplomacy to advance their interests. If one player is winning, the other players may each improve their chances of winning by ganging up on the leader. Games for 3+ players are often won by a manipulator who can control the other players, even if the manipulator is less skilled at the underlying game.
Something that many players (including me) love about zero-sum perfect-information games is that they win strictly by their wits without having to worry about charisma. Compared to games with diplomacy, zero-sum games feel pure and controlled.
On the other hand, games with diplomacy are much more social. They're great for getting more people into a game together and getting to know each other. Other players' unpredictability creates a wild experience that's great for those who like to just enjoy the ride without fixating on whether the "right" person won.
You can use the ordinary rules of Binary Homeworlds to play with more than two people.