I am part of a GM committee which organizes a yearly 3 day LARP near my hometown and it has been going on for quite a while. The setting is a custom made low-magic fantasy world, and the rule system (also custom made) is on the more combat-heavy end of the spectrum for European LARPs.
The core story used to focus on two or more groups (shires, cities, factions) who were essentially at war with one another — and the players could sign up for either of these groups with their registration. During adventures/quests/battles the players would mostly be in their groups (fighting against other groups), but during meals and the final feast (down time) everybody is sitting/living/eating in the same camp.
Now the issue I was observing considers how some players may take the hostility between their respective in-game factions to far, and let it 'shimmer through' in their interactions with players from other factions — even during down time.
Examples of such behavior would be taunts exchanged, being sore losers, 'bad vibes' — and in rare occasions overly aggressive combat (which triggered us to step in once or twice).
It is well established in psychology, that inter-group conflict can develop if groups of people — especially in antagonistic circumstances — are allowed to exist for a long enough period of time.
So far our solution was to essentially heavily change and disturb the available factions/groups every year or two, to avoid players becoming 'too attached' to their team. This has the advantage of mixing up the players and the groups and triggering new alliances and affiliations. The obvious disadvantage lies however in the heavy disruption of the story continuity, making it difficult to continue overarching plots from one year to the next.
What measures could we take — organizational or story-whise — to avoid in-character hostility progressing to out-of-character hostility while still keeping story continuity up and changing factions only slightly?
An afterword. So far the mentioned 'hostility' has not yet become a serious problem, at least not serious enough to merit us taking players aside and discussing the matter with them (which we will of course do, and have done in situations which called for it). But it takes careful planning in battles and combat scenarios to keep the different factions from 'heating up' too much. Also the camp vibe has suffered at times, which is something we would like to avoid, as it is valued by players, NPCs and GMs alike.
The rules of the LARP are very clear in terms of 'play fair and don't be a douchebag'. This receives special attention because we play our LARP without referees, i.e. the players are responsible for adhering to rules and GMs only intervene in very rare cases.
We have around 80 players, maybe 30 Staff/NPCs and 5-10ish GMs.
Best Answer
Both massively multiplayer online RPGs and live-action RPGs face this issue with player factions. In some cases, players identify very strongly with their factions and directly oppose others, in which case strong intergroup conflict can develop. However, in other cases there is hardly any conflict at all, even if the backstory sets up factions as foes. For example, my experience playing the City of Heroes MMORPG was that the hero and villain players hardly clashed at all and often associated as friends in the “downtime” zones.
Several game elements supported this friendly rivalry:
While the backstory set up the heroes and villains as foes, they rarely interacted that way in gameplay. Except for a handful of PvP zones, the players mostly dealt with their own problems, not the players from other factions.
Even in PvP play, the game wasn’t always set up as hero-versus-villain. The game included zones and arenas where players could play free-for-all or hero-versus-hero.
The game ran regular special events where enemy factions could team up toward a common goal. For example, on Valentine’s Day, heroes and villains could team up on special missions with unique rewards that you could only get by cooperating.
The overall story arc included common enemies like space aliens and parallel universes, which set up the factions as allies or rivals rather than enemies.
The Wikipedia article you linked talks about strategies for intergroup conflict reduction that correspond well to the methods used in City of Heroes.
Intergroup contact theory. The more you bring the groups into contact without conflict, the more you break down prejudice and conflict between them.
Superordinate identities. If you can gather the groups under an umbrella identity, like the “humans versus space aliens” story arc above, you foster social identification rather than conflict.
Interdependence. The more groups work together and depend on each other, like the Valentine’s Day special event, the more you foster cooperation rather than conflict.
If you want the factions to have some conflict, you can set them at odds for some tasks and missions, but use these techniques at other opportunities to moderate the conflict and make it more of a rivalry than enmity.