Hypothetical - Multiplayer Horror

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Guys this thing will never work as intended and it comes down to basic human instincts. And not only humans. There is a reason why people, wolves, dogs, birds etc...(animals) form groups. Not only are they stronger but most importantly they feel safe. Sometimes, this feeling is all that is needed to do things that you previously would have never imagined doing. That's why aggressive dogs are so unpredictable when they form a small group. That worm feeling of being safe is what effectively ruins horror.

Definitely. But I think that this feeling of being safe in a group could also be used to create even more fear in contrast. I'm thinking about how good horror games often include a few safe areas, so the player doesn't become too jaded from constant stress. (Back Hall in Amnesia for example, which was a save hub.)
So first you make the player feel all safe and cozy in his group - even better if you can manage to create emotional attachment to his fellow players (Journey-style ). Then you suddenly separate the players again. Maybe something that tears the group apart, like a portcullis crashing down, some sort of devious labyrinth that teleports you around or maybe just puzzles that require the group to split up.

Now I'm pretty sure that being alone after having gotten used to being safe in a group feels much more intense than just having been alone all along. Especially if the game established beforehand that you actually are safer in the group (Maybe there's some sort of enemy that attacks lone players but flees from groups?). Players would automatically set it as their goal to find their partners again, the relief when they found them again creating more emotion.

I think it could be very engaging if done well - I mean even in Journey there were a lot of players who panicked upon losing sight of their partner, even though there was no real punishment for playing alone, it was all just emotional attachment.

When it comes to gaming, if multiplayer horror rises as a game style you will see that most people will play with their friends and instead of being scared they will go all the way to find the monsters just to scream and laugh as they are being chased and of course video/audio recorded. Sometimes, this also applies to "skype let's plays" were people are constantly distructed by their talking or screaming friends resulting in a ruined immersion.

Jup, I agree. That's why I'd prefer a more anonymous approach with extremely limited communication. I guess you can't keep players from playing with their friends in the same room if they really want to, but at some point I think it's not the game dev's problem anymore. There are always people who will go out of their way to ruin a perfectly crafted experience, not only in multiplayer. As long as the game doesn't encourage such behavior, all is okay - after all, some people ruined Amnesia for themselves, too, by doing the same thing, but that doesn't mean Frictional did something wrong