Left 4 dead's AI director and player karma

I had heard that the AI director rewards teamwork on campaign maps. Specifically, your karma increases if you use health packs to heal teammates instead of everyone just healing themselves.

It seems like something Valve would want to do to improve the overall play experience. I have much more fun playing on a cohesive team than as a bunch of strangers passing in the night.

However, this could easily be an gaming urban legend.

Does the director AI reward the survivors based on karma?

You're thinking of the article on The Director from the L4D2 wiki, which claims:

In Left 4 Dead 2, the Director is sensitive to the amount of teamwork going on as measured by how many times teammates heal each other and rewarding more teamwork with more health kits throughout the map.

The director first populates the map with pain pills in predetermined locations according to the pain_pill_density cvar, which means that when the survivors are in the safe room initially, there are no health kits anywhere in the outside world. The director has the power to change these pain pills into health kits based on the performance of the team, modified by the director_convert_pills variable. This is why on harder modes, you'll see less health kits, even if the team is cooperating well.

So the answer to your question is yes, there is a "karma" value, though it's not known by that name.

Don't take my word for it, though. Play through Dark Carnival a few times and check the first aid station in the small room just before the coaster. You'll get different amounts of medkits and pills each game, based on your performance.

For more detailed information, see this presentation by Valve's Mike Booth: http://www.valvesoftware.com/publications/2009/ai_systems_of_l4d_mike_booth.pdf

I have never heard anything from Valve about "karma" as an attribute the Director tracks, nor can I find anything about this via Google.

All the technical info I've seen has just talked about the Director tracking the stress level of the players, sending enemies when appropriate to keep them stressed while also allowing short periods of downtime to recover.

Additionally, how exactly would the Director reward you? Sending fewer zombies in the next wave or delaying the spawn of a special zombie are the only ways I can come up with.

Only Valve can answer for sure, but I strongly doubt from what I've seen that the Director tracks a "karma" value for players.

The concept of "Karma" in Left 4 Dead 2 probably evolved from the meme (?) "Karma Charger". He even has a Facebook page.

Basically, when a single person acts more on his own and less as part of a team, the director might punish that person by spawning Special Infected near that person. However, I doubt that Karma, as you explained it, exists in the game.