Gaming by its nature is a social activity. Any social activity has a set of rules that govern the gathering, a standard agreement of social niceties. Every gaming group has not only these rules, but another set of rules that detail the group's expectations. An example would be a group trying to play heroes of the realm. Typically, it is expected that there are no characters that happen to be serial killers. Gamers across the Internet have labeled this concept social contract. These can be explicit or implicit depending on the group. This is not about defining social contracts, but if you are looking for more information here are some links.
This is a great concept for a gaming group, it levels the understanding of everyone in the group. The difficulty happens when someone in the group doesn't want to abide by these rules. This makes it uncomfortable for everyone. This is what I want to talk about.
Recently, I joined a gaming group already in progress. Which seems to happen to me a lot lately. The GM told me what the game was about and asked me to create a character that works within guidelines, that the group had a social contract of sorts. The first session was just one big combat so it was hard to understand the group dynamic from that limited experience. The second session was an eye opener for me. The GM described a campaign about freedom fighters and trying to make the world a better place. The party was varying degree on the freedom fighter theme with one anomaly character. One guy created a character who only wanted to murder/pillage. His character would help the group as long as he was paid. Also his character was the loudest player/character in the group, and thought it was his job to threaten every NPC we met. This made the session a disaster from my perspective. I thought of quitting, but the GM assured me that I was the type of character that he wanted. Another player came up to me and told me the same thing. I still play in this game which isn't really as fun as I would like even though the GM has a lot of good ideas and runs a decent game.
The reason I wrote this is that I wanted to warn people who think that a social contract will solve their groups problems. It will only solve the problem if everyone in the group is on board with it. The minute you have someone who feels they don't need to abide the social contract, you will have problems. Make sure everyone in the group is committed to it.