Coding Horror: The Bad Apple: Group Poison
What they found, in short, is that the worst team member is the best predictor of how any team performs. It doesn't seem to matter how great the best member is, or what the average member of the group is like. It all comes down to what your worst team member is like. The teams with the worst person performed the poorest.
in some of the groups, the fourth member of their team isn't a student. He's an actor hired to play a bad apple, one of these personality types: The Depressive Pessimist will complain that the task that they're doing isn't enjoyable, and make statements doubting the group's ability to succeed. The Jerk will say that other people's ideas are not adequate, but will offer no alternatives himself. He'll say "you guys need to listen to the expert: me." The Slacker will say "whatever", and "I really don't care." The conventional wisdom in the research on this sort of thing is that none of this should have had much effect on the group at all. Groups are powerful. Group dynamics are powerful. And so groups dominate individuals, not the other way around. There's tons of research, going back decades, demonstrating that people conform to group values and norms. But Will found the opposite. the worst team member is the best predictor of how any team performs.
Groups that have a "bad apple" perform worse. A good leader, who asks questions, solicits opinions and makes sure everyone is heard can make a difference. See also http://liberalorder.typepad.com/the_liberal_order/files/bad_apples_rob.pdf