So it turns out that social pressure has a similar effect to money - it too is a double-edged sword. It motivates people, especially in tasks that demand effort and no skill. But it can provide stress, too, and at some point that stress overwhelms the skill.Social pressure is good for things like tug of war, or hiking (you can hike longer, faster with a friend, than you can alone), but anything that requires detailed skill, well... too much pressure can hurt the skill.
Hat tip to Barking up the Wrong Tree, one of the most fascinating blogs I've found in a while. Head there for the full article, why people with large extrinsic rewards aren't always motivated to do better.