Wisdom of Crowds

As long as people are independently making decisions, the average results of a group of people are likely to be better than the results of any one person. Each person brings their own little bit of knowledge that helps get the answer closer to what it should be. However, if people share knowledge, before deciding, they are likely to negatively impact their decision making. The knowledge can taint an individual decision. This can also lead to people engaging in groupthink. Sometimes the best behavior is to follow the crowd that has wisely decided. (If everyone is carrying umbrellas, it may be wise to carry one also.) However, if this gets carried too far out, their may not be anybody making the decision, and we thus get a random herd.
Decision making can be improved by having all viewpoints aired before engaging in deliberation. The more diverse the group and the more willing they are to share differing opinions, the better the outcome is likely to be.