David Gurteen (tweet) unearthed this report on research: Why groups fail to share information effectively:
Again and again the results have shown that people are unlikely to identify the best candidate, make the best investment or spot who reallycommitted the crime. When asked to make a group decision, instead of sharing vital information known only to themselves, people tend to repeat information that everyone already knows.
In short, prejudgements and anxiety have a lot to do with it. So think about processes that give more space to say what they know before hearing from others, create enough time and space to go beyond the obvious.