My guess: People in the team need to communicate. This can be essentially achieved by two ways:

1) All team members voice their opinions openly.

2) Some team members don't voice their opinions, but other members are good at reading emotions, so the latter recognize when the former believe they know something relevant.

If this model is true, we would see that equal contribution (no one is silent) or emotional perceptiveness (other people recognize when the silent person wants to say something) increase the team output.