Generally, we assume that making decisions as a group is beneficial because groups can come together to make better choices than their members would alone. Is this true, or is group decision-making a case where a chain is only as strong as its weakest link? A new paper in Science this week has come to a conclusion: it depends.

Each trial of the experiment paired up two of 72 male participants. Over two timed intervals, the researchers used computer screens to show each of the two partners a visual field with six small circles. During one of the two intervals, one of the six circles appeared to be slightly darker than the other five. The participants each had to identify which circle was darker, and during which interval this difference occurred.

In the first section of the experiment, the partners saw identical images on their computers, made their choice, and then were given the chance to communicate before coming to a final, joint decision. By examining each participant's initial answers, the researchers could determine which of the two participants was individually better at the task. During this first set of trials, two heads were indeed better than one: the teams consistently chose the correct circle and interval more often than the more successful partner did on his own.

To determine whether the skill levels of the partners mattered, the researchers pulled a clever trick and secretly introduced "noise" into one of the partner's visual fields, making him perform more poorly at the task. Unlike the first set of trials, the pairs in these trials greatly underperformed when compared to the better participant (who saw no noise). This suggests that when one partner is performing poorly but can't recognize it, the pair's joint decisions will suffer.

In the final set of trials, the partners were not allowed to communicate at all, and only one person had the ability to make a decision based on both his choice and his partner's input. Here, the pairs again performed poorly because the final decision-maker had no information about how certain, or uncertain, his partner was. So, not only should partners be equally adept, they must also be able to communicate freely before making a decision.

The take-home lesson? Work with someone who's as good at their job as you are at yours, and make sure you're both willing to discuss your uncertainties.

Science, 2010. DOI: 10.1126/science.1185718 (About DOIs).