I sat in a board meeting recently as we debated a new strategic initiative. The downsides were enormous and everyone could see them. But, as we went round the table, it was clear that no one thought they could tell the CEO to stop. Until the last board member spoke.

"I think it's a bad idea and we should say, 'No'," he said bluntly.

At which point the tenor of the discussion changed. Reservations which had, previously, been hedged and muted suddenly became crisp and well-defined. The very idea of saying 'no' changed everyone's sense of what could and could not be said.

The sociologist Serge Moscovici wrote eloquenty about the disproportionate power of the single voice. Subsequent researchers have found that just knowing that there is a dissenting voice is enough to induce different cognitive processes that yield better judgments.

But I'd never seen this phenomenon so powerfully enacted before. Because one person had the courage to go against the majority, everything changed—and ultimately the initiative was abandoned.

Moscovici's lesson is critical to any group discussion. As much as we may believe we understand the power of groupthink, we also imagine ourselves immune to it. But sitting in the room that day, I saw how profoundly our knowledge of everyone else's perspective curtailed our own.

I too thought the initiative was a bad idea—but I was persuaded by those around me that we couldn't reject it. Knowing what others saw as possibilities blinded me to the full range of choices before us.

This is well-born out by neuroscience. If you ask volunteers in an fMRI scanner a simple question about a visual image, they will give the obviously wrong answer if they know that is what everyone else has done. But what is really important is that they will do so without the areas of the brain responsible for visual processing even lighting up. In other words, they quite literally won't see the obvious and correct answer. What we see is significantly determined by what we know those around us see.

Some people can resist this some of the time. Good decision making positively depends on it. Which is why minority voices are so essential in any group discussion. I'd always understood this in theory before. But seeing it in practice has sharpened my sense of how important it is to be skeptical of unanimity and courageous when confronted with it.