I've been musing about why, even when the consequences are harmless, some will and some won't say, "I don't know" about an issue of their obvious ignorance. If you're anything like most of us, it's a mixed bag.
Being a pragmatic soul, I'll ask why you should confess to ignorance. What's in it for you? Of course, it's an issue of simple honesty. But I'm unconvinced that simple honesty all the time is valued that much. It would be great if we could operate that way a bit more, but it doesn't always make for a job promotion.
The fact of the matter is that we all lie or cheat a little. Don't think so? In a charming study, the wry Dan Ariely of MIT, explored the subject of cheating, using--you guessed it--poker chips. To no surprise, he found that, yes, most of us will cheat a little, given the opportunity. But he also found that our consciences impose limits to our cheating.
It's the third conclusion that speaks to what's in it for you: when it's not about money, we'll cheat more. That means, for example, when we think our reputation is at stake, we're liable to cheat a bit and not say, "I don't know."
Take care when you fail to say, "I don't know" to a question someone asked. It may be to your advantage to fess up. Very solid research also reveals that we admire people whom we perceive to be likable and competent. Nice guys without competence eventually get ignored. And competent people who aren't likable get avoided.
But a nice guy who's also competent? Great stuff! Competence means that 90% of the time you know the answer to the question but the rest of the time, you may be guessing, fudging or cheating. Instead, use that extra 10% to to exemplify your honesty and confidence by admitting your not knowing. It's amazing the reputation for credibility you can earn with a simple answer to a question of your ignorance: "I don't know."