Have you ever noticed how few successful startups were founded by just one person? Even companies you think of as having one founder, like Oracle, usually turn out to have more. It seems unlikely this is a coincidence.

What’s wrong with having one founder? To start with, it’s a vote of no confidence. It probably means the founder couldn’t talk any of his friends into starting the company with him. That’s pretty alarming, because his friends are the ones who know him best.

But even if the founder’s friends were all wrong and the company is a good bet, he’s still at a disadvantage. Starting a startup is too hard for one person. Even if you could do all the work yourself, you need colleagues to brainstorm with, to talk you out of stupid decisions, and to cheer you up when things go wrong.

The last one might be the most important. The low points in a startup are so low that few could bear them alone. When you have multiple founders, esprit de corps binds them together in a way that seems to violate conservation laws. Each thinks “I can’t let my friends down.” This is one of the most powerful forces in human nature, and it’s missing when there’s just one founder.