Drew applied to YC (and was accepted) as a solo founder. Not sure when the co-founder came aboard.

Well that's the problem with this kind of analysis. While there was officially just one founder, Drew had his friend with him, Jeff Bezos his wife. Not exactly sure if this analysis makes any sense. No one can build something like Amazon by himself, so at one point people are hired and of course they have an influence on decision making, so the solo part vanishes real quick.

Yes, they stop being pure-solo quick, if successful, when they're already a "started-up". Very early stage funding is at an earlier phase.

I'm curious about how successful a start-up can be, before adding anyone. [problem: founders who want to grow will do so ASAP; founders who don't, won't publicise their niche and we won't know about it]

Does Minecraft count as a "start-up"? With a billion+ dollar exit, it should be a "unicorn". I think Notch was solo for a very long time.

A friend or a partner is not the same as a co-founder. If investors were saying they didn't fund solo founders who lack a support system, that would be different. But instead they say they don't fund solo founders.

Of course you cannot build Amazon on your own. The difference is whether you have the type of friends who can work for free for a year while they co-found a business with you (and would be interested in doing that), or whether you are capable of filling the second-head gap with a good hire.

FWIW, pg said that one of the reasons Drew was accepted was because he said in the application that finding a cofounder was a top priority.

That's a bit of a cop-out, since anyone can say it's a top priority on their app. Except not really: Drew was from MIT, and therefore had a large pool of potential cofounders to select from. Most people don't.

It also feels true to say that YC was more squeezed for talent back when Drew applied. Nowadays YC can afford to reject startups for not having two cofounders from the start, especially in borderline cases.

And he did find it there