There's several good reasons why single-person startups have a harder time or a lower chance of success. Still, each of them may turn out to not matter. What is relevant to your investors, I think depends a lot on the chemistry going on between the VC or angel and you, and on what you and they emphasize.

"Bus factor"

If I'm hit by a bus, my company is gone. That has already stopped a big customer from buying my product. It certainly is a valid concern, though I would like to see how big a danger for your investment car accidents are vs. screwing up the product or the marketing or vs. some competitor eating your lunch. I have heard of dozens of companies failing for all sorts of reasons, but I've never heard of one that failed because the founder actually died.

"Too hard", technically

I'm a one-man show, so far, and it is a lot of work which can be hard to do alone. But there's always something you can do. I outsource practically everything but programming and customer acquisition, and I can handle that. Also, it's sometimes surprising how many things you can get away with just not doing.

"Too hard", emotionally

I often feel that it would be a lot more fun if there was a second person on board who could share the fears, the joy, the problems and the successes. It's just very lonely sometimes. I'm lucky to have a wife who can share some of the experience without us actually working together, so it's OK.

"Couldn't convince anybody"

Although this is at the core of PG's reasoning for YCombinator, and I certainly respect him a lot, I think this one just isn't true. I know many people who think my product is a great idea, but I wouldn't get any of them to start a company with me, because they simply don't like the idea of being entrepreneurs, they explicitly prefer to be employed. On the other hand, the entrepreneurial people I know have a chock-full pipeline of their own ideas that they want to execute on.

I'm actively looking for partners and it's hard to find someone who is just in between those two scenarios, and is good enough, and you like each other, and you're both at an appropriate point in life (if you just took a job, it's different than when you just quit a job). Some of these things might be a lot easier if you're just out of college, so this argument may make more sense for YCombinator's main target audience.

Getting funded is hard to do even in the best of times. Anything that causes an investor’s eyebrow to go up, that causes them to pause to think, is an added hurdle that must be overcome. Can a single founder get funded? Sure, I have even invested in one. That said it is the exception not the rule. Making the buy decision easy for you customer - and that is exactly what an investor is to a start-up, a customer and the widget you are trying to sell is your business – is making it easy for yourself. Too many moving parts for any single person to be able to do it, and it is generally a huge red flag for me when the founder thinks they can do it all. I can’t begin to count the number of times I have said NO to a great engineer that believes he is also a great salesperson, great accountant, great marketing person, great executive, great manager. Even if there is someone out there somewhere who is great at each of those skills, they just won’t have enough hours in a day to get there from here. A team of founders cost less than a single founder with employees, (read more runway for the same dollars) a team of founders is more motivated than a team of employees, (read greater likelihood of success). Multiple founders is an EASIER bet to make as an investor. When trying to get funded – easy is a good thing.

Charles

My first startup was solely founded by myself. We ended up raising almost 1mil in angel money before the company was sold. The question of me being the only founder never came up. The focus was on the product, team and vision.

Rick

It doesn't matter how many "founders" there are. What matters to VCs (I am one) is that you have a team by the time you approach us and that there is enough equity allocated to your senior management team (both present and planned).

If you're looking for seed funding you can get away with just one senior person (you). For an A round I think you'll struggle without a good team behind you.

Having a second founder indicates a) that you can sell to at least one other person and b) that you understand that you can't do everything yourself.

Jesse,

I run a single-founder company, but I'm not seeking VC so I cannot speak directly to your needs. I don't think I could partner up with someone on a startup, but maybe I just haven't found the right partner yet. I am very independent and an introvert, which I think would make partnering even more difficult. If you are passionate and confident about your startup, don't feel pressured into finding a co-founder for the sole purpose of seeking VC.

I've seen firsthand many businesses fail because of clashes between partners. You have to trust your partner with your life and be confident in their abilities to lead the company just as you want it to be led. If it's your conception, it could be very hard sharing the company with someone else. After all, it's difficult enough sharing equity with investors!

You're a single-founder company and get funded. What are you going to do with the money? You're going to hire people? Uhm! That doesn't look like a startup.

I think you have to work with at least one co-founder in order to build the first prototype - It's hard to play alone. And potential early investors will dismiss you faster than well armed teams.

Maybe there are single-founder companies that got funded in the past and were successful but the odds are against you.

Look for a co-founder and good luck!

I agree on what Paul Graham says, 2 is the best combination although it's not always possible to find the second person.

It's not the case of other person is not believing in your idea, it's you who doesn't find a worthy person to be your partner. (technically, or business-wise or ambition-wise)

Personally my one-man-company get successfully funded and even accepted by 2 different investor groups, so I chose the one I like among them.

Mint.com was one founder.

It'd be very hard to do it alone.

Even if you can do everything your business needs - code, market, sell, administer - you're going to need to be able to do them all at once. That's a big ask!

If you can get funded really fast and get the team in, that'd work, but if you've got a long drag before you can scale up, then it might get a bit grim.

I think the reason single-founder-startups rarely get funded is that if the problem you're trying to tackle is "small" enough for you to be able to solve it reasonably well on your own, you're probably not VC-material.

VCs care about BIG problems that can make them MILLIONS of dollars. If your idea is BIG like that you'll likely not be able to even get to alpha on your own, you'll need people right off the bat.

If instead your idea is nice and focused and your end goal is to make a comfortable living with it, I'd say bootstrap it up, it's fun! :)

FWIW, I'm a single-founder and have been approached by a few VCs lately. I'm not interested right now so I don't know if they would have really made an offer in the end, but I guess it's not impossible to get funded even if you started by yourself.

Paul Graham said he looks for companies with at least two founders, because it proves that the idea was good enough to convince at least one person. That reasoning make sense to me: if you haven't been able to convince one person that your idea is going to work, your chances are significantly dimmer.

No, on account of the bus factor being 1. That is the minimum number of people dying that would derail the company.

Single founder companies do get funded. All the time. I've angel-invested in a few myself.

Having said that, I'd still advise finding one or two co-founders. Even if you are exceptional at various things (sales, product, marketing, finance, etc.), startups can become a lonely business and it's nice to have someone you like, trust and respect to work alongside with.

It also sends a positive signal to others that you were able to convince at least one other person to join your cause.

In fact, I've heard investors express that they look for startups with FEWER founders. Their thinking is that they want the founders to have enough equity to be highly motivated (after dilution from investors, senior staff, employees, etc). With too many founders, it's harder for each to maintain a "motivating" stake as several rounds of funding creates more and more dilution for them.

It's true that often they look for multiple founders, because you'll need multiple people and it's nice to see the team is already there, rather than having to fill a key function (like CTO).

But don't let that stop you. Just make it clear that you know the holes in your knowledge and that you expect them to plug them with people they like.

VCs and angels always have folks in the wings who have done this before.