Statistically, it seems that multiple founders correlates more highly with success than single founders.
Perhaps there's an underlying reason that the companies you've worked with have failed when there were multiple founders (e.g. personality clashes, inability to resolve disputes, etc.).
I keep hearing this stated, but I don't think I've ever seen the stats. Anyone got a cite?
However, assuming there is a correlation, this doesn't tell us much about causation. Perhaps a much larger percentage of single-founder startups aren't started seriously, and are therefore irrelevant to the numbers.
As a single-founder, what I want to know is: will adding a co-founder increase my probability of success? What if the co-founder is not a good friend?
The simple correlations mentioned above aren't very useful in answering these sorts of questions. If there is any good evidence either way I'd very much like to see it.
There is an underlying reason that those companies that failed failed- no two people are alike. When you take two people and give them equal say, you are sowing the seeds of conflict which will-- and this is a guarantee- end up resulting in someone leaving or the company failing.