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.