I think there will inevitably be a long "dating" period with a new co-founder who you're working with for the first time, and a fairly high attrition rate with

I think there will inevitably be a long "dating" period with a new co-founder who you're working with for the first time, and a fairly high attrition rate with these relationships, concentrated in the early days. In practice though, I do believe that if you work with someone for several months on a serious trial project you should be able to build a fair amount of confidence in the relationship's potential. I'd definitely still recommend talking through what a co-founder breakup would look like and signing documents that include a vesting cliff before actually starting something real together though. (That advice applies no matter how you met your co-founder.)