A possible "third variable" could be intelligence. It's been shown to be strongly correlated with success in general, and it seems likely people of similar intelligence would find it easier to relate to each other.
Tell that to the generation of autistics roaming America that were never properly diagnosed in their youth. High intelligence, often even high paying jobs, but all the other 'success' metrics will probably end up looking very wavy.
There are intelligent people who may not be necessarily be on the same scale socially and they do get burned by it.
> It's been shown to be strongly correlated with success in general
Has it? I was under the impression we could hardly define, much less measure that.