I might be able to find it again, but the essence of it was a statistical argument that indicated that the chances were very high that any given solar system would be sterilized by a cosmic cataclysm (nearby supernova, gamma ray burst etc) once every few hundred million years- the conclusion was that we were only here because of exceedingly good luck- we had won the lottery... the implication being that this statistical argument perhaps explained the Fermi Paradox...

I cant vouch for their assumptions, or the probabilities they assigned to various dis-asters - but it seems to me that solar systems in the more densely populated clusters would require a great deal more luck to avoid such cataclysms...