[ncmncm]

Civilizations that come into contact will tend to be those that are closer together, particularly if they make any effort to be inconspicuous.

The longer they go without contact, the wider their reach, and the greater their odds of contact.

We might suppose that the technology necessary to obliterate another civilization is arrived at early, and is hard to counter despite long subsequent development. Alternatively, civilizations that succeed in suppressing weaker ones eventually meet their match.