Just added a bit toSETI - the Search for ExtraTerrestrial Intelligence, in Wikipedia.
It goes something like this:
The fifth assumption behind SETI is that intelligent life does not always destroy itself. The duration of human beings in relation to earth time has been likened to the thickness of a piece of cigarette paper placed on the topmost railing of the Eiffel Tower. Earth-time is the Eiffel tower, human history is the paper. SETI assumes that two pieces of paper exist at exactly the same height, but the planetary towers on which we stand are in fact of a hugely different. If a human civilisation capable of sending electromagnetic signals continues for hundreds of thousands of years, the paper becomes a little thicker and the likelihood that we will exist simultaneously with another transmitting/receiving civilisation is increased. If however, our civilisation destroys itself through nuclear war or as a result of releases of greenhouse gases and other erosions of our life support system, and if other civilisations have the same proclivities, then the probability of two competent civilisations coinciding in time and making contact with each other becomes vanishingly small.