In the 1960s, Dr. Frank Drake, one of the founders of SETI, invented an elaborate equation for use in thinking about the possibility of finding and communicating with extraterrestrial life. The equation is supposed to allow scientists to quantify the uncertainty of all the factors which can determine the number of civilizations out there. The Drake Equation looks like this:
N equals the number of number of civilizations in our galaxy with which we could expect to communicate. The other pieces include the rate of star formation in our galaxy, the fraction of those stars which carry planets, the fraction of those planets which can support life, and so on in smaller, more complex pieces until you get to what I think is the most interesting part of Drake's equation: L. It is referred to as the "lifetime factor."
If our own is any indication, civilizations create technology that allows them to potentially communicate with extraterrestrial beings at approximately the same time that they create the means with which to completely annihilate themselves. Therefore N, considering L, equals those civilizations that don't blow themselves up.