"Fermi's Paradox" and "The Great Filter"
Within the annals of the philosophy of exobiology, there exists an axiom intended to try figuring out why we still seem to be alone – why we don't see any obvious signs of intelligent life all over the place. The question itself is commonly referred to as “Fermi's Paradox”, but the particular axiom to which I refer is called, “The Great Filter”.
Naturally, "Fermi's Paradox" itself is controversial in some circles, as no small number of people around the planet believe we have been visited by extraterrestrials, their presence both then and now has been visible and some of us are just too stubborn to admit it. But that's another discussion entirely, because whatever the truth of that situation is, it's pretty undeniable that "intelligent life" doesn't appear to show up all over the place. If there is intelligent extraterrestrial life visiting Earth, it doesn't appear to be common enough to be undeniably obvious in the mainstream media et al - for that kind of concealment, they would have to be working together, something which would seem unlikely if there were a plethora of species circulating all over the place as one might expect if the cosmos were literally teeming with intelligent, space-faring life. So "The Great Filter" still seems to be a necessary evil even among UFO believers and ET buffs.
You can read about "The Great Filter" here, and "Fermi's Paradox" here (Wikipedia articles), but in brief it's an hypothetical “phenomenon” (whatever its true nature may be) believed to cause most civilizations to become extinct before reaching the level of technology needed to venture effectively to the stars. Perhaps it's a war, plague, some civilization-destroying super-race doing pre-emptive exterminations – any number of possible candidates exist for what “The Great Filter” might possibly be.
Of course it isn't unreasonable to conclude that if there is a “Great Filter” effect at play in the cosmos, it's probably multi-threaded – i.e., many different things can easily contribute to the extinction of an emerging civilization (as humanity has come to realize). The most commonly-held theory is, of course, the dreaded “Zombie Apocalypse” which seems to be increasingly inevitible as Hollywood et al continue having their wicked, wicked ways with the scientific prognostication of humanity's fate.
Just kidding – checking to see if you're still paying attention. ;-)
Anywho... I tend to refer to this phenomenon as the “Filter Factor” - i.e., the total sum of all existent factors in a given civilization with any meaningfully significant probability of creating an extinction-level chain of events. Biological warfare, for instance, or industrial pollution. On a scale of “0” being “No risk factors” to “10” being “Imminent extinction”, I would rate humanity at about a 5 or a 6 right now. That might sound like “not too shabby” odds, but trust me – all it means is that humanity may survive from this point forward, but she's destined by now to be at least a little banged up by the time they regain their proper footing. So it's not a good thing by any stretch.
Humanity is actually about to enter into the worst odds of their entire existence. If humanity survives the next 100-200 years or so, it should be fine after that – but it's gonna' have the hurtin' put to it no matter what course things may take.
The increased danger is perhaps the most common cause of extinction for societies which have survived their industrial pollution phase (it still exists, but is no longer potent enough to be the most serious threat to your existence) – that of “common knowledge” slowly coming to include increasingly powerful and dangerous things, and many of these powerful and dangerous things becoming increasingly accessible to the common man. Ergo, you have common, ordinary people slowly being granted the ability to do increasingly potent, dangerous and non-ordinary things. At the height of this phase of The Great Filter, conditions essentially come to exist where just about any individual can obtain a weapon as potent as the Tzarina bomb – and that's just on the low-end of the power spectrum.
Given that the “common, ordinary population” of humanity includes no small number of deluded and insane individuals, this is definitely going to be humanity's biggest challenge. It's one reason humanity should be putting no small amount of “hurry up already!” into their efforts to establish colonies offworld. That is the one behavior in an emerging technorati that most often enables it to survive near-anihilation by such insanely easy access to such power. Virtually no species who experiences a serious attack from one of their own in possession of such weaponry and who has no meaningful presence offworld ever manages to survive. By staying so stubbornly transfixed to the surface of Earth, humanity is setting itself up for failure.
And yet, any species to succeed to an advanced stage had once faced the same problems as these, so it's no unusual thing. All are put to the fire, none make it through unscathed.
Good luck, and peace. :-)