Fairly psychedelic musings on The Great Filter by Peter Brannen in The Atlantic:

It was hard times for the bomber pilots that floated over Europe, their planes incinerating cities below, like birds of prey. Even as they turned the once-bustling streets beneath to howling firestorms, death had become a close companion to the crews of the Allied bombers as well. In fact, surviving a tour with the Bomber Command had become a virtual coin flip. While their munitions fell mutely from bomb bays, an upward sleet of fire from smoldering city grids and darkened farmland shot the planes out of the sky like clay pigeons. For recruits encountering the freshly empty bunk beds of dead airmen, morale was sapped before they could even get in the cockpit. Hoping to slow this attrition, Allied officers studied the pattern of bullet holes in returning aircraft for vulnerable parts to reinforce with armor.

It was natural to think that the bombers needed more armor where (it appeared) they were taking the most bullets. But the Hungarian-born mathematician Abraham Wald, and his colleagues at the Statistical Research Group at Columbia University, had a novel, if counterintuitive, prescription. Don’t protect the planes where they were taking the most damage, Wald said. Armor the planes where there were no bullet holes at all.

“You put armor where there are no holes, because the planes that got shot there didn’t return to the home base,” says Anders Sandberg, a senior research fellow at University of Oxford’s Future of Humanity Institute. “They crashed.”

The holes didn’t show where returning planes were likely to get hit, but only what it was possible for later observers to see. This is known as an observer selection effect, and the same sort of bias might apply not only to perforated planes, but to whole worlds as well.

What if, when we looked at our own planet’s past, we saw a similar pattern? After all, there are 100-mile impact craters on our planet’s surface from the past billion years, but no 600-mile craters. But of course, there couldn’t be scars this big. On worlds where such craters exist, there is no one around afterward to ponder them. In a strange way, truly gigantic craters don’t appear on the planet’s surface because we’re here to look for them. Just as the wounds of the returning planes could reflect only the merely survivable, so too for our entire planet’s history. It could be that we’ve been shielded from these existential threats by our very existence.

“Observer selection effects are really the kind of effects where the data you get is going to be dependent, in some sense, on survival, or that you as an observer exist,” Sandberg says. “Now this gets really interesting and scary when we apply it to our own survival.”