The idea that there might be gigantic alien structures orbiting a distant star just bit the dust.

After citizen astronomers spotted data in 2015 revealing that KIC 8462852, a star about 1,000 light years away, was dimming and brightening in a strange way, one of many explanations proposed by astronomers involved some sort of "megastructures" orbiting the star – perhaps built by aliens to harvest stellar energy.

That imaginative suggestion rocketed the star to fame. But Louisiana State University astronomer Tabetha Boyajian and colleagues collected more data on the star, nicknamed "Tabby's Star" for Dr. Boyajian, and they found that the star’s strange flickering was thanks to something much more mundane: ordinary dust.

"There's dust everywhere in our universe. We see it in many different ways, and the data that we took showed a clear signature of this being what we would see from dust," Boyajian says.

This may be a disappointing outcome for those hoping for proof of an alien civilization. But Tabby’s Star’s rise to stardom highlights a deeply entrenched human psychological quirk: When presented with a puzzling phenomenon, our knee-jerk instinct is to ask not what created it, but who. Scientists say that as social animals, we are evolutionarily predisposed to see agency and intentionality in the world around us. And when it comes to astronomical mysteries, aliens seem to fit.

"It's the duct tape of science," says Seth Shostak, senior astronomer for the SETI Institute. Because we don’t know what aliens might do, they could explain anything.

But why do we do that?

"It's not just aliens," says Christopher French, a psychologist and founder of the Anomalistic Psychology Research Unit at Goldsmiths, University of London. "We do have a natural tendency to assume that anything odd, or, superficially at least, inexplicable, that there must be some sort of intentionality behind it, some sort of intelligence, there must be a purpose, somebody or something has done that for a particular purpose. Aliens is one possibility, but of course another possibility would be ghosts, spirits, a whole range of as-yet-unseen entities."

UFO sightings are often laughed off, but there might be a less laughable reason for such a seemingly crazy reaction to the mysterious. A bias toward intentionality likely confers an evolutionary advantage, Dr. French suggests.

If a prehistoric human saw a bush rustling, for example, his first thought might be that it's a saber-tooth cat, even if it's much more likely that it is just the wind. Assuming some sort of agency tends to be a safer bet.

"It's better to err on the side of assuming intentionality," says Christine Looser, a social psychologist affiliated with Harvard University, agreeing with French.

Anthropomorphizing our world

Psychologists have long noted our tendency to map human characteristics onto inanimate objects, even when we know they’re not alive. In a classic experiment conducted in 1944, Smith College psychologists Fritz Heider and Marianne Simmel showed subjects different shapes moving around on a screen and then asked the viewer to describe what they had seen. The subjects described interactions between the shapes that were motivated by human-like thoughts and feelings. Instead of saying things like, "the triangle moved to the left of the screen," subjects described one triangle as being a bully, and other shapes as being afraid and wanting to hide.

"It kind of makes sense for us to try to interpret all the uncertainties in the world around us in terms of things that we do understand so that we are able to, to the best extent that we can, predict and control what's going on," French says. "And what do we understand more than anything else? Ourselves."

This tendency might not have only saved our ancestors from becoming lunch, but it also might have helped promote social cohesion, Dr. Looser says. In a study conducted with colleagues at New York University, she showed subjects images of human faces digitally merged with doll faces to different degrees along a continuum. Subjects were told either that the face was a member of their social group or a member of an outgroup. Looser and her colleagues found that outgroup faces required a higher threshold of humanness to be identified as having a mind.

Because we are not solitary animals, Looser suggests, identifying with and understanding the thoughts and feelings of others in our society must confer a survival advantage, too.

n = 1, for now

On Earth, our perception of what does and does not have agency is a helpful tool that relies on understanding our own agency and intelligence. But when it comes to alien civilizations, their intelligence might look nothing like ours.

In what astrobiologists refer to as the "n = 1 problem," we have just one data point of life. Because all life on Earth is related, scientists have just one model from which to extrapolate general biological principles to search for – and that sample size might be limiting.

The same is probably true of intelligence, says Susan Schneider, a philosopher of cognitive science affiliated with the University of Connecticut and a member of the Ethics and Technology group at Yale University. "We can kind of try to extrapolate from known laws of physics, and look for large-scale architectural features out there that maybe an intelligent civilization created," Dr. Schneider says. "But I think it's still inevitable that we're extrapolating from our own case. We have to be really humble as we think about these issues."

Still, we might not be at a total loss when considering what to look for when watching for intelligent aliens. We may already be creating a second model of intelligence: artificial intelligence.

"It will give us another datapoint, and a really different one," Schneider says. "And once we have one example of a highly intelligent AI, then we can tweak it so that we can make all kinds of other cognitive systems, and we can start to explore the scope and limits of cognitive systems. So we could start to explore this space of possible intelligences and maybe draw some principles of what other intelligences need to be like to succeed."

In fact, Schneider has suggested that AI might be the cosmic norm. The extraterrestrials we meet (assuming that they're out there) might actually be "post-biological," artificial intelligence created by an extinct life form that was more like us.

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The idea is that, if you extrapolate from our own trajectory, a civilization "graduates from biological intelligence" to synthetic intelligence, Dr. Shostak at SETI says. And because any being capable of intergalactic spaceflight is more advanced than we are, and we’re already building artificial intelligence, that certainly seems possible.

“It may be that what aliens do may not be terribly visible to us,” Shostak says. “I hope that's not true, because my job involves looking for signals that say they're present, but you really don't know what they're going to do. All you can do is say, whatever it is, it can't violate the laws of physics.”