When all is said and done we know absolutely nothing, nada, zip, zilch, about how other species across the cosmos would really evolve and behave. That’s especially true when it comes to species that might have sentience and sophisticated agency. We of course don’t even know if there are any such species (or whatever kind of equivalent categorization is suitable) anywhere else in the entire universe.

The upside to this appalling ignorance is that we have a lot of free range to speculate and hypothesize about what might be playing out elsewhere in the cosmos. That’s lots of fun, but to make any genuinely meaningful—scientifically useful- proposals about life elsewhere we have to tread extremely carefully and examine our own innate biases and predispositions.

When it comes to talking about the possibility of species traversing interstellar space and spreading across the galaxy we tend to set the bar pretty low. I can say that I’m guilty of this as much as the next scientist. In a recent lovely study of hypothetical interstellar settlement patterns and statistics, by Carroll-Nellenback, Frank, Wright, and myself we did our best to avoid the most heinous assumptions and implicit biases. But we nonetheless had to make some kind of assumption that there might be species that decide to seek “good planets”, settle them, and then send on probes and fellow organisms to new places if possible. Without that the whole question of interstellar spread is naturally moot.

However, these essential assumptions weren’t made lightly, and in the paper reporting that study we discuss some of the pitfalls. That includes the so-called “monocultural fallacy” – the notion that an entire species will think with one mind, especially when we humans clearly do not. But there’s another potential fallacy that is sometimes implicit in these discussions; that’s the notion that a technological species will, if they survive long enough, need to expand and spread because they can’t contain their growth.

Back in the 1960s we had visions of an Earth overrun by humans. A book like “The Population Bomb” by Ehrlich and Ehrlich in 1968, peddled the older Malthusian notion of population growth outpacing things like basic food supply. Obviously, there are elements of this that have indeed manifested themselves. But those elements are more to do with inequality and failures to disperse resources fairly around the globe than they are to do with global shortfalls.

Indeed, the concern in any rational person’s mind today is more to do with the shifting planetary climate state, which reflects a somewhat different abuse of resources –driven only in part by the sheer number of humans on the Earth.

But when it comes to motivations for expanding, for a civilization to look for more space, more resources, it’s not clear that humans have ever done this because we’ve reached a bursting point. Genghis Khan—with the largest land empire ever—doesn’t seem to have been motivated by overcrowding or immediate resource scarcity. The same is true for the Romans, the Russians, or the British. Yes, everyone wanted more resources, more wealth and power, but not because they started out with none.

And that’s interesting when it comes to thinking about both interplanetary and interstellar exploration and settlement. Perhaps, as Elon Musk is wont to suggest, a bigger motivation for going offworld is to “backup” our species. To ensure that even if a natural cataclysm (asteroid, super volcano, you name it) takes out Earth’s biosphere, or if we bomb or climate-change ourselves into oblivion, there is still a chance of our species (and others) continuing.

There might be exoplanetary systems that are more prone to existential challenges. That could include something as straightforward as planetary orbital instability—natural chaos that can destroy the nice rocky world you’ve evolved on, and its neighbors, but which doesn’t manifest itself for billions of years. In a scenario like that you might actually want to get out of town altogether and go to a different star.

Yet, if we look to human history as a guide, there could equally be an irrational, or egomaniacal motivation. Empire-building and zealotry seem to be pretty powerful forces. Perhaps that’s what it takes to push across interstellar space? Of course, as I mentioned above, we have zero idea of what other species would be like, or think like. But if Darwinian selection is a universal phenomenon, it stands to reason that all species will have variations in their individuals or groupings. And outlying variations could translate to minds that are more ruthless, more hellbent on an idea or principle, and more capable of accomplishing extreme things.

In other words, the wholesale expansion of species may never be driven by simple growth, but rather by things like existential threat or extremism.

That is not a comforting thought. Perhaps we should hope that we are the outliers in a galaxy of comparatively benign and calm life, and that’s the reason why no one’s come calling yet.