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Evolution, game theory, and (possibly) why we haven’t met any aliens yet: Ideas from the Three Body Problem books

11 min readApr 18, 2024

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The ideas discussed here are outlined in the Three Body Problem book trilogy by Cixin Liu, which I would highly recommend reading. There are also spoilers below.

We’re living in a sci-fi world: AI development accelerates towards AGI, self-driving cars roll out onto our streets, space travel is commercializing, and brain implants allow us to control things with our minds… So where are the aliens? This may be the last missing element needed to complete our sci-fi bingo.

There are around 2 trillion galaxies in the observable universe with an estimated 50 sextillion planets that may be hospitable to life. Life on Earth is only 3.7 billion years old — about a quarter of the age of the universe — and in that time it has advanced from single-celled organisms to large scale human societies. Shouldn’t there be lots of other life around too? This is the classic Fermi paradox: the apparent contradiction between the high probability of extraterrestrial life in the universe and the lack of evidence for, or contact with, such civilizations. What if the answer to this paradox lies not in the stars above, but in the fundamental principles that govern life itself?

Enter the dark forest theory. Born from the pages of Cixin Liu’s Three Body Problem sci-fi book series (though similar ideas have been floated by David Brin and others), the theory proposes an intriguing yet unsettling solution to the Fermi paradox. It draws upon the principles of evolution and game theory, defining simple axioms that, when fully extrapolated, lead to a framework for understanding the dynamics of life in the universe.

Evolution 101: Survival of the Fittest

What’s the underlying principle that informs our approach to vaccine development, dog breeding, antibiotic usage, pest control, development of new crop varieties, and our understanding of human genetic diversity? The answer, in a word, is evolution.

The theory of evolution by natural selection, first proposed by Charles Darwin in his groundbreaking work “On the Origin of Species,” has become the bedrock of modern biology. It provides a model for understanding the incredible diversity and complexity of life on Earth, from the intricate workings of a single cell to the grand sweep of the fossil record.

In biology, evolution refers to the change in the genetic makeup of a population of organisms over time, and natural selection is a mechanism by which this change occurs.

The principles of natural selection are elegantly simple:

  1. Populations contain variation: Generally, individuals are not identical; each possesses a unique combination of traits and characteristics.
  2. Traits are hereditary: Many of these traits are passed down from parents to offspring, encoded in the language of genes.
  3. Offspring compete: In each generation, more offspring are born than can possibly survive, given the finite resources available.
  4. Survival of the fittest: Only those organisms whose traits best equip them to survive and reproduce will pass on their genes to the next generation.

Over time, as this cycle repeats itself generation after generation, the genetic makeup of a population begins to change. Traits that confer an advantage become more common, while those that hinder survival gradually disappear. And as these changes accumulate, driven by the relentless force of natural selection, new species emerge, each adapted to its particular way of life. This simple process gives us a lens through which to examine and contextualize all of life on our planet.

But the implications of evolution extend far beyond the confines of Earth. If life exists elsewhere in the universe — and the vastness of the cosmos suggests it very well might — then it too will likely be subject to the inexorable logic of natural selection. The same principles that have guided the evolution of life on Earth for billions of years may also hold the key to understanding life in contexts beyond our planet.

What does this have to do with missing aliens? Ideas derived from the theory of evolution, applied on a cosmic scale, form one part of the dark forest theory, leading to a possible explanation for why we haven’t had contact with extraterrestrials. But to fully understand the dark forest, we first need to explore another fundamental theory that shapes strategic interactions: game theory.

The Prisoner’s Dilemma: A Crash Course in Game Theory

You and a partner are accused of a crime and held in separate cells. The prosecutor offers each of you a deal: if you rat out your partner (in this scenario, called defecting) while they stay silent, you walk free and they serve a hefty sentence. If you both tattle on each other, you each serve a moderate sentence. If you both stay silent (in this scenario, called cooperating), you each get a light sentence on a lesser charge. The catch? You can’t communicate with your partner to coordinate your strategy.

Welcome to the prisoner’s dilemma, a classic thought experiment in game theory that captures the essence of the conflict between self-interest and cooperation. Developed in the 1950s by mathematicians Merrill Flood and Melvin Dresher (and later formalized by Albert Tucker), this simple scenario has become a cornerstone of our understanding of strategic decision-making.

The prisoner’s dilemma is a game of trust and betrayal. The optimal outcome for both players together is to cooperate, but the fear of being betrayed by the other player creates a powerful incentive to defect. We can break the prisoner’s dilemma down into a few simple tenets:

  1. Self-interest: Each player aims to maximize their own payoff.
  2. Interdependence: The outcome for each player depends not only on their own choice but also on the choice of the other player.
  3. Risk of cooperation: For an individual player, cooperation yields the worst payoff if the other player defects, and still does not lead to the best possible individual payoff if the other player also cooperates.
  4. Collective suboptimality: If both players pursue their individual self-interest and defect, the outcome is worse for both than if they had cooperated.

The individually rational choice leads to a suboptimal outcome for all. Self-interest can lead to a breakdown of cooperation, even when cooperation would benefit everyone involved.

The prisoner’s dilemma has far-reaching applications in fields ranging from economics to international relations. It helps explain why countries engage in arms races, why fishermen overfish, and why it’s so difficult to tackle climate change. In each case, the pursuit of individual self-interest leads to a worse outcome for everyone involved.

Game theory is relevant for understanding interactions between intelligent, competing agents. It can therefore be useful to us when we consider how alien species may behave, or how humanity might respond to extraterrestrials. This is a major theme of the Three Body Problem series: the books reflect on how humanity might react after discovering the existence of an intelligent alien species, and what intergalactic relations between multiple intelligent species could look like.

The Dark Forest Theory: Synthesizing Simple Concepts to Model Cosmic Dynamics

In Cixin Liu’s books, humanity’s encounter with an intelligent alien civilization kickstarts the development of a new field: cosmic sociology, an attempt to understand how extraterrestrials might behave. The dark forest theory, proposed by the character Luo Ji, emerges as a central tenet of this discipline, offering a possible explanation for the Fermi paradox.

The two axioms of cosmic sociology, quoted from the book The Dark Forest, are as follows:

  1. Survival is the primary need of civilization
  2. Civilization continuously grows and expands, but the total matter in the universe remains constant.

The first axiom resembles the first tenet of the prisoner’s dilemma, where the primary focus of an intelligent being (or society) is their own self-interest. The second axiom hints at ideas taken from the theory of evolution: competition and survival of the fittest. It’s worth keeping in mind that the theory of evolution by natural selection as it describes life on earth applies to individuals rather than competing populations (there’s a much-debated fallacy called group selection that I won’t detail here). Nevertheless, we can understand the basic concept of finite resources leading to competition among societies of intelligent creatures.

The next component of Luo Ji’s theory is the chain of suspicion. This refers to the idea that, once one civilization becomes aware of the existence of another (alien) civilization, it cannot ascertain whether it has friendly or unfriendly intentions. Here’s where the prisoner’s dilemma comes in again, where one party’s uncertainty about the other’s intentions can result in overly defensive, or preemptively offensive, behavior. This suspicion is heightened by the challenge of communicating between alien species, which arises for several reasons: 1) the slow pace of communication across space, 2) language and cultural barriers that limit understanding between different societies, and 3) the lack of trust between different species that renders even clearly understood communication as suspect.

In the face-off between species, as in the prisoner’s dilemma, we can also see the phenomenon of interdependence. Each civilization’s fate depends not only on their own but also on the other’s decision of how to engage. A society must predict both their opponent’s intentions and their opponent’s interpretation of their intentions.

On top of the chain of suspicion, Luo Ji adds another factor into the mix which ups the stakes: technological explosion. While one alien civilization may be much more technologically advanced than another at a given moment in time, this may not remain the case because technological advancements can occur in rapid bursts. In the context of the prisoner’s dilemma, this increases the risk of cooperation; even if there is peace now, a technologically superior civilization cannot feel secure in their primacy. This can provide an incentive to act towards eliminating potential rivals preemptively.

Luo Ji reasons that, based on these simple assumptions, the most rational way for any species in the universe to react to the discovery of another is to immediately eliminate it. It’s too difficult to determine if they are friend or foe, and the consequences of miscalculation are too great. Like in the prisoner’s dilemma, the best way to reduce your own risk is to preemptively defect. Unfortunately, this leads to a suboptimal outcome for all involved: in the context of the universe, the dark forest. As stated in the book:

Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound…. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds other life…there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people. An eternal threat that any life that exposes its own existence will be swiftly wiped out.

The result of this cosmic prisoner’s dilemma, coupled with inter-civilizational survival of the fittest, is that each species in the universe takes care not to expose its own existence or location to others. Those who do reveal themselves are maladapted to existence in the universe and are swiftly eliminated by natural selection, in this case preemptive attacks by alien species. This further promotes the spread and growth of civilizations that have a tendency towards stealth and preventative aggression, changing the galactic inter-societal landscape and reinforcing the dark forest state.

This, posits Luo Ji, may be the long sought-after solution to the Fermi paradox: the reason that humanity hasn’t heard from intelligent aliens is that they’re intentionally hiding their own existence, limiting outward evidence of their activities and location in order to survive in the galactic dark forest.

Impact, Reflections, and Pitfalls of Anthropocentric Thinking

The implications of the dark forest theory are profound and unsettling. It suggests that the universe may be a far more hostile and unforgiving place than we’d like to believe, where the imperative of survival outweighs all other considerations. If the theory holds true, it means that our dreams of peaceful contact and cooperation with alien civilizations may be hopelessly naive and unrealistically romantic. Instead, we may be facing a future where the only way to ensure our own survival is to embrace the same ruthless logic of stealth and preventative elimination. But haven’t humans often over-romanticized what we don’t understand? Myths about the origin of humanity are often less harsh than the cold, semi-random, non-goal-directed reality of evolution. Yet I do not believe that this takes away from the wonder and beauty of nature and the emergence of the Earth’s existing biodiversity. Even if the dark forest theory holds true, this must not necessarily detract from the awe we feel when contemplating the cosmos.

It is also worth asking whether we as humans are blinded by our own behavioral tendencies, unable to imagine how different alien species might act. Perhaps the self-interest element of game theory and of the dark forest theory, rather than being a universal quality of intelligent beings, is not as common as we assume. We should consider that behavioral patterns can vary greatly between even closely-related species. Take chimps and bonobos: these two species of great apes are very similar to each other genetically, sharing over 99.5% of their DNA (they also each share about 99% with humans, and they’re our closest living relatives). Despite this, bonobos and chimps have very different social dynamics. Chimps live in male-dominated groups and are aggressive, with especially violent competition between groups; bonobos are female-dominated and much more peaceful (new research questions how peaceful, but there are nonetheless significant distinctions between species). Scientists believe that these differences may have evolved simply because bonobos lived in slightly more resource-rich habitats, with less competition from gorillas. If such similar species can evolve substantially different behaviors as a result of slight differences in environment, it may be extremely short-sighted of us to presume that extraterrestrial societies would behave as we do. And aliens are likely much more different from us than we can easily imagine (Arrival is an interesting piece of science fiction that explores this). It’s conceivable that, due to factors of their origin or evolution that we cannot even fathom, alien species might not readily react with violence or in aggressive self-interest.

It might be true that the existence of one powerful and aggressively selfish species in the universe could trigger a dark forest state by wiping out other more peaceful species, but it’s also possible that this situation might require very particular conditions to arise.

Regardless of whether it ends up being accurate, perhaps the most fascinating aspect of the dark forest theory is how it emerges from such simple premises. Like the theory of evolution, which revolutionized our understanding of life on Earth based on a few clear principles, the dark forest theory has the potential to reshape our entire conception of life in the universe. It extrapolates from basic tenets of evolution and game theory to provide a framework for understanding the dynamics of intelligent life on a cosmic scale.

This is the power of interdisciplinary thinking: by combining insights from seemingly disparate fields, we can arrive at new and startling conclusions. The dark forest theory stands as a testament to this power, weaving together ideas from biology, sociology, and mathematics to create a compelling and thought-provoking vision of the universe.

Of course, the dark forest theory remains, in truth, a simple hypothesis. Whether it accurately describes the reality of our universe is a question that may never be fully answered. But even as a thought experiment, it serves a valuable purpose. It challenges us to question our assumptions, to consider the potential consequences of life’s fundamental drive for survival, and to ponder our place in the vast and mysterious cosmos.

A message in a bottle morphs into a satellite, floating at the water’s surface with the vastness of space stretching out beneath
Scraps of evidence of human civilization — from electromagnetic radiation to discarded spacecraft — are already adrift in the cosmic sea, like messages in bottles waiting to be found by other species. Image adaped from one of my paintings

What do you think — is the dark forest theory a plausible explanation for the Fermi paradox?

(I’m one of many people to reflect on and write about this topic. You can find some other articles here and here)

This is the first piece of writing I’ve shared, but I’m planning to write more in the future (as time allows). I’m thinking of covering topics ranging from cephalopods, surrealism, neuroscience, creativity, biotech and more 🧠 You can also find me here: Instagram, LinkedIn, website

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Hannah Payette Peterson
Hannah Payette Peterson

Written by Hannah Payette Peterson

Biologist/neurotech VC, artist | Reflecting on science, art and weird ideas

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