Just-so story
Based on Wikipedia: Just-so story
Imagine you're at a dinner party, and someone confidently explains that humans prefer savanna-like landscapes because our ancestors evolved on the African plains. It sounds plausible. It feels true. But can anyone actually prove it?
Welcome to the world of the just-so story.
The Leopard's Spots and the Scientist's Dilemma
The term comes from Rudyard Kipling's 1902 collection "Just So Stories," a charming set of tales explaining how animals got their distinctive features. In "How the Leopard Got His Spots," an Ethiopian paints spots on a leopard so it can hide better in the dappled forest shadows. The story is delightful, deliberately fanciful, and—crucially—completely made up.
When scientists call something a "just-so story," they're not being complimentary. They're saying: this explanation sounds nice, but you've basically invented it to fit the facts you already know. It's an untestable narrative dressed up as science.
The accusation stings because it strikes at something fundamental about how we understand the world. We humans are pattern-seekers and story-tellers. We crave explanations. But not every compelling narrative is actually true.
Stephen Jay Gould's Devastating Critique
The paleontologist Stephen Jay Gould popularized this critique in 1978, and he aimed it squarely at evolutionary psychology—the field that attempts to explain human behavior through our evolutionary past.
Gould's skepticism ran deep. He questioned whether evolutionary psychology could ever provide objective explanations for human behavior. Even if it could, he doubted such explanations could be proven in a properly scientific way.
His criticism hit hard because it exposed an uncomfortable truth. It's remarkably easy to invent evolutionary explanations after the fact. Why do humans enjoy music? Perhaps our ancestors used rhythmic sounds to coordinate group activities! Why do we fear snakes? Obviously, those who avoided venomous creatures survived to reproduce! These explanations feel satisfying, but feeling satisfying isn't the same as being correct.
The problem is what philosophers call underdetermination: multiple different explanations can account for the same observed facts. If you already know that humans fear snakes, you can dream up a dozen evolutionary reasons why. But which one, if any, is actually true?
The Defense: All Hypotheses Start Somewhere
Evolutionary psychologists have fought back vigorously against the just-so label. Their arguments deserve a fair hearing.
David Barash makes a straightforward point: calling something a just-so story is really just a derogatory way of saying it's a hypothesis. And hypotheses are normal in science. They're how we start the process of investigation, not where we end it. Every scientific theory began as an educated guess that someone then tested.
John Alcock called "just-so story" one of the most successful derogatory labels ever invented. It's rhetorically powerful precisely because it's so dismissive. But dismissiveness isn't an argument.
The researchers John Tooby and Leda Cosmides pushed back more substantively. They argued that evolutionary psychology isn't just explaining facts already known—it makes predictions about things we haven't discovered yet. If a theory only explains what we've already observed, it's not very useful. But if it predicts what we'll find when we look in new places, that's genuine science.
The Crucial Distinction: Predictions Versus Post-Hoc Explanations
Here's where the debate gets interesting and genuinely important for understanding how science works.
Cosmides offered a compelling example. Take morning sickness during pregnancy. One hypothesis says it's just a byproduct of pregnancy hormones—essentially a side effect with no particular purpose. Another hypothesis says it's an adaptation: the body's way of protecting the developing fetus from potentially harmful substances in food during the vulnerable first trimester.
These two explanations make different predictions. If morning sickness is just a hormonal byproduct, you'd expect random food aversions. If it's a protective adaptation, you'd expect women to be specifically averse to foods most likely to contain pathogens or plant toxins. You can test this. You can gather data. You can distinguish between the hypotheses.
This is the key insight: a just-so story is unfalsifiable. It explains what happened but makes no predictions about what else should be true. A genuine scientific hypothesis, even one about our evolutionary past, generates testable predictions about the present.
Top-Down Versus Bottom-Up
Researchers Laith Al-Shawaf and colleagues drew a useful distinction between two ways of doing evolutionary psychology.
The bottom-up approach starts with an observation and works backward to explain it. You notice something about human behavior and then construct an evolutionary story to account for it. This approach is vulnerable to the just-so criticism because you're fitting your explanation to facts you already know.
The top-down approach works differently. You start with evolutionary theory, derive predictions about what should be true if the theory is correct, and then go looking for evidence. This is harder to dismiss as just storytelling because your predictions come before your observations.
Think of it like this: if I claim to be psychic and I "predict" yesterday's winning lottery numbers, you'd rightly be skeptical. But if I predict tomorrow's numbers and get them right, that's considerably more impressive.
The Historical Science Problem
One challenge facing evolutionary psychology is that it's partly a historical science. We can't run experiments on our Pleistocene ancestors. We can't directly observe the selection pressures that shaped human psychology over hundreds of thousands of years.
But Al-Shawaf and colleagues point out that this isn't unique to evolutionary psychology. Astrophysicists can't visit the Big Bang. Geologists can't observe continental drift in real time. Cosmologists can't rerun the formation of the universe. Yet we don't dismiss these fields as mere storytelling.
What makes any historical science legitimate is its ability to make testable predictions about what we can observe today. The Big Bang theory predicts the cosmic microwave background radiation—and we find it. Plate tectonics predicts matching geological formations on separated continents—and we find them.
Similarly, evolutionary psychology doesn't require time travel. It requires theories that predict patterns we should see in present-day human psychology—patterns that can be tested with experiments and observations we can actually conduct.
A Concrete Example: Why Heights Frighten Us
Lisa DeBruine offers a striking example of evolutionary psychology generating genuine, testable predictions.
The evolved navigation theory hypothesized that humans would systematically misjudge vertical distances. Specifically, it predicted that people would overestimate vertical distances compared to horizontal ones. And more specifically still, it predicted that vertical distances seen from above would be overestimated more than those seen from below—because the danger of falling makes the view from above more consequential.
This wasn't obvious beforehand. It wasn't something researchers already knew and were just explaining. It was a prediction derived from theory and then tested empirically.
The prediction was confirmed. People do systematically misjudge heights in exactly this way. This is evolutionary psychology doing what science is supposed to do: making specific predictions and having them come true.
The Counter-Counter-Argument: Just Not So Stories
In a clever rhetorical move, some defenders of evolutionary psychology have accused their critics of telling "just not so stories."
The claim is that critics often accept any alternative explanation as long as it's not evolutionary. Someone might reject an evolutionary explanation for a behavior while uncritically embracing a purely cultural or sociological one. But if the standard for accepting an explanation is simply that it's not the evolutionary one, that's not rigorous science either.
Steve Stewart-Williams makes a related point: the ability to construct post-hoc explanations isn't unique to evolutionary psychology. Sociocultural explanations can be invented just as easily. If people in different cultures behave differently, we can always say "cultural differences explain it." If they behave similarly, we can say "universal human needs explain it." The same flexibility that makes evolutionary stories suspicious applies to their competitors.
Gould's Own Just-So Story?
David Buss turned Gould's critique back on itself. Gould sometimes suggested that traits evolutionary psychologists called adaptations were actually "spandrels" or "exaptations"—byproducts or features co-opted for new purposes rather than directly selected for.
But here's the catch: proposing that something is a byproduct or a co-opted feature carries its own explanatory burdens. You have to identify what the trait was originally for, what it was a byproduct of, and how and why it was co-opted for its current use. Simply asserting "it's probably a spandrel" without this supporting evidence is itself a just-so story.
Buss argued that Gould never actually met these evidentiary burdens. He made alternative claims without providing alternative evidence. By his own standards, Gould's counterproposals were as vulnerable to criticism as the adaptationist explanations he attacked.
The Deeper Issue: What Counts as Explanation?
Behind this debate lies a fundamental question about the nature of scientific explanation.
We want to understand why things are the way they are. That's the whole point of science. But "why" questions are tricky. They can be answered at different levels.
Why do we feel fear? A neuroscientist might point to the amygdala. A psychologist might describe learned associations. An evolutionary psychologist might invoke ancestral selection pressures. A philosopher might question whether any of these constitutes a true explanation.
None of these answers is wrong, exactly. They're operating at different levels of analysis. The danger comes when we mistake a compelling narrative for a tested causal mechanism.
The Real Test: Novel Predictions
Robert Kurzban perhaps said it best: "The goal should not be to expel stories from science, but rather to identify the stories that are also good explanations."
Stories are how humans think. We understand the world through narratives. The question isn't whether we tell stories—we can't help it—but whether our stories make predictions that can be tested and potentially proven wrong.
A just-so story in the pejorative sense is a story that's been crafted to fit existing facts while remaining immune to any possible test. A scientific hypothesis is a story that sticks its neck out, making predictions that could prove it wrong.
The difference isn't always obvious at first glance. Both might sound equally plausible when you hear them at that dinner party. But one can be checked against reality, and the other is just a leopard's spots painted on after the fact.
A Factual Homage
In 2014, Lewis Held Jr. wrote a book about evolutionary developmental biology called "How the Snake Lost Its Legs." The title deliberately echoes Kipling. But unlike Kipling's stories, Held's book describes actual science: the genetic and developmental mechanisms that explain why snakes don't have limbs.
This captures something important. We've moved from fanciful stories about Ethiopian painters spotting leopards to understanding the actual molecular biology of pigmentation. The urge to explain hasn't changed—humans still want to know how the snake lost its legs—but our standards of evidence have.
The just-so critique remains valuable precisely because it keeps us honest. Every time someone proposes an evolutionary explanation for human behavior, we should ask: Does this make predictions we can test? Could anything possibly prove it wrong? Or is this just a satisfying story that fits what we already know?
Because in science, unlike in Kipling's stories, explanations shouldn't be "just so." They should be testably, demonstrably, empirically so.