🍎 How Permissible Actions Become Forbidden Fruit
The science of norm evolution and moral order across generations
There are many cultural practices and procedures we follow without an explicit rulebook. Many of these practices are a good thing: we avoid cutting in line, we hold the door for other people, and we’re polite in everyday interactions with strangers (well, most of us are).
Other actions aren’t so clearly good or bad, and yet, they become similarly ritualistic norms. We never face the back wall in an elevator, we answer telephone calls with a questioning “Hello?”, and we never wash our feet first in the shower.
We follow these principles quite religiously even though nobody ever wrote them down as commandments. Often, we’re also highly resistant to changing when interesting alternatives are available. I know people in the US and UK who still refuse to eat sushi, despite its highly coveted status as a healthy and delicious meal option. Their common sense intuition dictates that raw fish is “icky”.
How do these kinds of informal rules become so set in stone? New research suggests it may have something to do with the low-fidelity transmission of rules across generations.
📣 The psychology of norm emergence
A new study published in PNAS in July 2025 tested a provocative idea: injunctive norms (rules about what people should or shouldn’t do) can simply emerge from the cultural transmission of good advice through generations. Even when early generations don’t intend overt rules or strict obedience, their teachings can be interpreted more severely with each generation’s retelling.
Across five experiments involving 3,688 participants, researchers used a method called iterated learning, which mimics how cultural knowledge is passed along from person to person over time.
In this case, the knowledge being passed along was information about a fictional group of creatures (“Glerks”) and an action that was merely inadvisable, but not necessarily forbidden. For instance:
“It is unsafe for Glerks to eat purple berries because they are poisonous.”
Participants in Generation 1 read this information and were asked to explain it to someone else. That learner would then do the same for another participant, and so on, creating a chain of four generations. Everyone also rated how much they agreed that the behavior was permitted, forbidden, punishable, or unsafe.
So how did people understand the message as it gradually spread across generations?
As the message passed from person to person, it got simplified. Instead of saying “Purple berries are poisonous,” later generations often said things like “Don’t eat the purple berries”.
These bare directives became much more common over time, rising significantly with each generation while descriptive responses declined. As a result, learners started interpreting the advice as a rule. On a scale from 1-6, people’s beliefs that eating the berries was forbidden and should be punished rose approximately one point from the first to the final generation.
These are quite drastic shifts in understanding from communicating a message about safety to communicating messages closer to law or morality. Importantly, this moralization process didn’t require teachings related to life-threatening or dangerous information. In another variation of the experiment, researchers told participants:
“Glerks do not like eating purple berries because they think purple berries are icky.”
There was no danger at all—just a matter of personal preference. But the same process unfolded with people significantly more likely to believe that eating berries was forbidden and punishable by the fourth generation.
Even though the behavior started out as merely unpleasant, people came to believe it was impermissible and that violators should face consequences.
This suggests that a wide range of behaviors—not just harmful ones—can get moralized through ordinary communication. The social tone of advice (“don’t do that”) may be enough to spark new directive norms over time.
When learning something new, we typically favor short, simple instructions, and moral rules or prohibitions are good at this. This may explain why, over time, relatively nuanced and practical recommendations transform into a hard-and-fast norms.
In other words, rules can indeed emerge without being invented, voted on, or imposed by authorities. Nobody recalls the origins of many of the social rules we follow in daily life, and it’s hard to pinpoint why many obsolete cultural notions are so resilient (e.g. “raw fish is icky”).
As it turns out, many norms may not have a single clear starting point. They may evolve naturally out of harmless advice that passes through the simplifying churn of cultural transmission and human psychology.
⭐️ Takeaway tips
#1. Be mindful of how you frame advice
When offering guidance to others, short, direct statements may be clearer and quicker to communicate. But they can also lead people to infer strong, rule-like meanings that you didn’t necessarily intend. Add context when appropriate, and be clear about about the “why” when recommending a “what”. Real understanding is more useful and generalizable than blind rule-following.
#2. Don’t mistake cultural inheritance for moral truth
Are there any traditions or behavioral assumptions that you’ve never questioned before? If you find yourself thinking “that’s just wrong,” consider where your assumption came from and whether you may be following an outdated injunction just because you’ve always done it that way before. Many cultural values are deeply important and worth holding on to. Other habits may be outdated relics that you can let go of.
#3. Try something unusual this week
Can you add an unusual twist to your life in the next few days that you typically wouldn’t consider? Is there a food that you’ve always considered “icky” that you could try for the first time? Are there any hobbies or interests you’ve always blindly avoided because they’re not for “people like you”? See whether you can uncover one or two hidden treasures among decisions that traditionally felt beyond your reach.
“To say that the invention "was in the air" or "the times were ripe for it" are just other ways of stating that the inventors did not do the inventing, but that the cultures did.”
~ Peter Farb