Self-domestication
Self-domestication refers to the idea that a species has made itself tamer over very long periods of time, without anyone actively breeding it. Usually this refers to humans: those who were more peaceful and cooperative had more offspring.
When humans select and breed animals over many generations, those animals change. This is how wolves became dogs: smaller teeth, more colorful coat patterns, less aggressiveness. This process is called domestication, meaning taming through targeted selection. Self-domestication refers to the same outcome without an outside breeder. The selection happens within the group itself: those who are aggressive are cast out or avoided and have fewer offspring. Those who are agreeable find partners and allies. Over tens of thousands of years, this shifts the average of the entire species.
Why researchers consider humans to be tamed
The term comes up mainly in the question of why humans are able to cooperate so well. No other large mammal can withstand living in cities by the thousands without constant fighting breaking out. Chimpanzees on a full airplane would be a bloodbath. Humans sit there peacefully side by side for hours.
Also striking is how the human body has changed over the past roughly 300,000 years. Faces became flatter, brow ridges smaller, teeth finer. These exact traits also appear in domesticated animals compared to their wild relatives. Experts speak of the domestication syndrome, a bundle of traits that keeps appearing together.
An important distinction must be made: tame does not mean peaceful. The self-domestication hypothesis explains why spontaneous outbursts of rage are rare. It does not explain why humans wage wars. The anthropologist Richard Wrangham therefore distinguishes between hot-blooded, reactive aggression, which has strongly declined in humans, and planned violence, which has if anything increased.
The mechanism behind taming
The decisive point is who decides over reproduction. In the case of dogs, it was humans. In self-domestication, it is the group. In small hunter-gatherer communities, several members could band together against a tyrant. Language made this possible: people could coordinate without the victim noticing. Anyone who constantly threatened others lost their standing or their life.
On the level of biology, there is an explanation for this. As an embryo grows, a particular group of cells, the neural crest, migrates through the body. Among other things, it gives rise to parts of the face, the pigment cells of the skin, and the adrenal glands, which produce stress hormones. If slightly fewer of these cells arrive, the animal becomes both less easily startled and different in appearance at the same time. Aggression and appearance are thus linked by the same thread.
A frequently cited example is Russian silver foxes. Starting in 1959, only the tamest animals were bred there over decades. After just a few generations, the foxes had floppy ears, spotted coats, and curled their tails. No one had selected for appearance, only for behavior. This exact connection is what makes the hypothesis plausible for humans.
From bonobos to the debate over AI systems
Outside of human history, the term comes up mainly with regard to bonobos. These great apes are closely related to chimpanzees but noticeably more peaceful, and they likewise show smaller teeth and childlike facial features. Something similar is also discussed in some bird species. In textbooks and documentaries, the topic usually comes up under the question of whether humans are naturally good or evil.
In technology and AI texts, the term has appeared for a few years now as a comparison. When language models are subsequently trained to respond politely and harmlessly, some authors call this a domestication of the systems. The comparison is flawed, however. With AI models, a company deliberately chooses which behavior gets rewarded. That is breeding from the outside, not self-selection within a group.
The term should therefore be read with caution. Self-domestication is a hypothesis, not a proven fact, and it is contested within the scientific community. Critics point out that flatter faces can also be explained by changes in diet. Anyone who comes across the term in an article should check whether actual evolution is meant, or just a nice-sounding image.