Siberian fox experiment adds a line bred for aggression

The Siberian fox experiment began in 1959 at the Institute of Cytology and Genetics in Novosibirsk, led by Soviet geneticist Dmitri Belyaev. His team bred silver foxes on fur farms, choosing only the animals that reacted least fearfully or aggressively to humans. Over generations, the foxes became increasingly tame.

Around 1970, the researchers added a second line. Instead of selecting only for tameness, they also began breeding the foxes that showed the strongest aggression toward people. This produced two populations descended from the same farm stock but with sharply opposite behaviour, alongside a conventional farm-bred group.

The addition mattered because it allowed direct comparison. Scientists could study how selection for a single behavioural trait shaped differences in the animals' biology, including stress hormones, physical appearance and, later, genes and brain function. Belyaev believed that selecting for temperament could explain changes seen during animal domestication. The paired tame and aggressive lines have since been a key resource for researchers examining the biological basis of social behaviour, and they form the background to later brain studies of these animals.