[0]: https://en.wikipedia.org/wiki/Dmitry_Belyayev_(zoologist)#Be...
[0]: https://en.wikipedia.org/wiki/Dmitry_Belyayev_(zoologist)#Be...
"At seven or eight months, when the foxes reach sexual maturity, they are scored for tameness and assigned to one of three classes. The least domesticated foxes, those that flee from experimenters or bite when stroked or handled, are assigned to Class III. (Even Class III foxes are tamer than the calmest farm-bred foxes. Among other things, they allow themselves to be hand fed.) Foxes in Class II let themselves be petted and handled but show no emotionally friendly response to experimenters. Foxes in Class I are friendly toward experimenters, wagging their tails and whining. In the sixth generation bred for tameness we had to add an even higher-scoring category. Members of Class IE, the “domesticated elite,” are eager to establish human contact, whimpering to attract attention and sniffing and licking experimenters like dogs."
Pretty hard and fast criteria. Hard to let a slightly bushier tail affect your evaluation of whether a fox just snapped at you.
The tests for tameness took the following form, which was still in use as of 2009. "When a pup is one month old, an experimenter offers it food from his hand while trying to stroke and handle the pup. The pups are tested twice, once in a cage and once while moving freely with other pups in an enclosure, where they can choose to make contact either with the human experimenter or with another pup. The test is repeated monthly until the pups are six or seven months old." At the age of seven or eight months, the pups are given a tameness score and placed in one of three groups. The least domesticated are in Class III; those that allow humans to pet and handle them, but that do not respond to contact with friendliness, are in Class II; the ones that are friendly with humans are in Class I.[5] After only six generations, Belyayev and his team had to add a higher category, Class IE, the "domesticated elite", which "are eager to establish human contact, whimpering to attract attention and sniffing and licking experimenters like dogs. They start displaying this kind of behavior before they are one month old."