> Capuchin monkey fairness experiment.
Right. Humans are social primates so we expect humans to have prosocial instincts, and we expect those instincts to be derived from earlier examples of social primates.
(Googles... )
Hey, Wikipeida has a whole article:
https://en.wikipedia.org/wiki/Inequity_aversion_in_animals
> In controlled experiments it has been observed, in varying degrees, in capuchin monkeys, chimpanzees, macaques, marmosets, dogs, wolves, rats, crows and ravens. No evidence of the effect was found in tests with orangutans, owl monkeys, squirrel monkeys, tamarins, kea, and cleaner fish. Due to inconclusive evidence it is assumed that some bonobos, baboons, gibbons, and gorillas may be inequity averse. Disadvantageous inequity aversion is most common, that is, the animal protests when it gets a lesser reward than another animal. But advantageous inequity aversion has been observed as well, in chimpanzees, baboons and capuchins: the animal protests when it gets a better reward. Scientists believe that sensitivity to inequity co-evolved with the ability to cooperate, as it helps to sustain benefitting from cooperation.
As I said, that's pretty much it for the strongest interpretations of tabula rasa: We have innate morality, our ancestors had it back to the dim and distant pre-human prehistory, and the fact it's possible to train kids out of it doesn't prove it wasn't there to begin with. The fact some people don't have it (cf antisocial personality disorder) is proof of a disease process, an acquired or congenital illness, not that those people are "purer" for having no built-in moral sense, or that without society indoctrinating a specific morality we'd all be utterly callous.