This is a perfectly reasonable question, for it is indeed the case that some anarchists expect a remarkable change in human nature to follow (or precede?) the establishment of an anarchist society. This assumption partially explains the frequent lack of explanation of how an anarchist society would handle crime, dissenting individualists, and so on.
The belief in innate human virtue is normally found only among left-anarchist thinkers, but of course it does not follow, nor is it true, that all left-anarchist thinkers believe in humanity’s innate human virtue.
Anarcho-capitalists have a very different picture of human nature. While they normally believe that people have a strong capacity for virtuous action (and it is to people’s moral sense that they frequently appeal when they favour the abolition of the state), they believe that it is wise and necessary to cement moral virtue with material incentives. Capitalism’s system of unequal wages, profits and losses, rent and interest, is not only morally justified but vitally necessary for the preservation and expansion of the economy. In short, anarcho-capitalists believe in and indeed must depend on some reasonable level of human morality, but prefer to rely on material incentives when feasible. (Similarly, they morally condemn crime and believe that most people have no desire to commit crimes, but strongly favour some sort of criminal justice system to deter the truly amoral.)