The ancient Chinese philosopher Mengzi and the early modern French philosopher Rousseau both argued that human nature is good. The ancient Chinese philosopher Xunzi and the early modern English philosopher Hobbes argued that human nature is not good.
I interpret this as an empirical disagreement about human moral psychology. We can ask, who is closer to right?
1. Clarifying the Question.
First we need to clarify the question. What do Mengzi and Rousseau mean by the slogan that is normally translated into English as "human nature is good"? There are, I think, two main claims.
One is a claim about ordinary moral reactions: Normal people, if they haven't been too corrupted by a bad environment, will tend to be revolted by clear cases of morally bad behavior and pleased by clear cases of morally good behavior.
The other is a claim about moral development: If people reflect carefully on those reactions, their moral understanding will mature, and they will find themselves increasingly wanting to do what's morally right.
The contrasting view -- the Xunzi/Hobbes view -- is that morality is an artificial human construction. Unless the right moral system has specifically been inculcated in them, ordinary people will not normally find themselves revolved by evil and pleased by the good. At least to start, people need to be told what is right and wrong by others who are wiser than them. There is no innate moral compass to get you started in the right developmental direction.
2. Mixed Evidence?
One might think the truth is somewhere in the middle.
On the side of good: Anyone who suddenly sees a child crawling toward a well, about to fall in, would have an impulse to save the child, suggesting that everyone has some basic, non-selfish concern for the welfare of others, even without specific training (Mengzi 2A6). This concern appears to be present early in development. For example, even very young children show spontaneous compassion toward those who are hurt. Also, people of different origins and upbringings admire moral heroes who make sacrifices for the greater good, even when they aren't themselves directly benefited. Non-human primates show sympathy for each other and seem to understand the basics of reciprocity, exchange, and rule-following, suggesting that such norms aren't entirely a human invention. (On non-human primates, see especially Frans de Waal's 1996 book Good Natured.)
On the other hand: Toddlers (and adults!) can of course be selfish and greedy; they don't like to share or to wait their turn. In the southern U.S. about a century ago, crowds of ordinary White people frequently lynched Blacks for minor or invented offenses, proudly taking pictures and inviting their children along, without apparently seeing anything wrong in it. (See especially James Allen et al., Without Sanctuary.) The great "heroes" of the past include not only those who sacrificed for the greater good but also people famous mainly for conquest and genocide. We still barely seem to notice the horribleness of naming our boys "Alexander" and "Joshua".
3. Human Nature Is Nonetheless Good.
Some cases can be handled by emphasizing that only "normal" people who haven't been too corrupted by a bad environment will be attracted to good and revolted by evil. But a better general defense of the goodness of human nature involves adopting an idea that runs through both the Confucian and Buddhist traditions and, in the West, from Socrates through the Enlightenment to Habermas and Scanlon. It's this: If you stop and think, in an epistemically responsible way (perhaps especially in dialogue with others), you will tend to find yourself drawn toward what's morally good and repelled by what's evil.
Example A. Extreme ingroup bias. For example, one of the primary sources of evil that doesn't feel like evil -- and can in fact feel like doing something morally good -- is ingroup/outgroup thinking. Early 20th century Southern Whites saw Blacks as an outgroup, a "them" that needed to be controlled; the Nazis similarly viewed the Jews as alien; in celebrating wars of conquest, the suffering of the conquered group is either disregarded or treated as much less important that the benefits to the conquering group. Ingroup/outgroup thinking of this sort typically requires either ignoring others' suffering or accepting dubious theories that can't withstand objective scrutiny. (This is one function of propaganda.) The type of extreme ingroup bias that promotes evil behavior tends to be undermined by epistemically responsible reflection.
Example B. Selfishness and jerkitude. Similarly, selfish or jerkish behavior tends to be supported by rationalizations and excuses that prove flimsy when carefully examined. ("It's fine for me to cheat on the test because of X", "Our interns ought to expect to be hassled and harrassed; it's just part of their job", etc.) If you were simply comfortable being selfish, you wouldn't need to concoct those poor justifications. If and when critical reflection finally reveals the flimsiness of those justifications, that normally creates some psychological pressure for you to change.
It's crucial not to overstate this point. We can be unshakable in our biases and rationalizations despite overwhelming evidence. And even when we do come to realize that something we eagerly want for ourselves or our group is immoral, we can still choose that thing. Evil might still be commonplace: Just as most plants don't survive to maturity, many people fall far short of their moral potential, often due to hard circumstances or negative outside influences.
Still, if we think well enough, we all can see the basic outlines of moral right and wrong; and something in us doesn't like to choose the wrong. This is true of pretty much everyone who isn't seriously socially deprived, regardless of the specifics of their cultural training. Furthermore, this inclination toward what's good -- I hope and believe -- is powerful enough to place at the center of moral education.
That is the empirical content of the claim that human nature is good.
I do have some qualms and hesitations, and I think it only works to a certain extent and within certain limits.
Perhaps oddly, the strikingly poor quality of the reasoning in recent U.S. politics has actually firmed up my opinion that careful reflection can indeed fairly easily reveal the lies behind evil.
Related: Human Nature and Moral Education in Mencius, Xunzi, Hobbes, and Rousseau (History of Philosophy Quarterly 2007).