This probably refers to the discussion of misanthropy in Plato's Phaedo, 89d-90b. Socrates points out that misanthropy comes from being too naive and trusting and then turning to the opposite extreme after getting burned a few times. A knowledgeable rational man, on the other hand, would realize from the start that in this, as in many other things (misanthropy is analogized to misology in this passage, i.e. good/bad men to good/bad arguments), the extremes are rare and most cases fall in the middle:
"For misanthropy arises from trusting someone implicitly without sufficient knowledge. You think the man is perfectly true and sound and trustworthy, and afterwards you find him base and false. Then you have the same experience with another person. By the time this has happened to a man a good many times, especially if it happens among those whom he might regard as his nearest and dearest friends, he ends by being in continual quarrels and by hating everybody and thinking there is nothing sound in anyone at all. Have you not noticed this?”
“Certainly,” said I.
“Well,” he went on, “is it not disgraceful, and is it not plain that such a man undertakes to consort with men when he has no knowledge of human nature? For if he had knowledge when he dealt with them, he would think that the good and the bad are both very few and those between the two are very many, for that is the case.”
“What do you mean?”
“I mean just what I might say about the large and small. Do you think there is anything more unusual than to find a very large or a very small man, or dog, or other creature, or again, one that is very quick or slow, very ugly or beautiful, very black or white? Have you not noticed that the extremes in all these instances are rare and few, and the examples between the extremes are very many?”
“To be sure,” said I.
“And don't you think,” said he, “that if there were to be a competition in rascality, those who excelled would be very few in that also?”
“Very likely,” I replied."
[Plato in Twelve Volumes, vol. 1, translated by Fowler]