Defamation is the making of a false statement that creates a negative image of another person. At a superficial level the point of anti-defamation laws are to prevent such false statements. But false statements by themselves are not damaging unless they do harm to the subject’s reputation. For that, the statement must be credible.
If the direct effect of an anti-defamation law is to reduce the number of false statements made, an indirect effect is to enhance the credibility of all of the false statements that continue to be made. Because a member of the public who cannot assess the veracity of a given statement will begin with the presumption that the statement is more likely to be true since a larger fraction of all statements made are true. This of course encourages more false statements, undermining the original direct effect of the law.
Indeed it is impossible to eliminate false damaging statements without making them even more damaging.
Nevertheless, in equilibrium the net effect of an anti-defamation law is to increase the truthfulness of public discourse. The marginal slanderous statement is the one which is just damaging enough to compensate for the expected cost of a lawsuit. When that cost is higher, the previously marginal statement is crowded out.
But that just says that the proportion of statements that are false goes down. Another effect anti-defmation laws are to reduce the number of truthful statements. Even a truthful statement has a chance of being judged false and damaging. There will overall be fewer things said.
Furthermore, since a defamatory statement must be proven to be false and some falsehoods are easier to demonstrate than others, the incidence of anti-defamation laws on various types of lies must be considered. A libelous claim will be made if and only if the cost of the potential lawsuit is outweighed by the value of making it. For statements whose explicit intention is to defame, that value increases as the overall credibility of public discourse increases. Among those statements, the ones that are hardest to prove false will actually be said more and more often.
In fact as long as the speaker is creative enough to think of a variety of different ways to defame, the main effect of anti-defamation laws will be to substitute away from verifiable lies in favor of statements which are more difficult to prove false. This will be so as long as a sufficiently large segment of the public cannot tell the difference between statements that can be verified and statements that cannot.