New York Times v. Sullivan:

The First Amendment "prohibits a public official from recovering damages for a defamatory falsehood relating to his official conduct unless he proves that the statement was made with 'actual malice' -- that is, with knowledge that it was false or with reckless disregard of whether it was false or not."

Gertz v. Welch:

"We hold that, so long as they do not impose liability without fault, the States may define for themselves the appropriate standard of liability for a publisher or broadcaster of defamatory falsehood injurious to a private individual."
"[W]e hold that the States may not permit recovery of presumed or punitive damages, at least when liability is not based on a showing of knowledge of falsity or reckless disregard for the truth."

Dun & Bradstreet, Inc. v. Greenmoss Builders:

We have never considered whether the Gertz balance obtains when the defamatory statements involve no issue of public concern. To make this determination, we must employ the approach approved in Gertz and balance the State's interest in compensating private individuals for injury to their reputation against the First Amendment interest in protecting this type of expression. This state interest is identical to the one weighed in Gertz. There we found that is was "strong and legitimate." The First Amendment interest, on the other hand, is less important than the one weighed in Gertz. Speech on matters of private concern is of less First Amendment concern. In such a case, "[t]here is no threat to the free and robust debate of public issues; there is no potential interference with a meaningful dialogue of ideas concerned self-government; and there is no threat of liability causing a reaction of self-censorship by the press." In light of the reduced constitutional value of speech involving no matters of public concern, we hold that the state interest adequately supports awards of presumed and punitive damages - even absent a showing of "actual malice."