united-states
In the United States, public officials cannot recover damages in a defamation action unless the defendant made the statement with "actual malice"—"that is, with knowledge that it was false or with reckless disregard of whether it was false or
not" (New York Times Co. v. Sullivan, 376 U.S. 254, 279��83 (1964)).
The justification was that:
[a] rule compelling the critic of official conduct to guarantee the truth of all his factual assertions—and to do so on pain of libel judgments virtuall unlimited in amount—leads to a comparable "self-censorship."
The Court was of the opinion that libel law would only be consistent with the First and Fourteeth Amendments if a public official were required to prove actual malice. The Court also noted this goes hand-in-hand with the absolute privilege that a public official enjoys when they are sued for libel by a private citizen for utterances made "within the outer perimeter" of the official's duties (p. 282).
See ohwilleke's answer explaining how this standard has been extended to public figures more generally.
canada
Canada has rejected the "actual malice" standard (Hill v. Church of Scientology of Toronto, [1995] 2 SCR 1130, para. 137). Instead, it relies on flexible defences of fair comment, qualified privilege, and responsible communication on matters of public interest, as well as anti-SLAPP legislation in several provincial jurisdictions. All of these give heightened protection to communication on matters of "public interest."
Paragraphs 127–36 of Hill present critique of the rule in Sullivan from American authors and judges, and from courts and law reform commissions in England and Australia. Another brief comparison was made by the Supreme Court in Grant v. Torstar Corp., 2009 SCC 61, para. 68 (citations removed):
Commonwealth courts have rejected the precise balance struck in Sullivan between free expression and protection of reputation. However, the law has begun to shift in favour of broader defences for press defendants, most prominently in England, but also in Australia.