Kind leading men, an aw-shucks American Idol, cute advertising: smiles are spreading across the culture. Have a nice day.

In the hit comedy “I Love You, Man,” Paul Rudd plays Peter, a sunflower of a guy: cheerful, welcoming and reassuring, if a little squeamish. Peter blushes over references to pubic hair, winces at flatulence and refuses to kiss and tell, even when the girls do.

Mr. Rudd has made a career of playing soft-spoken nice guys, and in another era that persona might have relegated him to a second tier of celebrity status, along with the likes of Donny Osmond and Pat Sajak. But today he is a leading man, even a matinee hunk, who has appeared in a string of Judd Apatow romantic comedies like “The 40-Year-Old Virgin” and “Knocked Up,” as well as the current “I Love You, Man,” which has taken in just over $70 million at the domestic box office as of May 22.

Or what about Kris Allen, the polite and aw-shucks singer from small-town Arkansas, who has been called a latter-day Pat Boone, and who on Tuesday was voted by the public, casting almost 100 million ballots, a record, as its latest American Idol.

That amiable guys and uncomplicated sweethearts could be today’s pop heroes is one sign of an outbreak of niceness across the cultural landscape — an attitude bubbling up in commercials, movies and even, to a degree, the normally not-nice blogosphere.

“We are now in an age of nice,” said Eric G. Wilson, an English professor at Wake Forest University, who, as the author of “Against Happiness: In Praise of Melancholy,” qualifies as a professional curmudgeon. But even Mr. Wilson sees no end of smiley faces. He cites as avatars of a new niceness the Obama administration, which has been criticized for being too friendly to some repressive world leaders; advocates of political correctness who still hold sway in many public forums; and the director-writer-producer Mr. Apatow, whose era-defining comedies feature “nice guys who finish first — a great hope for non-threatening puerile males,” Mr. Wilson said.

“There’s more spark to nice — it is really in,” said Graceann Bennett, the director of strategic planning at the Chicago office of the advertising agency Ogilvy & Mather. She said clients are shying away from the arch and sardonic campaigns that were in vogue when the economy was flush. Ogilvy recently pitched an ad for Truvia, a new sweetener, showing two mounds of white powder meant to suggest female breasts, one more perfectly shaped than the other. The proposed tagline: “The difference between real ones and fake ones is obvious.” The agency wanted to make the point that Truvia is derived from stevia leaves, a South American herb, not from a laboratory. But executives at Truvia’s manufacturer, Cargill, thought the ad was mean to the competition and to women with breast implants. They rejected it.

“I’m in marketing meetings with clients all the time,” Ms. Bennett said, “and they’re always coming back with, ‘I don’t want mean.’”

Other brands seek to be known for putting on a happy face rather than having an attitude. Pepsi la-bored for decades to position itself as the younger, sexier alternative to Coca-Cola, but last fall it unveiled a logo that has been widely interpreted as a stylized smiley face (some thought it mimicked the logo for the Obama campaign, which arguably makes the same point in a different way).

Volkswagen, too, shifted away from a series of sardonic commercials featuring an acerbic German narrator; the current campaign stars Max, a cute talking 1964 Beetle, and his talking Microbus friend. The black bug is so innocent it blushes when Heidi Klum makes a suggestive remark. And then there are Internet companies that embrace positivity. On Twitter, every user who uploads a photograph from that latest trip to Cancún is greeted with a pop-up message that chirps, “That’s a nice picture!” Skype, the Internet telephone service, greets users with cutesy cloud motifs and bubble-letter graphics, as if the entire brand image was lifted out of a 13-year-old girl’s bedroom.

Niceness in the culture spikes when the real world is full of trouble, theorized Catherine Ryan Hyde, the author of “Pay It For-ward,” the 1999 novel that gave a marketable name to the idea of doing good deeds for strangers with the expectation that they would then do the same for others. (The book inspired a 2000 movie star-ring Kevin Spacey and a charitable foundation, run by Ms. Hyde.) When Ms. Hyde’s book was published, during the gilded dot-com era, many people responded to its overweening earnestness with a collective groan, she said. But in the months after 9/11, earnestness and altruism were in fashion, and Ms. Hyde saw a surge in interest in her foundation, as well as a proliferation of other like-minded grass-roots groups.

The same thing happened again after the financial crisis escalated last fall, she said. “When do you ever see people making sweeping changes when everything is going fine? They don’t. They change when the roof caves in.”

Yes, there is no shortage today of brands and famous people who are sticking with sarcasm as their public image. A few recent campaigns, including those for Miller High Life and JetBlue, are cut-ting against the grain and trying mocking ads, which some analysts believe will tap into people’s frustrations over the economy. And for every star like Paul Rudd and Kris Allen who kill audiences with kindness, there is an adder-tongued Simon Cowell or Bill O’Reilly.

But consider that the snarly blog world has made room for some new make-nice citizens. While sites devoted to politics, celebrities and the media are rife with invective, an expanding parallel universe of blogs catering to young women and mothers unfailingly offer encouragement and support.

Pepsi unveiled a logo that has been widely interpreted as a stylized smiley face. OperationNice.com links to news articles about good deeds and asks bloggers to take an oath of niceness. The blog was created 10 months ago by Melissa Morris Ivone, a 28-year-old graphic designer in Cinnaminson, N.J., who was inspired after a stranger held the door for her in an elevator. She wanted to create an oasis of good vibes online.

“Blogs, message boards and comments sections are just set-tings for people to verbalize their (usually negative) opinions,” Ms. Ivone wrote in an e-mail message. “Personally, I wanted a little corner that was free of all that.” So far, more than 1,100 readers, many of them fellow bloggers, have taken her oath of niceness, allowing them to post an Operation Nice seal of approval on their blogs and Facebook pages.

Maybe that Nice seal will come to rival the original bright-yellow smiley logo, which became a popular sensation in the early 1970s, a period of acute societal division over war, race and politics, when people apparently needed to force a smile. In 1971 —the year the Weather Underground bombed the Capitol and Charles Manson was convicted — more than 50 mil-lion smiley-face buttons were sold. The symbol remained a perennial on jeans patches and coffee mugs throughout the Watergate years.

“The men who marketed it imagined it as a successor to the peace sign,” according to Bruce J. Schulman, a history professor at Boston University and the author of “The Seventies: The Great Shift in American Culture, Society and Politics.” In an e-mail message, he added that smiley represented “the ‘live and let live’ message of the counterculture in a tame, depoliticized and eminently commercial form.”

Whether today’s “nice” celebrities and friendly brands reflect or influence people’s behavior in the real world is impossible to say. There is no scientific niceness index. But what can it mean that use of the word “idiot” in blog posts has declined by nearly half in the last six months, according to Blog-Pulse, a Nielsen service that tracks online data?

Some businesses whose stock in trade was hauteur seem to be slapping on a grin. Josh Ozersky, a restaurant editor for Citysearch.com, said he has detected a tonal shift at elite Manhattan restaurants that treated unfamous diners indifferently. “Now you see chefs working the room rather than just a few V.I.P. tables, and hostesses instructed to thank everybody on the way out, rather than just treating them like nonentities,” Mr. Ozersky said. 

The Santos Party House on Lafayette Street has been em-braced as the nightclub of the moment because of an open-door policy and its overt friendliness, as expressed by signs throughout that read “Thank you for partying!” and “You have a beautiful face!”

Shaving off the edges does have its rewards. Research indicates that people who might be categorized as nice tend to have lower blood pressure and lower divorce rates, said Linda Kaplan Thaler, an author of “The Power of Nice: How to Conquer the Business World With Kindness.” Doctors who take an extra few minutes to talk with patients are sued less often for malpractice.

“Companies that have the highest retention have the nicest atmospheres,” said Ms. Kaplan Thayer, who is also chief executive of the Kaplan Thaler Group, an advertising agency that employs about 175 people. “And in a situation where people are losing their jobs and you have an option of whom to hire, you’re going to hire the per-son who is complimenting your tie. Nice becomes a competitive edge.”

Not everyone is buying it. Professor Wilson of Wake Forest, for one, will not relinquish his disdain for the smiley face. All this undue politeness, he says, “produces ubiquitous and maybe dangerous mediocrity.”

“‘Have a nice day.’ ‘That’s a nice car.’ ‘Be nice.’ What do these sayings mean but let’s make everything perfectly agreeable?” he said. “Which is to say, predictable and vaguely reassuring, like easy-listening radio or greeting-card sunrises or Tom Hanks.”