The Chicago Tribune's plans to charge readers for some of its online content, along with McClatchy's announcement last week that it will expand its paywall testing to four more sites, are further proof that a majority of large newspaper companies now believe in some form of paid online content.

Ken Doctor expects about 20 percent of the 1,400 or so daily papers in the U.S. to charge for online access by the end of the year. That seems small when you look at the major newspaper chains; it's getting harder to find one that isn't at least experimenting with some kind of paid access.

What's dawned on newspaper executives, Doctor told me, is that "a metered system doesn't have to — if you do it in the right way — get in the way of your digital revenue."

Poynter business analyst Rick Edmonds outlined nine reasons that newspapers have changed course so quickly (relative to the newspaper industry's normal rate of change, that is). Among them: Porous paywalls separate the loyalists who are likely to pay from the drive-by users; users become accustomed over time to the idea of having to pay; it turns out that traffic usually doesn't drop enough to affect online advertising revenue.

The naysayers are still out there; some of them commented on David Simon's Columbia Journalism Review post in which he said that more papers will go the way of The Times-Picayune unless they adopt paywalls. The principle argument against charging is that readers won't pay for news and that newspapers need to get creative about finding other digital revenue streams instead.

Journal Register Co., where Digital First CEO John Paton has been a vocal opponent of paywalls, is still holding out. When Paton assumed control of MediaNews as head of Digital First Media, he told paidContent's Staci Kramer that it was too early to say what would happen to paywalls at MediaNews sites.

Likewise, I didn't find paywalls for Hearst's 16 newspaper sites (including the online-only, although The Bay Citizen reported in April 2011 that the company was mulling a move to charge users of the San Francisco Chronicle's website. Hearst does charge for digital editions of its magazines.

Cox Media Group hasn't started to charge for online access to or other sites, but users have to pay for the Atlanta Journal-Constitution's iPad app and replica edition.

Freedom Communications' toe is still in the water, but just barely. The company tried paywalls at two sites a couple of years ago, said Bob Emmers, a spokesman for the company. It dropped the paywall at the Valley Morning Star in Harlingen, Texas because traffic dropped so much. The paywall at The Lima News in Ohio is still in place, although users can access the site by participating in Google Consumer Surveys. (Both newspapers are part of a sale Freedom announced a couple of weeks ago.)

More and more, though, those companies are outliers. Newspaper chains that have implemented paywalls at one or more sites:

It's been tough to get a firm count on how many newspaper sites have added paywalls. PEJ's State of the Media put it around 150; Press+ said it has about 250 daily newspaper clients. News & Tech has a running list of newspaper sites that charge for access.

Correction: This post originally stated that McClatchy is expanding paywalls to five sites, but that included one site that has had a paywall since last year.