Pornographic websites must check that users in Louisiana are over 18 years old or risk being sued, a new law in the US state says.
Republican state legislator Laurie Schlegel, who introduced the bill, says she was inspired by musician Billie Eilish, who has spoken of the effect viewing porn as a child had on her.
But US adult industry campaigners argue the law is the wrong solution.
Age checks are also a key part of the UK's Online Safety Bill.
Under the Louisiana legislation, most users are expected to use a smartphone app, called LA Wallet, to show they are over 18.
The app requires a driving licence or an official state identity card to establish a person's age.
Users of one of the largest pornographic websites, Pornhub, are now directed to an age verification process, according to a video posted on Twitter.
"Louisiana law now requires us to put in place a process for verifying the age of users who connect to our site from Louisiana," the on-screen text says.
The message also says that the site "guarantees" it does not collect data during this process.
Not retaining data is written into the bill, according to an official summary.
The law does not make it illegal to allow children to access pornographic websites, but enables people to sue sites that do not have age checks.
A federal law giving regulators the power to mandate age-verification country-wide was proposed last month by Republican US senator Mike Lee, from Utah. The Shielding Children's Retinas from Egregious Exposure on the Net Act would make US regulators require sites to age-check their users.
But some lawyers question whether the Louisiana legislation is compatible with US constitutional protections for freedom of speech. Campaigners note a previous law was ruled unconstitutional by the Supreme Court.
Mike Stabile, of the Free Speech Coalition, which campaigns for adult industry rights, told the BBC: "We should all work to prevent access to adult content by minors, but the simple, free filters available on most devices would accomplish the same thing."
The legislation was a dangerous step towards censorship and should be seen in the context of an effort by conservative and faith-based groups to "remove sex and information about sex from the public square", said Mr Stabile.
Ms Schlegel says she is a "certified sex addiction therapist" including "addiction" to pornography.
The legislator told USA Today the bill was not designed to prevent adults from accessing pornography, but was just about protecting children.
Ms Schlegel said she was inspired to act by Eilish, who said in a 2021 interview she watched pornography from age 11, and felt it helped her to be "one of the guys".
Eilish said the experience led her to "not say no to things that were not good" when she began having sex.
In the UK, the Online Safety Bill, which is currently progressing through Parliament, will order pornographic websites to prevent children from accessing explicit content.
Previous legislative efforts have been abandoned and many privacy campaigners argue age verification systems can be easily circumvented using virtual private networks.
An Ofcom report last year found one in 20 children (6%) already got around parental controls restricting access to certain sites.
And critics say such systems collect large amounts of highly personal data about people's sexual preferences.
Jason Kelley, of digital civil liberties group the Electronic Frontier Foundation, told the BBC: "Age verification systems like those required by the Louisiana law force users to hand private data over to a third party simply to use a website.
"There's no guarantee that this data won't be retained."
But providers of age check systems disagree there is a privacy risk.
Ben Keirle, of UK-based age verification platform 1account, said all that an adult website would see using its system "is a simple yes/no response to the request to verify a user prior to admitting access".
He said information used to prove age was deleted once a person was validated and was in any case not stored by the platform and was never made available to the adult website.
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