PHILADELPHIA – About 1 percent of Web sites indexed by Google and MSN are sexually explicit, and 6 percent of random search queries yield at least one sexually explicit site, according to a U.S. government-commissioned study.

Government lawyers introduced the study in court this month as the Justice Department seeks to enforce the 1998 Child Online Protection Act, which would criminalize allowing children to access “harmful” material.

But the American Civil Liberties Union, which challenged the law on behalf of a broad range of Web publishers, said the study supports their argument that filters work well.

The study, conducted by a University of California at Berkeley professor, concludes that the strictest filter tested, AOL’s Mature Teen, blocked 91 percent of the sexually explicit Web sites in indexes maintained by Google Inc. and Microsoft Corp.’s MSN.

Filters with less restrictive settings blocked as little as 40 percent of sexually explicit sites, according to the study of random Web sites by Berkeley statistics professor Philip B. Stark.

“Filters are more than 90 percent effective, according to Stark,” ACLU attorney Chris Hansen said Thursday, during a break in the trial. “Also, with filters, it’s up to the parents how to use it, whereas COPA requires a one-solution-fits-all (approach).”

Stark apparently prepared the report based on information the Justice Department obtained through subpoenas sent to search engine companies and Internet service providers.

Google refused one such subpoena for 1 million sample queries and 1 million Web addresses in its database, citing trade secrets. A judge limited the amount of information the California-based company had to surrender.

Justice Department lawyers Theodore Hirt and Raphael Gomez declined to comment Tuesday on Stark’s findings.

Stark also examined a random sample of search-engine queries. He estimated that 1.7 percent of AOL, MSN and Yahoo! search results and 1.1 percent of Google and MSN search results are sexually explicit. About 6 percent of searches yield at least one explicit Web site, he said.

The most popular queries return a sexually explicit site nearly 40 percent of the time, Stark found.

But filters blocked 87 to 98 percent of the explicit results, Stark found.

The law’s opponents argue that one of its flaws is that people who publish pornography from overseas would be out of the reach of the federal government. But the Justice Department argued in court papers that it believes popular pornography sites are generally published in the United States.

Stark found that about half the sexually explicit Web sites found in the Google and MSN indexes are foreign. But he agreed that the most popular sites are domestic.

“COPA – right out of the bat – doesn’t block the 50 percent (posted) overseas,” Hansen said. “So COPA is substantially less than 50 percent effective.”

Closing arguments in the four-week, non-jury trial before Senior U.S. District Judge Lowell Reed Jr. are expected Monday.

The law, signed by then-President Clinton, requires Web sites to get credit card information or some other proof of age from adults who want to view material that may be considered “harmful to children.” It would impose a $50,000 fine and six-month prison term on commercial Web site operators that allow minors to view such content, which is to be defined by “contemporary community standards.”

The law has yet to be enforced. The U.S. Supreme Court upheld a preliminary injunction, ruling in June 2004 that the plaintiffs were likely to prevail.

The plaintiffs, including Salon.com, say they would fear prosecution under the law for publishing material as varied as erotic literature to photos of naked inmates at Abu Ghraib.