Downloading from illegal sources has increased instead of decreased six months after a court ordered ISPs in the Netherlands to block access to The Pirate Bay, according to research from the University of Amsterdam (UvA) and Tilburg University, released Thursday.

As of May 2012, more than 80 percent of Dutch Internet subscribers were unable to access The Pirate Bay directly through their ISP due to two court rulings that lead to the torrent search site being blocked at the biggest ISPs in the Netherlands. A block of the site at the ISP level was called for because an estimated 90 percent to 95 percent of the material offered via The Pirate Bay is illegal, the court ruled.

Antipiracy foundation Brein, which represented rights holders in both cases, argued in court that a block of The Pirate Bay, based on evidence from Italy and Denmark, could significantly reduce the number of unique visitors to the site.

"However, the economically more relevant question is not whether blocking access to TPB decreased the number of visitors to this particular website, but what the effect of the blocking is on online copyright infringement as a whole," the researchers wrote in their paper.

Instead of decreasing the percentage of subscribers who downloaded from illegal sources, the percentage increased after The Pirate Bay was blocked, the researchers wrote.

For the ISPs UPC, KPN, Tele 2 and T-Mobile, 18.4 percent of subscribers had turned to illegal sources six months after the block, compared to 15.7 percent just before it went into effect, For ISPs Ziggo and XS4ALL, who were ordered block access to The Pirate Bay three months earlier, the percentage of customers that had downloaded from an illegal source in the preceding half year increased from 22.5 percent three months after the block to 25.2 percent 10 months after the intervention, the researchers said.

Those results are based on two surveys that were of representative samples of the Dutch population aged 16 years and over, the researchers said. The first survey was conducted in May 2012, the second was held seven months later at the end of 2012. By that time, Ziggo and XS4ALL had been blocking access to The Pirate Bay for 10 months, while the other ISPs had been doing so for six months.

"The percentage who downloaded films & series, games and books from illegal sources in the preceding six month has increased between May and November-December 2012, while the percentage downloading music from illegal sources remained constant," the researchers said. "This implies that any behavioral change in response to blocking access to TPB has had no lasting net impact on the overall number of downloaders from illegal sources, as people learn to use alternatives to TBP."

The findings of the survey corroborate with a second method used by the researchers to assess the impact of blocking access to The Pirate Bay. They also used a BitTorrent monitoring technique to gauge its effects.

"The two measurement techniques provide complementary insights," the researchers said, adding that the monitoring method only measured small changes in the distribution of Dutch peers over different ISPs, implying limited effects of the intervention on BitTorrent file sharing.

"Legal action against file sharing often has an immediate effect which tends to fade out after a period of typically six months, as illegal supply and demand find other places to meet," the researchers said, adding that this is in line with conclusions from similar research.

When access is first blocked consumers may start downloading less in part because they learn more about issues that could arise from downloading from illegal sites, and the decrease may continue for a time, they wrote. However, consumers may learn how to circumvent the blocking and the effects of becoming educated about illegal downloading may wear off, they said.

"We do not find strong indications of the long-lasting effectiveness of such an approach in preventing consumers from making use of illegal sources. Therefore it is unlikely that the increased use of legal channels, as we found for music, was caused by this intervention," they said.