The number of people downloading music in the US has halved since the Recording Industry Association of America (RIAA) began filing lawsuits against individuals last September.

According to a survey by the Pew Internet & American Life Project, the threat of legal action is beginning to discourage downloaders from sharing copyrighted music files online.

The survey revealed that the number of people using peer-to-peer (P2P) sites such as Kazaa and Grokster fell from 35 million in April and May 2003 to around 18 million during November and December.

There has been a traditionally steep decline in the numbers of heavy P2P users, such as students. The research also found that one-fifth of those who continue to use such sites have eased up, citing the threat of legal action.

The RIAA has warned that it will continue to file lawsuits against P2P users, while a recent decision by a US appeals court, which ruled that ISPs do not have to reveal the identities of subscribers, may make things worse for file sharers.

In a statement posted on its website, RIAA president Cary Sherman explained: "This decision ... unfortunately means we can no longer notify illegal file sharers before we file lawsuits against them to offer the opportunity to settle outside of litigation."

With the RIAA seeking up to $150,000 per violation, many people have taken the opportunity to settle out of court for around $5,000.