Americans who spy against the U.S. are increasingly motivated by ideology rather than by money, with nearly half of the known spies since the end of the Cold War showing allegiance to another country or cause, according to a government report.

Prior to 1990, just a fifth of Americans spying for others were ideologically motivated.

The March report, obtained by the Federation of American Scientists' Secrecy in Government project, compares trends among the 173 Americans known to have spied against the U.S. since 1947, of which 37 began their spying since 1990.

Only five of those 37 spies are known to have received payment for their work. Of the 11 spies identified since 2000, none was paid — although five had hoped to be. But almost half of those spying against the U.S. since 1990 demonstrated an allegiance to a foreign country or cause.

Increasingly, that cause is wrapped up in the war on terror.

"It's sort of an opportunist thing," said Katherine Herbig, who conducted the study for the Defense Personnel Security Research Center. "People in search of customers for classified information now have terrorists in mind."

The finding appears to upend the conventional wisdom in intelligence circles, which for years has held that money is by far the most important motivator for spies. The other common motivators are excesses of ego or thrill seeking, ingratiation or an attempt to curry favor, and coercion or compromise by a foreign agent — blackmail, for instance.

The report found that 17 of the 37 Americans known to have spied against the U.S. since 1990 were either solely or primarily motivated by ideology or allegiance to a foreign power or cause. Just 10 were solely or primarily motivated by money. The rest had other motives.

In earlier periods, money has proven to be a much more powerful sole motive, especially compared to ideology or divided loyalties. Before 1990, 77 spies in the database were solely or primarily motivated by money, compared to 22 who were motivated by ideology.

Herbig said the 11 cases since 2000 are too small a sample to determine real trends in spying. However, they show shifts that may be of interest to U.S. intelligence agencies, which are loosening security clearance regulations to allow more non-native-born Americans — particularly those from the Middle East and Central Asia — to translate, recruit spies, and collect and analyze intelligence.

Nine of the 11 most recent spies had security clearances, but three of those had hidden parts of their past lives, made up false identities or used false documents when they applied for the clearances. Four were naturalized citizens, and six had foreign relatives, close friends or business attachments. Only two were recruited to spy; the others volunteered. Four had serious mental or emotional problems.

Six of the 11 most recent cases have involved terrorism, either as potential or actual recipients of information, or by translators at the Guantanamo Bay detention camp for accused terrorists, or as a protest of conscience against the treatment of those detainees.

In one case, a Navy lawyer at Guantanamo Bay printed the names of the 550 prisoners being held at jail from a secret database and mailed it to a legal defense group that was filing habeas corpus lawsuits on behalf of prisoners but was stymied by a lack of names. The government later released the names under a Freedom of Information Act request filed by The Associated Press.

Lt. Cdr. Matthew Diaz, who served a six-month jail term, was the first American convicted of passing classified information to an American rather than a foreign organization.

"His case illustrates the expansion and reframing of the application of espionage by U.S. authorities since the terrorist attacks of 9/11 that resulted in the declaration of a "global war on terror" and the establishment of a detention center at Guantanamo Bay," the report states.

The new spies are also using the Internet: Seven of the 11 cases since 2000 used the Internet to initiate offers of espionage, and 10 of 11 used computers to collect intelligence.