Some have done it for love, and some for money. Others have become spies for the thrill of living a double life. A Navy ensign tried to sell national secrets because he needed cash to buy a car. An aerospace executive recounted that after he got a divorce and declared bankruptcy, he was an easy target for Polish intelligence agents who "found a fool who needed money."

One Pentagon aide confessed to authorities that he decided to sell classified documents about Asia "to prove . . . I could be a man and still be gay."

Some spies have appeared to their neighbors to be typical Americans who paid their mortgages, mowed the lawn and drove their children to soccer practice. But the recent history of American espionage also is filled with morose beings, misfits, malcontents and alcoholics. Hostile intelligence agencies often exploit their hang-ups brilliantly.

These spies have badly damaged national security in various ways over the past generation, an examination of recent espionage cases shows. The Walker spy case and the recent arrests of former CIA employe Sharon M. Scranage and Ghanaian national Michael A. Soussoudis are only the latest in what authorities say is a disturbing trend.

The United States has seen an upsurge in spy cases lately because Soviet bloc agents have become increasingly aggressive in recruiting spies, and because of a mid-1970s government decision to prosecute more cases, intelligence experts say. There were only a few spy prosecutions in the 1960s, as federal authorities often concentrated on turning spies into double agents. There have been 38 prosecutions since 1975.

Committing espionage against the United States can be a lonely existence and, judging by the tearful pleas of convicted spies at sentencing proceedings, a devastating one.

"The sorrow and remorse I feel are beyond words," Northrop Corp. engineer Thomas Cavanagh told a judge last year after pleading guilty to trying to sell blueprints for the Stealth bomber to the Soviets. "I shamed my father's honorable name."

Federal Judge Matthew Byrne Jr. wondered aloud why a good family man would do what Cavanagh did. "What we ponder is what made this not-bad man do this extremely bad thing," Byrne said. Then he sentenced him to two life terms.

While some American spies get big money from their spymasters -- James Harper received $250,000 from Polish intelligence for information about U.S. missiles, for instance -- William Kampiles' experience was more common. A former CIA trainee, Kampiles sold the Soviets an extremely sensitive manual on a reconnaissance satellite for $3,000.

Intelligence specialists say that the damage done by spies usually is not known until a war, when a nation may discover that its enemy easily jams its radar or outmaneuvers its weaponry.

It is "standard" for United States officials to stress how low-level a spy was, and the insignificance of his or her betrayal, according to a recent book, "The New KGB," by two former U.S. intelligence officials, William R. Corson and Robert T. Crowley.

Playing down spy cases serves not only to spare embarrassment to officialdom, but also to reassure the public, the book says.