A new study by a Defense Department contractor shows that divided loyalty, usually on the part of naturalized Americans with roots in a foreign land, has become the dominant motive [for spying against the US, a historic shift].

From 1947 to 1990, the study found, fewer than 1 in 5 Americans charged with spying were acting solely or primarily out of patriotic, as opposed to ideological, loyalty to a foreign country. Since 1990, according to the study’s author, Katherine L. Herbig, divided loyalty has been the sole or primary motive in about half of all cases.

“Dual loyalty is a problem we haven’t seen on such a scale since the Revolution,” when many colonists swore allegiance to the British king, said Joel F. Brenner, the top counterintelligence official in the office of the director of national intelligence.