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Before the public loses interest in the case of CIA turncoat Aldrich H. Ames, it ought to add one more lesson to the list of those that should have been learned from this distasteful, embarrassing episode.

The lesson: Don't rely on polygraph or so-called lie detector tests to safeguard security at any level of government or in the private sector.That much should be clear from the fact that Ames passed CIA lie detector tests in 1986 and again in 1991 even though he was selling American secrets to the Russians.

But then the same lesson should have been apparent from the fact that a 1987 report from the House Intelligence Committee recommended reduced reliance on the polygraph and from studies showing that success rates for these tests range from 95 per cent to just above 50 per cent. Moreover, the CIA teaches agents going into the field how to beat the enemy's lie detectors.

The machines can be fooled by inveterate liars who no longer react emotionally to lying, by people who take certain drugs beforehand, or by those who have convinced themselves that their lies are the truth. Another way to fool the test is to raise one's blood pressure by pressing one's toes hard against the floor whenever one is telling the truth.

We make these points just now because reliance on lie detector tests persists in some quarters even though Congress six years ago prohibited most private employers from using polygraphs to screen job applicants or randomly test workers.

Exempted from the prohibition are federal, state and local governments, firms that do sensitive work for agencies like the CIA and FBI, companies that provide security services, and those that make or distribute controlled substances.

How many more cases like the one involving Ames must there be before those with legitimate needs for tight security learn that the lie detector should not be considered a substitute for thorough background investigators and other security procedures?