“Pretty fair... Mr. Mason, I want to ask you something.”
“Go ahead,” Mason told her.
“What’s a lie detector?” she asked.
Mason studied her and failed to find any expression in her eyes. “Why the question?” he asked.
“I wanted to know, that’s all.”
“Any particular reason?”
“Well,” she said, “I’m interested from a psychological standpoint, that’s all.”
Mason said, “It’s really not much more than an instrument for taking blood pressure, the theory being that when a witness gets ready to lie, he sort of mentally braces himself, and that shows in a change of blood pressure, which, in turn, shows on a needle. Telling the truth is easy and effortless. Telling a lie involves mental effort.”
“Are they of any real value?” she asked.
“Yes,” Mason said, “their value, however, depends on the skill of the man who does the questioning. In other words, the machine registers what you might call a psychic change in the individual. The skill of the questioner accentuates those psychic changes and makes them significant.”