Abstract: A method of detecting psychological stress by evaluating manifestations of physiological change in the human voice wherein the utterances of a subject under examination are transduced to electrical signals and processed to emphasize selected characteristics which have been found to change with psycho-physiological state changes. The processed signals are then displayed, as on a strip chart recorder, for observation, comparison and analysis. An especially useful characteristic is an infrasonic modulation in the voice. Apparatus for performing detection of this type includes a transducer, a magnetic recorder, a series diode, a plurality of integrating capacitors, an amplifier and a chart recorder. A second apparatus includes filter means, an FM discriminator and a detector, a waveform integrator, an amplifier and a recorder for producing a visible record.