The occupant of a vehicle seat is characterized by computing a power spectrum function of data obtained from a seat sensor such as a pressure sensor in a fluid-filled seat bladder or a frame-mounted vertical accelerometer. The spectrum function is integrated over a specified frequency range, and compared to a threshold value that is calibrated to compensate for vehicle speed related variability. The presence of a cinched child seat is indicated when the threshold is exceeded, and the presence of a normally seated adult is indicated otherwise.