In a 2004 study on pain perception, researchers K. Ibolya, A. Brake, and U. Voss asked participants to immerse their hands in icy water for as long as they could tolerate. The study provided evidence that the experimenter's sex can act as an extraneous variable: male participants tolerated the pain longer when tested by a female experimenter, and female participants tolerated it longer when tested by a male experimenter.