Self-reports can be biased by a number of factors, whereas measures of physiological responses can be made directly and are often presumed to be free of bias.

Physiological measures such as heart rate or brain activity can be obtained directly and objectively, without being influenced by a person's subjective interpretation or biases. Self-reports, on the other hand, are based on a person's perception and memory of their experiences, which can be influenced by a range of factors such as social desirability bias, demand characteristics, or memory biases. Therefore, physiological measures are often considered more objective and accurate than self-reports.