Decreased sensitivity to self-inflicted pain.

There is anecdotal and incidental research evidence suggesting that self-inflicted injury is experienced as less painful than when the same injury is applied by another person. We tested the hypothesis that the sensitivity and the ability to tolerate pain differs depending on the person applying the painful stimulus. Self-selected healthy undergraduate students were obtained from the University of Stirling participant panel. None were suffering chronic pain or taking any form of analgesic drug. The participants applied a pressure algometer to themselves and to other participants. Depending on the type of trial, each was terminated when the participant experienced the algometer as either "painful" (for threshold reading) or "unbearable" (for tolerance reading). Both measures of pain, threshold and tolerance, were significantly higher when the algometer had been self-applied compared with when it was applied by another person. The mean difference for pain thresholds was 0.27MPa (95% confidence interval 0.10-0.44, P=0.002), and the mean difference for pain tolerance readings was 0.25MPa (95% confidence interval 0.03-0.48, P=0.028). An unexpected finding was that the mean tolerance score was less when females applied the algometer (P<0.01). When a painful stimulus was self-inflicted this resulted in significantly less pain and a greater ability to tolerate the pain compared with when the same stimulus was applied by another person. If the findings generalized to a clinical context, certain painful or discomforting procedures, such as mammography, removal of wound dressings and lancet withdrawal of blood, should be adapted for self-application by patients.