Sex differences in facial encoding of pain.

UNLABELLED: There is substantial evidence that men and women differ in their perception and experience of pain. However, research on sex differences in pain has mainly relied on self-report ratings, whereas little is known about sex differences in facial expression of pain. The aim of the present study was to investigate: 1) whether men and women differ in their facial expressiveness of pain; and 2) whether sex modulates the relationship between self-report and facial pain responses when tonic experimental pain is applied. Forty young and pain-free individuals (male n = 20, female n = 20) were investigated for their subjective and facial responses to tonic heat stimulation at both painful and nonpainful levels. Tonic heat stimulation was tailored to the individual pain threshold. Self-report was assessed via visual analog scales. Facial expression was objectively examined using the Facial Action Coding System. Correlation analyses for the relationship between self-report and facial expression of pain were conducted. Men and women differed neither in self-report ratings nor in facial responses during tonic heat stimulation. However, sex had a considerable impact on the relationship between these variables. Whereas no significant correlations at all were found for men, we obtained several significant correlations in woman. For that reason, future studies investigating the relationship between self-report and nonverbal pain behaviors should consider sex as an important modulating factor. PERSPECTIVE: The findings of the present study suggest that facial responses to pain can be used as estimates of the intensity of subjective pain in women better than in men.