Sex differences in pain perception have been noted in multiple studies, with women typically displaying lower pain tolerance than men, but it is unknown whether the mechanisms underlying these differences are hormonal, genetic or psychosocial in origin. For example, some researchers have suggested that men are more motivated to express a tolerance for pain because masculine stereotyping encourages it, while feminine stereotyping encourages pain expression and lower pain tolerance.
"These findings suggest that motivation does not account for the sex difference in pain tolerance," says study author Roger B. Fillingim, Ph.D., of the Department of Operative Dentistry at the University of Florida and the Gainesville VA Medical Center in Gainesville, Fla.
Fillingim and colleagues enlisted 81 undergraduates at the University of Alabama at Birmingham, with roughly equal numbers of males and females, as study participants. Half of the students were assigned to a low-incentive group, in which students received just 5 cents for every 15 seconds they kept their hand submerged in ice water. The remaining students in a high incentive group received $1 for every 15 seconds their hand remained submerged.
Before enduring the cold water, students were asked to predict how painful the experience would be, how they would perform and how motivated they were by the monetary reward. During the experience, in which they submerged their right hand in cold water up to their wrist until they could no longer tolerate the pain, the researchers measured their pain and took blood pressure and heart rate measurements. After removing their hand from ice water, the students completed questionnaires that measured their pain, stress, perceived performance, and motivation to keep their hand submerged.
Like in previous pain studies, males had higher pain thresholds and tolerances than females, with the female students providing higher pain ratings 30 seconds after their hand was submerged, as well as on the questionnaire afterward. The study results are published in the current issue of Psychosomatic Medicine.
The researchers predicted that participants with a greater incentive to endure the cold water would exhibit a more robust heart rate or higher blood pressure responses, but this did not happen. Instead, blood pressure and heart rate measures increased equally across all groups during the cold water submersion. But the researchers did note differing patterns of responses depending on incentive.
"The monetary incentive did not influence pain responses, but the relationship between cardiovascular measures and pain responses was influenced by the incentive manipulation," says Fillingim. Specifically, low incentive subjects with higher blood pressure at the start of the study period tended to tolerate pain better. But this association was not seen in the high incentive subjects. For the high incentive subjects, a leap in blood pressure (which is a sign of being engaged in a task) was associated with having higher pain tolerance.
The researchers point out that psychological factors can influence the association between physiological responses -- such as heart rate and blood pressure -- and pain responses. It is important to understand "these complex interactions among biological and psychosocial variables, rather than artificially separating their effects," says Fillingim says.
"Additional research is needed to replicate these findings and to further elucidate the relationships among motivation, gender roles, and pain responses," he concludes.
This study was supported by National Institutes of Health.
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