It is generally accepted that gonadal sex hormones contribute to greater clinical and experimental pain experiences in women as compared to men [
Implicit and often unavoidable social experiences occur in laboratory and clinical settings in ways that can modulate the heuristical expression of verbal and nonverbal pain behaviors (see [
In the current study, we explore the hypothesis that experimenter gender influences experimental pain sensitivity differently for women depending on whether they are naturally cycling (i.e., versus hormonal contraceptive users) and whether they are at a relatively low or high fertility phase of their menstrual cycles. In theory, female-male interactions (and to a lesser extent female-female interactions) have the highest relevance to biological fitness and are thus the most important during peak fertility stages of the menstrual cycle. We therefore anticipated that the gender of experimenters who provide basic procedural instructions for conducting an experimental discomfort task (CPT) will have latent influences on pain reporting, particularly among women at high fertility phases of their cycles as compared to women at low fertility phases or women using a hormonal contraceptive. We estimated fertility state based on the probability of conception according to the participants’ current cycle day (reverse counting method), and we simulated minimal procedural interactions with research personnel by limiting participant-experimenter interactions to only a brief consenting process and scripted experimental instructions. No researchers were present during the actual pain task. The experimental paradigm therefore allowed us to measure whether minimal social interactions with either a male or female researcher has latent influences on experimental pain reporting in healthy young women depending on the fertility state.
The study protocol was approved by the University of New Mexico’s Institutional Review Board, and informed written consent was obtained from all participants. Undergraduate students received extra credit for an introductory psychology course for their participation. Participants who self-identified contraindications to the CPT were excluded from the study; these included taking any pain medications or having a problem that would increase risk of the CPT, including illnesses related to a cardiovascular disorder (e.g., high blood pressure, heart problems, or heart rhythm concerns), history of fainting or seizures, history of frostbite, having an open cut, sore or bone fracture on the limb to be immersed in water, or a history of Reynaud’s phenomenon. All participants signed up via the University of New Mexico Sona website for psychological research volunteers. To be included in the analysis, females had to be naturally cycling (i.e., nonhormonal contraceptive use) or using a hormonal contraceptive and had to have not yet experienced menopause. The final sample consisted of 132 women with complete data for inclusion in the study (18–48 yrs,
Experimenters (7 females and 12 males) assisted participants through the protocol. The gender of the researcher was operationalized as the outward expression or appearance of culturally defined masculinity and femininity. Forty-seven percent of participants (
Experimenters followed a scripted protocol in order to minimize any possible influences from interpersonal factors (e.g., duration of conversations, eye contact, and personal factors) that are not contributing to researcher’s gender identity. After participants were finished with the consenting process, they were measured for physical characteristics (e.g., height, not used in the current study), which requires minimal and noninvasive physical contact between the subject and the experimenter, and subjects provided information about their menstrual functioning, which usually took between 3 to 5 minutes to complete. Afterwards, participants were guided to an assessment room where they were left alone to fill out a demographic questionnaire and watch a short video regarding the instructions for the CPT. The video depicted a male researcher performing the experiment with explicit directions provided in a female voice. The video also described the directions for using the cold pressor apparatus and how to indicate the various pain measurements. Again, to minimize exposure to the participant, the experimenter was not present while the participants completed the demographic survey and video portion of the experiment; this portion of the experiment usually lasted between 20 and 30 minutes.
Next, the participants were escorted to the room containing the cold pressor apparatus and a small laptop with a user interface pain assessment program. This program recorded the development of pain as participants indicated (by pressing a corresponding icon) when the discomfort sensation began to feel painful (pain threshold) and when they quit the task (pain tolerance). Finally, the researcher addressed any questions the participants had about CPT instructions, and the participants were told that they could choose when to begin the task. The experimenter then left the room and closed the door behind her/him. The participant performed the CPT without the presence of the researcher in order to minimize the possible influence of audience effects. Researchers monitored the participants via a hidden video camera during the task to ensure the adherence to instructions. None of the resulting CPT sessions were interrupted mid experiment by an experimenter. In total, the participants interacted with the researchers for roughly 5–10 minutes (e.g., for the consent process, to take body measurements, to escort participants to the various laboratory rooms, and for answering questions) throughout the experiment up to the CPT.
A basic questionnaire created by our lab gathered information about sex, age, ethnicity, education, and family background. The menstrual-related information included whether or not the participant was currently menstruating, usage and type of hormonal contraceptives, average number of days in their typical menstrual cycle, and number of days since their last menstrual cycle (from the date of assessment). Participants were provided a calendar to calculate their responses.
Participants were seated in a chair between the cold pressor apparatus (on the participant’s left side) and the laptop computer (right side) in a small room (2.0 m × 2.5 m). The mechanical CPT device was an Isotemp 6200R28 refrigerated bath circulator (reservoir size: 29 cm × 16.5 cm × 22.4 cm). The machine circulates the water automatically and maintains a consistent water temperature by dual heating and cooling actions. The water temperature was set to 5°C (known to produce a range of pain tolerance levels with only minimal ceiling effects [
The pain assessment program displayed an initial screen with the general CPT instructions. The researcher verbally reiterated the instructions by describing that when participants choose to both begin (after the researcher left the room) and end the task (at maximum pain tolerance) they were to perform two simultaneous actions. To begin the task (and initiate the pain assessment program), participants were instructed to first indicate their baseline (premanipulation) pain severity along a standard visual analog scale (VAS, 0–10 from
The CPT was observed on a video monitor from a remote location, and the researcher returned to the experimental room to debrief the participant once they retracted their hand from the water or after the maximum duration of 5 minutes had occurred (the participants were not informed of this time limit before beginning the pain task). Following debriefing, participants were asked to rest for five minutes to ensure they no longer felt any physical discomfort from involvement in the study and that their heart rate had returned to resting.
Individual-level fertility level was calculated using the Wilcox findings which provide a precise estimate of the probability of conception based on a standardized 28-day cycle [
The pain scores included the participant’s pain threshold and pain tolerance (measured in seconds after submersion). Lower threshold and tolerance scores are interpreted as indicating greater CPT pain sensitivity as is common in the pain literature. Multilevel (random-effects) models were used to examine the separate effects of experimenter’s gender (coded 0 = male, 1 = female) and fertility stage of the participant’s menstrual cycle (coded 0 = not fertile, 1 = fertile) on the pain scores separately in the naturally cycling and hormonal contraceptive-using women; age, baseline pain intensity (prior to CPT), weight, and experimenter’s ethnicity (coded 0 = White, 1 = Hispanic) were entered as covariates. Multilevel models were preferred over linear regression or analysis of covariance because the same 19 experimenters were used across all 132 pain tasks; our multilevel models account for the repeated measures across experimenters. We also reported Cohen’s
All of our analyses were conducted in R v3.0.2 [
In order to examine whether gender of the experimenter interacts with fertility stage to influence pain reports, a multilevel model was estimated for each of the pain scores (pain threshold and pain tolerance) separately for natural cycling and hormonal contraceptive-using women. For these models, experimenter gender, fertility stage, and the gender × fertility stage interaction terms were entered as predictor variables along with the covariates (age, baseline pain, weight, and experimenter ethnicity). For women on a natural cycle, a significant experimenter gender × fertility stage interaction term emerged for pain tolerance,
Follow-up analyses examining the effect of experimenter gender on pain tolerance reports separately for naturally cycling and hormonal contraceptive-using women at the high and low fertility stages (entering the covariates) showed a significant effect of experimenter gender for the naturally cycling women at high fertility only,
Plot of significant fertility × experimenter gender interaction for pain tolerance in naturally cycling women. Higher values on the
Follow-up analyses examining the effect of experimenter gender on pain threshold separately for naturally cycling and hormonal contraceptive-using women at the high and low fertility stages (entering the covariates) failed to show any effect of experimenter gender for naturally cycling women at high fertility,
This study shows for the first time that the gender of persons in the immediate social context influences how a female research participant subsequently reports pain experiences differently across fertility phases of the menstrual cycle at the time of participation. At a fertile phase of their cycle, naturally cycling women showed significantly higher pain tolerance following interactions with a male, but not a female, experimenter. These differences were not found for women at a relative infertile phase or if they were using hormonal contraceptives. In general, these findings build on previous research on social relationships and menstrual-dependent changes in pain sensitivity [
These results are consistent with the broader literature showing that females have evolved numerous adaptations for experiencing fertility-dependent changes in basic components of social cognition (e.g., feelings, dispositions, and judgments) in ways that could modify affiliation and avoidance with others, particularly men. Menstrual-related changes in social cognition have been found across several domains of psychological functioning, including romantic relationship preferences, relationship satisfaction, risk avoidance, and social decision-making [
The first explanation for reduced pain sensitivity around ovulation in the presence of a male is that reduced pain sensitivity facilitates sexual interaction and successful insemination by reducing pain intensity associated with the copulation act itself. Previous research has shown that at least one-third to half of females experience pain during their first sexual encounter [
The second explanation for reduced pain sensitivity around ovulation in the presence of a male is that decreased pain sensitivity following exposure to unrelated males operates alongside other psychological and somatic changes across the menstrual cycle that evolved to reduce the risks of rape, ultimately facilitating rape avoidance [
The proximate mechanisms that may process social experiences on pain perception across the menstrual cycle remain unclear. For example, one possibility is that women may experience elevations in oxytocin in the presence of unfamiliar males. It is well established that oxytocin is involved with social behaviors such as social distance and eye contact (see [
The current findings may also have direct implications for health care providers and research investigators. Pain is often considered the fifth vital sign alongside temperature, heart rate, blood pressure, and respiratory rate, and pain measurement is central for effective and patient-centered care. Many implicit and currently unavoidable social cues, such as examiner characteristics, have been shown to contribute to measurement error in patient pain reporting [
In addition to these implications, a discussion of our study’s limitations is warranted. One limitation is the relatively small sample size, disallowing examination of additional social factors such as quality of peer and romantic relationships, which have been shown to covary with experimental pain reports in women [
Nonetheless, the current findings have wide-reaching implications for (a) standardizing pain measurement protocols, (b) understanding basic biopsychosocial pain-related processes, (c) addressing clinical pain experiences in women, and (d) understanding how social interactions influence felt pain. Previous investigations delineated both significant and negligible associations between menstrual cycling and pain intensity, and the current study highlighted a significant methodological-level confounding factor, namely, the gender of laboratory personnel interacting with participants, which has not been controlled in most previous studies. Finally, the current results have implications for understanding how social experiences and the motivation to either affiliate or avoid selective social affiliates in the immediate context may induce changes in somatic (e.g., sensory and perceptual) functioning, including exogenous pain sensitivities.
The authors declare no financial or other conflicts of interests in the design and interpretation of study results.
The authors thank David Wilson at the University of North Florida for designing the pain measurement software.