Widespread findings in the basic and applied pain literatures that have attracted significant clinical attention and financial resources (e.g., in terms of federal funding aimed at reducing health disparities) are those of higher self-reported pain intensity among people with ethnic minority identities as compared to non-Hispanic Whites (NHWs) [1-3]. Current explanations of the ethnic differences in reported pain vary widely depending on the specialties and perspectives of the researchers at hand. Conventional models for explaining ethnic disparities in pain experiences focus on physiological causes (e.g., epigenetic profiles) [4, 5], as well as various and often inter-related psychological (e.g., social learning; coping strategies), sociological (e.g., health promotion behaviors, access to health services), and clinical factors including institutional-based models that highlight the role of under-treatment of pain in ethnic minority patients [6-10]. Another plausible yet overlooked explanation for the ethnic differences in pain sensitivity may be that they are actually the result of (often unavoidable) methodological confounds such as implicit social factors (ISFs) that influence how pain is commonly measured in scientific and medical research settings [11].

There are several converging lines of evidence in support of the hypothesis that the social context in which pain occurs affects the degree of pain that individuals perceive and report [11-13]. ISFs, such as characteristics of people in the immediate situational context, affect nearly every aspect of patient-provider and participant- experimenter interactions, including how people subjectively experience and report their pain, as well as how evaluators voluntarily (e.g., measure) and involuntarily respond (e.g., empathize) with the pain suffering of others [14]. Characteristics of experimenters, including gender and authority role, and even the passive and simulated presence of others (e.g., listening to a stranger’s voice) can influence subjective pain ratings [15-21]. Unfortunately, the majority of previous research on pain sensitivity has been unable to control for these implicit ISFs, even in experiments that were otherwise operating under controlled laboratory conditions. It remains largely unknown how additional ISFs, especially those that play a foundational role in person perception, such as the ethnic identities of the persons evaluating pain in others, may influence pain reporting.

As described above, a common assumption is that racial / ethnic minority persons generally show higher pain sensitivity than NHWs; however, most of this research has been conducted comparing African-Americans to NHWs [3, 8, 22-24]. Less is known about how other ethnic groups, such as Latin-Americans / Hispanics, report their pain, and most of the previous studies on ethnic differences in pain sensitivity have not considered how characteristics of the experimenters themselves affect pain outcomes. Few studies have examined whether researcher ethnicity influences participants’ pain reporting [25-27], and none of this work focused on Hispanics.

This is the first controlled study to examine how Hispanic and NHW women report cold pressor task (CPT) pain sensitivity after interacting with research experimenters with either a concordant or discordant ethnic identity. Three discomfort conditions that varied in stimuli intensity (Condition 1: mild pain; Condition 2: moderate pain), and distraction components (Condition 3) were designed to resemble expedient procedural interactions with intake healthcare/laboratory staff by limiting participant-experimenter interactions to only brief consenting and providing instructions; the experimental paradigm minimizes the potential confound of audience effects, because no researchers are present during the actual CPTs. Similar to previously reported findings [23, 24, 28], it was predicted that Hispanics would show greater pain sensitivity than NHWs with increasing pain intensity, yet that the pain reports would be influenced by the ethnicity of the experimenters who interacted with the participants prior to the actual CPT. It is possible, for example, that participants may respond to the ethnic identity of experimenters differently, depending on the participant’s own ethnicity, which if observed, would confound the ability to reliably measure pain reports in clinical and experimental settings. This potential confound in pain reporting may be particularly pertinent to women due to their generally higher pain reports than men [14, 29], and findings that women may be more sensitive than men to contextual cues in research settings during standard discomfort tasks [20, 30]. Other research shows that the influence of experimenter ethnicity may be different for females and males [27], therefore, the current study focused on the role of examiner ethnicity among women across three discrete experimental conditions.

The final sample consisted of 161 women (54% Hispanic; 18–51 yrs., M_Hispanics = 20.0, SD = 4.4; M_NHWs = 19.9, SD = 4.2). A series of multilevel models were estimated, one for each of the two dependent variables (pain tolerance and pain intensity) across each of the three experiments using the participants’ and experimenters’ ethnic identity separately and the Participant Ethnicity x Experimenter Ethnicity interaction term as predictors (controlling for baseline pain score, experimenter gender, and participant self-esteem.) The results of these multilevel models are shown in Table 1. The first three columns of Table 1 show the results for pain tolerance. In the mild pain condition (Condition 1), there was no main effect of experimenter ethnicity or of participant ethnicity, but there was a significant interaction between the two ethnicity variables in predicting pain tolerance (a so-called “pure” interaction) [40]. A simple- slope analysis revealed that NHW participants tolerated pain for longer after interacting with an NHW experimenter rather than a Hispanic experimenter (B = -95.0 [SE = 63.2], z = -1.50, p = .133, d = .88), whereas the pattern was reversed for Hispanic participants, who tolerated pain for longer after interacting with a Hispanic rather than NHW experimenter (B = 63.4 [SE = 46.5], z = 1.36, p = .173, d = -.66). However, neither effect taken individually reached statistical significance. Stronger effects emerged in the moderate-pain condition (Condition 2; see Table 1). In that condition, there was a main effect of experimenter ethnicity, averaging across NHW and Hispanic participants, participants who interacted with a Hispanic experimenter tolerated pain longer than participants who interacted with a NHW experimenter. However, this experimenter effect varied across NHW and Hispanic participants (again indicated by a significant interaction term). A simple-slope analysis revealed that the ethnicity of the experimenter had no effect on pain tolerance for NHW participants (B = 18.0 [SE = 34.4], z = 0.52, p = .600, d = -.15), whereas Hispanic participants tolerated pain for longer after interacting with a Hispanic experimenter rather than an NHW experimenter (B = 152.5 [SE = 34.8], z = 4.39, p < .001, d = -.89). Fig. (1) shows the pattern of this interaction.

Fig. (1). Predicted pain tolerance (in seconds) in the moderate-pain condition as a function of experimenter ethnicity (x axis) and participant ethnicity (lines) when baseline pain score is 0 and self-esteem is average, averaging across experimenter genders.

There were other significant predictors of pain tolerance in the moderate-pain condition as well. Controlling for the other predictors in Table 1, participants who interacted with a female experimenter rather than a male experimenter tolerated pain for a shorter time. Also, baseline pain was negatively associated with pain tolerance, such that participants with higher levels of baseline pain withdrew from the pain task earlier. Finally, self-esteem was also associated with pain tolerance during the cold-pressor task, such that participants with lower self-esteem withdrew from the task earlier. In contrast, there were no significant predictors of pain tolerance in the distraction condition (Condition 3). The last three columns of Table 1 show the results for pain intensity. In the mild pain condition (Condition 1), as was the case for pain tolerance, there was no main effect of either experimenter or participant ethnicity, but a significant interaction emerged between the two ethnicity variables. In particular, a simple-slope analysis showed that NHW participants reported lower pain levels after interacting with an NHW experimenter rather than a Hispanic experimenter (B = 3.3 [SE = 1.6], z = 2.07, p = .039, d = -1.45), whereas no such experimenter effect was observed among Hispanic participants (B = -1.0 [SE = 1.2], z = -0.83, p = .405, d = .30). A similar result was obtained in the moderate pain condition (Condition 2). In that condition, there was again an interaction between experimenter and participant ethnicity in predicting pain intensity. A simple-slope analysis showed that NHW participants had a tendency to report lower pain levels after interacting with a NHW rather than Hispanic experimenter (B = 1.4 [SE = 0.8], z = 1.69, p = .092, d = -.56), whereas the reverse pattern was observed among Hispanic participants, who reported experiencing lower pain intensity after interacting with a Hispanic rather than NHW experimenter (B = -2.1 [SE = 0.8], z = -2.76, p = .006, d = 1.05). Fig. (2) shows the pattern of this interaction. This is consistent with the pattern shown in Fig. (1), where Hispanic participants also displayed lower pain sensitivity (in the form of greater pain tolerance) after interacting with a Hispanic experimenter. Finally, there were no significant predictors of pain intensity in the distraction condition (Condition 3; see Table 1).

Fig. (2). Predicted pain intensity (self-reported pain level) in the moderate-pain condition as a function of experimenter ethnicity (x axis) and participant ethnicity (lines) when baseline pain score is 0 and self-esteem is average, averaging across experimenter genders.

This is the first study to show that the ethnic identity of laboratory experimenters who engage in minimal procedural interactions with research participants prior to an experimental pain task affects how people report their pain sensitivity. Specifically, we observed a general tendency of Hispanic participants to demonstrate lower pain tolerance and report higher pain intensity following interactions with a NHW rather than a Hispanic experimenter. This effect was not observed among NHW participants, was magnified in the moderate pain condition versus the mild pain condition, and appeared to be extinguished by a simple distraction technique. These unique results build on numerous previous investigations of ethnic differences in pain perception [2, 3] and stand in contrast to three previous investigations which did not find significant interactions between the ethnic identities of participants and experimenters (African American vs. NHW) and pain sensitivity [25-27]. This pain study was also among the first to focus on the influence of ethnicity during minimal procedural interactions with research experimenters, enabling us to account for the large proportion of variance in pain outcomes that was due to latent influences of experimenter characteristics. Nearly one-fifth of the variance in the pain tolerance scores was attributable to characteristics of the experimenters, demonstrating that multiple traits of experimenters who interact even minimally with research participants can affect experimental pain reporting under otherwise controlled laboratory conditions.

In the majority of experimental pain research studies, experimenter characteristics are not described in sufficient detail to gauge the likelihood of confounding effects, and experimenter effects are rarely the focus of investigation. Caution should therefore be applied when interpreting clinical pain assessments and experimental pain reporting when using paradigms that do not tightly control or account for interactions with different researchers and other ISFs. For example, because most previous research examining ethnic differences in pain reporting did not account for examiner characteristics, and given that the examiners in those studies were more than likely NHW, reevaluation of previous findings may be warranted. If anything, considerations of these results should be made when sampling patients for a clinical drug and other therapy trials, as a change in the experimental staff between initial appointments and follow-ups may result in misinterpreted changes in patient pain levels.

The current findings have several potential implications for health care providers. Pain is often considered the 5^th vital sign (joining temperature, heart rate, blood pressure and respiratory rate) and its measurement is considered central for effective and patient-centered care. Because many social cues likely influence how pain is perceived and reported, they likely obscure reliable pain measurements involving individuals and groups (e.g., patient prognosis, institutional outcomes). Variation alone in pain intensity and tolerance or even the reporting of pain intensity and tolerance could have treatment confounding consequences by causing illusory changes (improvement or a decline) in patient well-being, leading to unnecessary and potentially unhelpful fluctuations in care received by patients. Both the continued provision of care by doctors unaware of their own influences on patients’ pain reports, as well as when health providers are on the side of caution, e.g., engage in “defensive care,” can result in excessive care. These seemingly innocuous habits can be not only financially costly but also increase the risk of over-prescription of pain medication, prolong treatment periods, and increase insurance and coinsurance payments, possibly benefiting healthcare providers but certainly not benefiting patients or payers. Over-prescription of certain drugs such as opioid-based treatments (i.e., ‘pain killers’) can also carry unavoidable consequences such as medical and mental disturbances, risk of dependency, withdrawal, dysfunction and limitations in behavior, and risk of accidents (e.g., due to drowsiness) [41-46]. On the other hand, the results could also imply that alleviating pain for some patients might be achievable simply by changing the clinical personnel with whom that patient interacts. The possibility that the ethnicity of the experimenter or health care staff may affect Hispanics’ pain ratings differently than NHWs is particularly important given that Hispanics are one of the largest growing ethnic groups in the United States [47] and thus highly representative of future patient-provider interactions.

While the underlying reasons for the influence of experimenter’s ethnicity on subjective pain reporting remain speculative, they are broadly consistent with the psychological literature on social rejection, in-group/out- group perception, and basic social behavior strategies. Ethnic distribution falls along a social class gradient [48], and people may use some components of ethnicity as a heuristic to approximate in-group/out-group identities according to historical power roles in the local ecology [49]. Previous research suggests that individuals who describe themselves as having less societal influence attend (e.g., visually) to more hostile social threats in their environment [50, 51]. It is possible that a greater attention paid to contextual threats may very well manifest as a heightened awareness of pain, thus accounting for the decreased pain threshold and increased pain sensitivity of the minority participants when evaluated by NHW experimenters. Relatedly, due to the power imbalances between Hispanic and NHW in the U.S [48], Hispanics may show lower pain tolerance following interactions with NHW experimenters as an example of a reflexive and submissive signaling behavior, designed to avoid conflict with competitors when one is in a disadvantaged position [12, 13, 52]. The idea that behavioral adjustments are related to the demonstration of social dominance [48] is supported by overlapping neurobiological substrates of physical (experimental) pain and social pain reporting associated with interpersonal rejection and perceptions of social threat [53, 54].

The notion of stereotype threat [55] may be of particular importance when framing the results above. A stereotype threat occurs when cues in the environment highlight an individual's devalued group membership and results in a decrement in that individual’s performance. The classic research example is racial / ethnic participants exhibiting poorer performance on an academic test when the instructions included implications of the test for measuring intelligence but not when instructions stated it would not reflect intelligence [55]. One possibility may therefore be that Hispanic participants experience stereotype threat or perceived inferiority biases following interactions with a NHW experimenter, thus resulting in ‘poorer’ pain performance, or ability to withstand the discomfort task (though the challenge of enduring the pain task was never made explicit). Another possibility is that the current participants had a tendency to perceive the NHW experimenters as more authoritarian than the Hispanic experimenters, which could have potentiated the impression that the experiment entailed greater (controlled) risk and intensity of noxious stimuli. Despite these uncertainties, the current findings are consistent with several converging lines of evidence that support the hypothesis that the social context in which pain occurs affects the degree of pain that individuals perceive and report [11-13]. Numerous other salient person perception characteristics (referred to here as ‘ISFs’) such as gender and physical stature likely also affect pain reporting in experimental and clinical settings [14, 27], with ethnicity being but one confounding factor.

Despite these potentially important implications, a discussion of the limitations of the study is warranted. A general limitation in the field is that most racial/ethnic studies are conducted with racial/ethnic groups in European and U.S. settings where NHW persons are the majority, so the current results may not generalize to countries where Latinos represent a majority rather than minority population. However, in the few studies that examine differences in ethnic populations where participants are tested in majority contexts (i.e., their home country), prototypical differences were minimal or even reversed [56, 57]. Therefore, although self-reported Hispanic ethnicity was used as a variable in this study, it is important to recognize that the Hispanic population is composed of many different cultural groups with diverse genetic make-ups [58-62], and the lack of measurement of acculturation [63] and other differences across Latino cultural groups (e.g., Mexican American, Cuban American, etc.) are limitations as well. It should be noted that the study was comprised of participants attending a State university with relatively similar cultural and ethnic backgrounds (i.e., with historical family origins from the Southwestern United States).

Finally, pain reactions caused by the CPT could be different than other types of pain sensations experienced in laboratory discomfort tasks and clinical settings, potentially limiting the generalizability of the observed effects of ethnicity. Methodologically, the study did not control for handedness, which has been shown to influence CPT measurements [64], as well as other individual-level characteristics and normative behaviors (e.g., exposure to cold water) of participants that may correlate with pain reporting. The sample was also comprised of healthy, young, female university students, whose responses may be different than other demographic groups. On the other hand, some benefits of using a relatively homogeneous sample is that it helped control for variability in pain reporting due to cultural background, financial status, comorbidities, and other potential confounding effects (e.g., participant gender).

Taking these potential limitations into account, the current findings challenge the belief that the concept of ethnicity exists in a vacuum, by showing it can have immediate or even latent influences on subjective pain reporting. Unless directly measured or controlled (e.g., via automated pain assessment procedures), examiner characteristics and other ISFs will likely remain confounding factors for the ability to reliably measure subjective pain reporting, provide cost-effective and equitable patient care, and understand the underlying associations between pain and disease.