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Abstract

The Behavioral Immune System (BIS) drives disgust-based avoidance, but its response to immunological changes during pregnancy is understudied. The Compensatory Prophylaxis Hypothesis (CPH) predicts heightened disease-avoidant social behavior in early pregnancy due to immunosuppression. We examined whether first-trimester women show reduced willingness to engage with outgroup members or individuals displaying infection signs compared to later pregnancy stages and postpartum. We also assessed moderation by perceived vulnerability to disease (PVD) and perceived COVID-19 threat. Data were collected in Poland during the COVID-19 pandemic via online surveys in two studies. In the cross-sectional Study 1 (N = 425, age 18–46, M = 29.64, SD = 5.37), pregnant participants at different stages of pregnancy assessed their willingness to engage in social contact with individuals displaying or not displaying infection cues shown in photographs, and then completed measures of PVD and Perceived Threat of COVID-19. The longitudinal Study 2 (N = 112, age 22–41, M = 30.14, SD = 4.05) employed the same procedure, administered four times—during the first, second, and third trimesters, and postpartum. Findings did not support CPH. Social avoidance was unrelated to the pregnancy stage but significantly associated with PVD and COVID-19 threat. Results underscore the situational and individual flexibility of BIS responses.

Keywords

behavioral immune system compensatory prophylaxis hypothesis perceived vulnerability to disease COVID-19 threat social contact

Introduction

Throughout evolutionary history, humans have faced the persistent threat of pathogens. In response, organisms developed both physiological immune defenses (Nicholson, 2016) and behavioral mechanisms known as the Behavioral Immune System (BIS; Ackerman et al., 2018; Curtis et al., 2011; Murray & Schaller, 2016). The BIS may be particularly relevant during periods of reduced immunocompetence, such as early pregnancy, when heightened disease vulnerability could shape social behavior (Fessler et al., 2005; Fessler & Navarrete, 2003) as well as during periods of elevated pathogen risk, such as the COVID-19 pandemic. This project explores how BIS activation manifests socially throughout pregnancy.

Complexity of the BIS

Unlike the physiological immune system, which acts after infection and is metabolically costly (Abbas et al., 2014; Khansari et al., 2009; Konsman et al., 2002; LeVine et al., 2001), the BIS evolved to reduce exposure to pathogens in the first place (Ackerman et al., 2021). It includes a broad repertoire of avoidance behaviors, with disgust functioning as a core mechanism (Curtis et al., 2004; Curtis & Biran, 2001; Fessler & Navarrete, 2003; Gassen et al., 2018; Landry et al., 2021; Oaten et al., 2009). Disgust plays a significant role in shaping interpersonal behavior, influencing various aspects such as hygiene practices (Curtis et al., 2011; Wormley & Varnum, 2023), social distancing (Stevenson et al., 2009, 2011; Szymkow et al., 2021a; Tybur et al., 2009; Wicker et al., 2003), and the enforcement of social norms (Curtis, 2011; Tybur et al., 2009). It affects social attitudes, leading to outgroup dehumanization (Buckels & Trapnell, 2013), prejudice (van Leeuwen et al., 2023; Zakrzewska et al., 2019), and reduced social trust (Aarøe et al., 2016).

The BIS modulates social interaction patterns, especially when environmental cues indicate a risk of infection; for example, people tend to maintain greater physical distance when others show signs of illness (Ackerman et al., 2021; Brown et al., 2017; Fan et al., 2022; Schaller & Neuberg, 2012; Shook et al., 2020; Szymkow et al., 2021a). These effects are often triggered by visually salient cues, such as rashes, skin color changes, or coughing (Park, 2015). However, they can also occur even when there are no obvious symptoms, as individuals living with HIV may be avoided due to the stigma associated with AIDS (Fauk et al., 2021).

COVID-19 likely intensified BIS-related responses. The pandemic, caused by the SARS-CoV-2 virus, emerged in late 2019 and was declared a global pandemic by the World Health Organization in March 2020 (Cucinotta & Vanelli, 2020; World Health Organization [WHO], 2020). It posed substantial threats to public health, with high infection and mortality rates worldwide. In Poland, COVID-19 accounted for 6% of all deaths in 2020, and the government implemented measures including lockdowns, mandatory mask-wearing, and social distancing (Sagan et al., 2022). Perceiving the virus as a significant threat has been associated with increased social prejudice, including homonegativity (Szymkow et al., 2021b). Even subtle cues, such as group membership, can activate BIS responses. According to the outgroup-as-pathogen-cue hypothesis, unfamiliar individuals may be perceived as potential carriers of novel pathogens (Bressan, 2021; Brown et al., 2019; Diamond, 2017; Schaller & Neuberg, 2012). This framework helps explain findings that exposure to disease-related cues increases prejudice against immigrants (Faulkner et al., 2004) and strengthens ethnocentric attitudes (Navarrete & Fessler, 2006).

Importantly, pathogen disgust sensitivity has been shown to predict negative attitudes toward ethnic minorities across various populations (Aarøe et al., 2017). However, some studies suggest that visible pathogen cues may have a greater influence on social avoidance than group membership alone (Fan et al., 2022; van Leeuwen & Petersen, 2018). Bressan (2021), in a reanalysis of data from van Leeuwen and Petersen (2018), showed that perceived dissimilarity, rather than categorical group status, might be a more relevant trigger for social avoidance. Furthermore, outgroups may be avoided if they are perceived as uncooperative (van Leeuwen et al., 2023).

The Flexibility of the BIS in the Context of the Compensatory Prophylaxis Hypothesis (CPH)

The BIS adapts flexibly to various environmental, situational, and individual factors (Schaller & Park, 2011). Importantly, contextual environments can also calibrate disgust responses, with individuals showing heightened avoidance in high-risk situations and reduced avoidance when pathogen exposure is unlikely (Cepon-Robins et al., 2021). This dynamic adjustment helps determine when and how pathogen-avoidance mechanisms, including those proposed by the CPH, are expressed.

These contextual effects are reflected in observable social behaviors. For instance, the BIS reduces social interactions in settings with high disease risk, such as during the COVID-19 pandemic (Bacon & Corr, 2020; Meleady et al., 2021; Miłkowska, Galbarczyk, Klimek, et al., 2021; Miłkowska, Galbarczyk, Mijas, et al., 2021; Stevenson et al., 2021; Szymkow et al., 2021a, 2021b). These circumstances likely heightened perceived vulnerability to disease (PVD), making this period particularly relevant for testing the CPH. Additionally, individual differences—such as disgust sensitivity, personality traits, hormone levels, and conservativeness—affect how the BIS responds. For example, men with strong immune markers (characterized by high testosterone and low cortisol) tend to prefer mates who exhibit lower vulnerability cues (Kandrik et al., 2017), and individuals higher in conservativeness tend to show greater avoidance of potential pathogen sources (Tybur et al., 2016).

The CPH (Fessler & Navarrete, 2003) posits that disgust compensates for temporary decreases in immunocompetence, particularly during early pregnancy. Indeed, Fessler et al. (2005) found that women in their first trimester exhibited heightened sensitivity to disgust, a finding that was later replicated by Żelaźniewicz and Pawłowski (2015).

However, more recent research reframes rather than rejects the notion of immune adaptation during early pregnancy. While earlier perspectives emphasized immunosuppression to prevent fetal rejection (e.g., Fessler & Navarrete, 2003; Żelaźniewicz & Pawłowski, 2015), current evidence suggests a more dynamic immunomodulation that balances fetal tolerance with necessary inflammatory responses supporting implantation and placental development (Abu-Raya et al., 2020; Hové et al., 2020; Kwon et al., 2025; Racicot et al., 2014). For example, Kwon et al. (2025) found that pregnant women who experienced stronger odor and food aversions, as well as nausea and vomiting, exhibited a shift toward a pro-inflammatory Th1 cytokine profile. This pattern suggests that such physiological and behavioral changes may co-occur with transient immune activation supporting early embryogenesis rather than reflecting general immune suppression. In this view, the maternal immune system is not uniformly downregulated but shifts between pro- and anti-inflammatory states across gestation.

Consistent with this, research examining hormonal correlates of disgust, particularly progesterone, has yielded inconsistent results (Jones et al., 2018; Rafiee et al., 2022; Stern & Shiramizu, 2022; Timmers et al., 2018). The CPH posits that higher progesterone levels (e.g., during the luteal phase of the menstrual cycle) suppress physiological immune responses, leading to increased behavioral prophylaxis through heightened disgust toward pathogen cues to compensate for reduced immunity. Whereas some research confirms these predictions (Fleischman & Fessler, 2011; Miłkowska et al., 2021a, 2021b; Żelaźniewicz et al., 2016), other studies report null results (Jones et al., 2018; Rafiee et al., 2022; Stern & Shiramizu, 2022; Timmers et al., 2018). Elevated disgust in early pregnancy may therefore reflect not direct hormonal effects but heightened vulnerability during organogenesis (Fessler et al., 2005). Supporting this interpretation, recent studies have found that disgust sensitivity tends to increase when immune markers are low (Kaňková et al., 2022), when hCG levels are reduced (Kaňková et al., 2023), after recent illness (Dlouhá et al., 2023), or in contexts of elevated pathogen risk, such as during the COVID-19 pandemic (Kaňková et al., 2023). Thus, heightened disgust may represent an adaptive behavioral response parallel to immune modulation, enhancing avoidance of potential pathogens during a particularly vulnerable developmental period.

Present Studies

Activation of the BIS is considered crucial during early pregnancy, but its social consequences for pregnant women remain understudied. In two studies (one cross-sectional and one longitudinal) conducted among women at different stages of pregnancy, we examined their readiness to engage in social contact with ingroup and outgroup members, both with and without visible signs of infection. Based on previous research documenting heightened pathogen avoidance during early pregnancy (Navarrete et al., 2007), increased social withdrawal in response to pathogen cues (Dlouhá et al., 2023), and greater sensitivity to infection threats among pregnant women (Żelaźniewicz & Pawłowski, 2015), in Study 1, we hypothesized that social avoidance—reflecting pathogen-avoidance mechanisms—would be most pronounced during the first trimester. We expected lower readiness to interact with outgroup members compared to ingroup members, especially when signs of infection were present. Additionally, we predicted that these effects would be stronger among women with higher PVD (Faulkner et al., 2004; Park et al., 2003) and among those experiencing greater threat related to COVID-19.

Study 2 tested the same hypotheses as Study 1 but also included women up to 4 months postpartum—a critical period for newborn protection—and we therefore hypothesized that heightened disgust sensitivity and social avoidance would be particularly pronounced during this stage, especially in the context of the COVID-19 pandemic.

All data and additional analyses are available on Open Science Framework—OSF Files. The study was approved by the Institutional Ethical Review Board at SWPS University.

Study 1

Method

Participants

A total of 425 pregnant women from Poland, aged 18 to 46 (M = 29.64, SD = 5.37), were recruited via the Ariadna research panel in April 2021, during the early phase of the third COVID-19 wave. All participants were ethnically European (White/Caucasian). The recruitment process focused solely on pregnancy status, without considering gestational age. Participants received 50 PLN (approx. US$13). Trimester classification was based on the gestational week, which was calculated from the conception date recorded in their pregnancy books. The first trimester (T1) included women who were up to 12 weeks pregnant (n = 130; 30.59%; M_WoP = 8.07, SD_WoP = 3.29), the second trimester (T2) from 13 to 24 weeks of pregnancy (n = 191; 44.94%; M_WoP = 19.83, SD_WoP = 4.14), and the third trimester (T3) at more than 24 weeks (n = 104; 24.47%, M_WoP = 33.44, SD_WoP = 3.52). For a more detailed description of the study sample, see Supplementary Materials.

Materials and Procedure

Participants were informed that the study aimed to examine pregnant women's attitudes toward current social phenomena. We asked participants to assess their readiness to engage in social contact with individuals depicted in the photos. After the main part of the study, they completed the PVD Scale (PVD; Duncan et al., 2009) and the Perceived Threat of COVID-19 Questionnaire (PCTQ; Conway et al., 2020). Finally, they answered demographic and health-related questions (e.g., recent infections, vaccinations, autoimmune diseases) and provided pregnancy-related information, including gestational age, fetal sex, previous pregnancies, frequency of nausea, and treatment options.

Experimental Manipulations

Each participant was randomly presented with a subset of 20 pictures. This set included 16 faces selected from 48 individuals in the Pathogen-Relevant Face Database (Tołopiło et al., 2021), following a 2 (European/ingroup vs. Asian/outgroup) × 2 (female vs. male) × 2 (without vs. with infection signs) design. Additionally, four neutral faces (two male, two female) from the Warsaw Set of Emotional Facial Expression Pictures (Olszanowski et al., 2015) were included as buffer stimuli (see Figure 1). The infection cues included in the stimuli consisted of changes in skin color and rashes, consistent with the visually salient cues of infection described in previous research on BIS (e.g., Ackerman et al., 2018; Curtis et al., 2011; Murray & Schaller, 2016; Park, 2015). These features are well-established elicitors of pathogen-related disgust and allow for testing responses to infection-relevant facial signals. The use of European (Polish/ingroup) versus Asian (outgroup) faces was theoretically motivated by Poland's high ethnic homogeneity: all participants in our studies were ethnically European (White/Caucasian), and over 97% of Polish residents identify as such (GUS, 2021). Consequently, participants have very limited daily exposure to individuals of visibly different ethnic backgrounds, such as East Asians. Previous research suggests that faces deviating from the prototypical morphology of the local population are more likely to be categorized as outgroup members and elicit stronger behavioral immune responses (Bressan, 2021). Therefore, contrasting local European faces with Asian faces allowed us to test responses to ethnic outgroup cues in line with pathogen avoidance theory.

Figure 1.

Schema of example faces presentation used to measure readiness for social contact in Studies 1 and 2.

Dependent Variable: Readiness for Social Contact

Participants’ readiness for social contact was measured by averaging responses to three questions about each person pictured: “How much would it be ok for you to”: “Shake this person's hand?, “Sit next to this person on the bus?”, “Lend this person your mobile?”. Participants rated each of these situations using an 11-point scale (from 0 = definitely would not be ok to 10 = definitely would be ok). As averaged responses for each statement constituted reliable measures (all Cronbach's α > .90), we created four overall indexes of readiness for social contact with: ingroups with no infection signs (α = .90), ingroups with infection signs (α = .92), outgroups with no infection signs (α = .95), and outgroups with infection signs (α = .92).

Moderator 1: PVD

The subjective perception of susceptibility to diseases was measured by the PVD Scale (Duncan et al., 2009). The scale comprises 15 statements regarding subjective beliefs about an individual's susceptibility to catching diseases. Each statement was assessed by respondents on a 7-point scale (from 1 = strongly disagree to 7 = strongly agree). The scale consists of two subscales, namely Perceived Infectibility (PI; α = .80) and Germ Aversion (GA; α = .71). The PI subscale pertains to an individual's beliefs regarding their susceptibility to contracting infectious diseases (e.g., “If an illness is ‘going around’, I will get it”). The GA subscale assesses the emotional discomfort experienced in situations with a high risk of transmitting pathogens (e.g., “It does not make me anxious to be around sick people'’).¹

Moderator 2: The Perceived Threat of COVID-19

To measure the subjective threat of COVID-19, we used a short version of the Perceived Coronavirus Threat Questionnaire (PCTQ; Conway et al., 2020). Participants assessed their agreement with three statements on a 7-point scale (from 1 = strongly disagree to 7 = strongly agree). Within this scale are subsumed items measuring to what extent participants perceive the coronavirus as a threat to their life (e.g., “I am afraid of COVID-19”). Ratings for these items were averaged to form an index of the perceived threat of COVID-19 (α = .93).

Data Analysis Strategy

We used multilevel modeling (Linear Mixed Models) with restricted maximum likelihood estimation (REML) to account for fixed and random effects. Analyses were performed in R using the lme4 and lmerTest packages (Bates et al., 2015; Kuznetsova et al., 2017), applying Type III Satterthwaite approximations for degrees of freedom and Bonferroni correction for multiple comparisons. Random intercepts and slopes (for ethnicity and infection signs) captured participant-level variability.

We followed a stepwise model-building strategy: starting with a baseline model (ethnicity, infection signs, pregnancy trimester), then adding moderators (PVD and COVID-19 threat) to assess their individual contributions. Model selection was based on the Akaike Information Criterion (AIC) and likelihood ratio tests. We extracted marginal and conditional R² (via the performance package; Lüdecke et al., 2021) to evaluate variance explained by fixed and random effects (Hox et al., 2017).

Results

First, we conducted correlation analyses between all continuous variables. Considering that the sample consisted of different participants for each trimester, we first conducted Pearson's correlation analyses between readiness for social interactions and other variables for the three trimesters together, as well as for each trimester independently. For all three trimesters combined, the correlations indicated a negative relationship between readiness for social interactions and Germ Aversion (GA; r = −.16, p < .001), as well as perceived infectability (PI; r = −.14, p < .001). The relationship between readiness for social interactions and the COVID-19 threat was not significant (r = .01, p = .885). However, the results varied depending on the pregnancy trimester (for detailed statistics see Table 3 in Supplementary Materials), with a negative correlation between readiness for social interactions and GA being significant only in the third trimester (r = −.25, p = .01) and PI only in the second trimester (r = −.20, p = .005).

The Baseline Model

The baseline model included only fixed factors—pregnancy trimester (first vs. second vs. third), ethnicity (ingroup vs. outgroup), and infection signs (present vs. absent)—along with random intercepts and random slopes for infection signs. Model performance was R² = .90 (conditional) and R² = .08 (marginal), which refers to a weak effect size (Cohen, 1988).

Contrary to our expectations, the main effect of the trimester was not significant (p = .281). However, there was a significant effect of ethnicity, with lower readiness for social interactions with outgroups (M = 2.25, SE = 0.10) than with ingroups (M = 2.47, SE = 0.10), F(1, 844) = 36.85, p < .001, η_p² = .04. A significant effect of infection signs was also observed, as participants were less willing to interact with individuals displaying facial infection signs (M = 1.75, SE = 0.12) compared to those with non-infected faces (M = 2.97, SE = 0.09), F(1, 422) = 240.23, p < .001, η_p² = .36.

Importantly, there was a significant interaction between trimester and infection signs, F(2, 422) = 6.35, p = .002, η_p² = .03. The simple effect analysis revealed one significant result: independently of ethnicity, readiness for social interactions with infected-looking individuals was particularly lower in the first trimester (M = 1.41, SE = 0.21) compared to the second trimester (M = 2.07, SE = 0.17; see Figure 2), t(422) = 2.46, p = .038, d = 0.93. However, the differences between the third trimester compared to the first and second trimesters were not significant (ps > .05). The interaction effects involving ethnicity, as well as the three-way interaction, were not significant (ps > .05).

Figure 2.

Mean readiness for social interactions with individuals who displayed (or not) facial infection signs presented separately for each trimester in Study 1.

The PVD as a Moderating Variable

To test the moderating effect of PVD (Duncan et al., 2009), we extended the baseline model by including two PVD subscales—Germ Aversion (GA) and PI—as separate continuous factors, leading to two potential four-way interactions. The best-fitting model included random slopes for ethnicity and infection signs, with performance of R² = .92 (conditional) and R² = .12 (marginal), indicating a weak-to-moderate effect size (Cohen, 1988).

The analysis revealed that only GA (not PI, p = .233) was a significant predictor of readiness for social interactions, F(1, 416) = 6.21, p = .013, η_p² = .01. Importantly, GA levels did not differ across pregnancy trimesters (ps > .05).

Unlike the baseline model, the main effects of ethnicity, infection signs, and their interaction were no longer significant (ps > .05). However, a significant interaction emerged between infection signs and GA, F(1, 416) = 31.31, p < .001, η_p² = .07, as well as the interaction of pregnancy trimester, infection signs, and GA, F(2, 416) = 5.42, p = .005, η_p² = .03 (see Figure 3). As the two-way interaction was further qualified by the higher-order interaction, we focused our interpretation on the latter.

Figure 3.

Mean readiness for social interactions with individuals who displayed (or no) facial infection signs depending on the germ aversion level in Study 1.

The simple effects analysis of pregnancy trimester, infection signs, and GA interaction showed that, against our hypothesis, both for non-infected and infected faces there were no significant differences between trimesters depending on the GA levels (ps > .05). However, separate analysis for each trimester indicated that in the first trimester, higher GA levels were associated with lower readiness for social interactions, but only with individuals displaying infection signs, F(1, 129) = 18.88, p < .001, η_p² = .13. In the third trimester, higher GA levels led to reduced social readiness overall, F(1, 101) = 6.47, p = .012, η_p² = .06, with a stronger effect for individuals displaying facial infection signs (infection signs × GA interaction: F(1, 101) = 5.61, p = .02, η_p² = .05). No such effects were observed in the second trimester (ps > .05).

The Perceived Threat of COVID-19 as a Moderating Variable

To examine the moderating effect of the perceived threat of COVID-19, we extended the baseline model in the same manner as for the PVD by incorporating COVID-19 threat as a continuous factor, which led to a new potential four-way interaction. The best-fitting model, including random slopes for infection signs, had conditional R² = .90 and marginal R² = .08, indicating a weak effect size (Cohen, 1988).

The COVID-19 threat did not directly predict readiness for social interactions (p = .966). Also, similar to GA, the COVID-19 threat levels did not differ across pregnancy trimesters (ps > .05). Again, unlike the baseline model, the main effect of ethnicity and its interactions with other fixed factors were not significant (ps > .05). The effect of facial infection signs was significant, although it was relatively small, F(1, 419) = 4.64, p = .032, η_p² = .01. Importantly, a significant interaction emerged between infection signs and COVID-19 threat, F(1, 419) = 11.08, p < .001, η_p² = .03, as well as the interaction of pregnancy trimester, infection signs, and COVID-19 threat, F(2, 419) = 3.84, p = .022, η_p² = .02 (see Figure 4). Similarly to GA, we focused on the three-way interaction.

Figure 4.

Mean readiness for social interactions with individuals who displayed (or no) facial infection signs, depending on the COVID-19 threat level in Study 1.

There were no significant effects of infection signs, depending on the COVID-19 threat levels, between pregnancy trimesters (ps > .05). However, separate analysis for each trimester revealed that the moderating effect of the COVID-19 threat was significant only in the first trimester, where higher COVID-19 threat was associated with lower readiness for social interactions, particularly with infected-looking individuals, F(1, 129) = 9.65, p = .002, η_p² = .07. In the second trimester, only the main effect of facial infection signs was significant, F(1, 189) = 10.21, p = .002, η_p² = .05, aligning with the main effect for this factor. No such effects were found in the third trimester (ps > .05).

Models Comparison

In the final step, we compared the performance of all three models (the baseline and the two moderation models) using χ² likelihood-ratio tests. The analysis revealed that the PVD Model exhibited significantly better performance than both the baseline model, χ²(27) = 94.88, p < .001, and the COVID-19 threat model, χ²(15) = 74.02, p < .001. However, no significant difference was found between the COVID-19 threat model and the Baseline Model, χ²(12) = 20.86, p = .052. This indicates that incorporating PVD, and specifically, GA as a moderator, significantly enhanced the model's ability to account for the variability in participants’ readiness for social contact. In contrast, adding the COVID-19 threat level did not significantly improve the model's performance.

Study 2

Method

Participants

A total of 112 pregnant women from Poland aged 22 to 41 (M = 30.14, SD = 4.05) were recruited via parenting websites, social media, and leaflets in gynecologist clinics between February 2021 and April 2022 (the third wave of the COVID-19 pandemic). All participants were ethnically European (White/Caucasian). Participants received a 250 PLN (approx. US$62) Sodexo voucher. Four participants withdrew (e.g., due to relocation or miscarriage), yielding a final sample of 108 women without pregnancy-related health issues. For a more detailed description of the study sample, see Supplementary Materials.

Materials and Procedure

The study included four stages. In the first stage, participants were told that they were completing an online study on the attitudes of pregnant women towards current social phenomena. They were provided with an online survey link, which was administered at their three different trimesters during the pregnancy (T1-T3) and once after delivery—postpartum (PP4). The first link was sent to participants in their first trimester (up to 12 weeks of pregnancy; T1; M_WoP = 9.12, SD_WoP = 1.97), the second link was scheduled for participants in their second trimester (14 to 21 weeks; T2; M_WoP = 19.50, SD_WoP = 2.16), and the third link was scheduled in their third trimester (27 to 33 weeks; T3; M_WoP = 33.13, SD_WoP = 2.19). The fourth link was sent approximately 8 weeks after delivery (PP4; M_{weeks after delivery} = 12.52, SD_{weeks after delivery} = 4.01). We presented participants with the same study procedure as in Study 1 in all four stages.

Experimental Manipulations

The experimental manipulations were the same as in Study 1. Each participant was randomly presented with a unique subset of 20 pictures, varying for each stage of the study.

Dependent Variable: Readiness for Social Contact

The readiness to make social contact was measured in the same way as in Study 1. As averaged responses for each of the three statements constituted reliable indexes (all α > .90), we created four overall indexes of readiness to social contact with: ingroups with no signs of infection (α = .79), ingroups with signs of infection (α = .92), outgroups with no signs of infection (α = .83), and outgroups with signs of infection (α = .92).

Moderator 1: PVD

The subjective perception of susceptibility to disease was measured by the PVD scale (Duncan et al., 2009), as in Study 1 (Perceived Infectibility: α = .80, and Germ Aversion: α = .71).²

Moderator 2: The Perceived Threat of COVID-19

To measure the subjective threat of COVID-19, we used a short version of the Perceived Coronavirus Threat Questionnaire (PCTQ; Conway et al., 2020), as in Study 1 (α = .89).

Data Analysis Strategy

Similarly to Study 1, we utilized multilevel modeling (Linear Mixed Models) with restricted maximum likelihood estimation (REML), with Type III Satterthwaite approximations for degrees of freedom, while Bonferroni correction was applied in cases of multiple comparisons. We followed a structured, stepwise model-building strategy similar to that used in Study 1, with a minor difference. We specified a baseline model that included only the manipulated variables (ethnicity and infection signs), as well as the study stage—but not pregnancy trimester—as predictors. This is because Study 2 included additional measurements during the postpartum period.

Results

First, we conducted correlation analyses between all the continuous variables (see Table 4 in Supplementary Materials). When all study stages were analyzed together, readiness for social interaction was negatively associated with Germ Aversion (GA; r = −.44, p < .01), PI (; r = −.19, p < .01), and perceived threat of COVID-19 (r = −.23, p < .01). However, the strength and pattern of these associations varied across pregnancy stages (see Table 4 in Supplementary Materials for detailed statistics). In the first trimester, readiness for social contact was negatively correlated only with GA (r = −.24, p < .01). In the second trimester, readiness was negatively associated with GA (r = −.37, p < .01), PI (r = −.29, p < .01), and COVID-19 threat (r = −.23, p < .01). In the third trimester, readiness remained negatively correlated with GA (r = −.48, p < .01) and PI (r = −.18, p < .01), but not with COVID-19 threat. Finally, in the postpartum period, readiness for social contact was negatively associated with GA (r = −.45, p < .01), PI (r = −.37, p < .01), and COVID-19 threat (r = −.30, p < .01).

The Baseline Model

Similar to Study 1, the baseline model included only fixed factors—study stage (first vs. second vs. third vs. postpartum), ethnicity (ingroup vs. outgroup), and infection signs (present vs. absent)—along with random intercepts and random slopes for study stage and infection signs. Model performance was R² = .88 (conditional) and R² = .11 (marginal), which refers to a weak-to-moderate effect size (Cohen, 1988).

Results revealed a significant effect of study stage, F(3, 79) = 4.63, p = .005, η_p² = .15, with decrease in readiness for social interactions from the second trimester (M = 3.19, SE = 0.17) to both the third trimester (M = 2.84, SE = 0.20), t(82) = 2.77, p = .034, d = 0.47, and the postpartum period (M = 2.76, SE = 0.20), t(71) = 2.91, p = .025, d = 0.57. There were no significant differences between the first trimester and all other study stages, as well as between the third trimester and the postpartum period (ps > .05).

A significant effect of ethnicity was also observed, with lower readiness for social interactions with outgroups (M = 2.90, SE = 0.16) than with ingroups (M = 3.11, SE = 0.16), F(1, 847) = 27.44, p < .001, η_p² = .03. Moreover, the effect of infection signs was also significant, as participants were less willing to interact with individuals displaying facial infection signs (M = 2.34, SE = 0.21) compared to those with non-infected faces (M = 3.67, SE = 0.13), F(1, 106) = 72.39, p < .001, η_p² = .41.

Importantly, there was a significant interaction between study stage and infection signs, F(3, 875) = 14.31, p < .001, η_p² = .05 (see Figure 5). For infected-looking individuals, compared to the second trimester (M = 2.65, SE = 0.22), readiness was significantly lower in both the third trimester (M = 2.20, SE = 0.25), t(124) = 3.25, p = .008, d = 0.98, and the postpartum period (M = 1.85, SE = 0.25), t(100) = 5.04, p < .001, d = 1.08. Readiness for contact was also significantly lower postpartum than in the first trimester (M = 2.64, SE = 0.20), t(103) = 4.17, p < .001, d = 1.07. However, this was not the case with non-infected faces, as the readiness to interact with these individuals did not differ significantly across all study stages (ps > .05; see Figure 5). The interaction effects involving ethnicity and the three-way interaction were not significant (ps > .05).

Figure 5.

Mean readiness for social interactions with individuals who displayed (or no) facial infection signs presented separately for each study stage in Study Trimester 2.

The PVD as a Moderating Variable

As in Study 1, to test the moderating effect of PVD (Duncan et al., 2009), we extended the baseline model by incorporating two subscales of PVD as separate continuous factors. The best-fitting model included random slopes for study stage and infection signs, with performance of R² = .88 (conditional) and R² = .22 (marginal), indicating a moderate effect size (Cohen, 1988).

Again, the analysis revealed that only GA (not PI, p = .723) was a significant predictor of readiness for social interactions in general, F(1, 271) = 33.18, p < .001, η_p² = .11. Similar to Study 1, the GA level did not differ across study stages (ps > .050) with one exception: the GA level in the postpartum period (M = 4.61, SE = 0.11) was significantly higher than that in the first trimester (M = 4.34, SE = 0.10), t(228) = 3.30, p = .006, d = 0.38.

Again, including the moderator caused most of the effects from the baseline model to become nonsignificant (ps > .05). While the effect of facial infection signs remained, it was relatively small, F(1, 467) = 5.28, p = .022, η_p² = .01. However, a significant interaction emerged between study stage and GA, F(3, 94) = 3.26, p = .025, η_p² = .09, as well as the interaction of infection signs and GA, F(1, 755) = 26.72, p < .001, η_p² = .03, and interaction between study stage, infection signs, and GA, F(3, 868) = 3.57, p = .014, η_p² = .01 (see Figure 6). As the two-way interactions were further qualified by a higher-order interaction, we focused on the latter.

Figure 6.

Mean readiness for social interactions with individuals who displayed (or no) facial infection signs, depending on the germ aversion level in Study 2.

For non-infected faces, there were no significant differences across pregnancy stages based on GA levels (ps > .05). However, for faces displaying infection signs, significant differences were observed, F(3, 548) = 7.14, p = .001, η_p² = .05. Specifically, willingness to engage in social interactions with infected-looking faces significantly decreased between the second and third trimesters among participants with high GA levels, t(102) = 3.88, p = .004, d = 0.66. This reduced willingness persisted into the postpartum period, as the difference between the second trimester and postpartum remained significant (p = .001), but the difference between the third trimester and postpartum was not significant (p = .667).

Separate analysis for each study stage showed that in first, F(1, 91) = 5.20, p = .025, η_p² = .05), and second, F(1, 75) = 7.12, p = .009, η_p² = .09, trimesters, higher GA levels were associated with lower readiness for social interactions, but only with individuals displaying infection signs (see Figure 6). In the third trimester, F(1, 77) = 32.30, p < .001, η_p² = .30, and during the postpartum period, F(1, 69) = 34.60, p < .001, η_p² = .33, higher GA levels led to reduced social readiness overall, with a stronger effect for individuals with facial infection signs (third trimester: F(1, 77) = 16.32, p = .001, η_p² = .17; postpartum: F(1, 69) = 21.63, p < .001, η_p² = .24).

The Perceived Threat of COVID-19 as a Moderating Variable

To examine the moderating effect of the perceived threat of COVID-19, we extended the baseline model in the same manner as for the PVD by including the COVID-19 threat as a continuous factor. The best-fitting model, including random slopes for study stage and infection signs, had conditional R² = .88 and marginal R² = .13, indicating a moderate effect size (Cohen, 1988).

In contrast to Study 1, the COVID-19 threat was a significant predictor of readiness for social interactions, F(1, 279) = 7.53, p = .006, η_p² = .03. Additionally, the COVID-19 threat levels significantly increased between the third trimester (M = 3.65, SE = 0.15) and the postpartum period (M = 4.16, SE = 0.15), t(219) = 3.75, p = .001, d = 0.68.

This time, only one effect from the baseline model became nonsignificant—the main effect of ethnicity (p = .126). As the effects of the study stage, F(3, 98) = 3.36, p = .022, η_p² = .01, and infection signs, F(1, 106) = 72.39, p < .001, η_p² = .41, remained significant, the results pattern was the same as in the Baseline Model. Additionally, the results showed a significant interaction between facial signs of infection and the COVID-19 threat, F(1, 831) = 4.08, p = .041, η_p² < .01; however, the effect was relatively small (see Figure 7).

Figure 7.

Estimated readiness for social interactions with individuals who displayed (or no) facial infection signs, depending on the COVID-19 threat level in Study 2.

First, to facilitate interpretation of the interaction between infection signs and COVID-19 threat level, the threat level was probed at three representative levels corresponding to the 25th, 50th, and 75th percentiles of its distribution (i.e., low, M = 3.00, medium, M = 4.00, and high, M = 5.00, respectively). The simple effects analysis for COVID-19 threat levels depending on infection signs showed that in the case of infected faces, higher COVID-19 threat levels were associated with lower readiness for social interactions. Specifically, at low threat level, readiness was significantly higher (M = 2.54, SE = 0.21) compared to medium threat level (M = 2.32, SE = 0.20; t(435) = 2.99, p = .008, d = 0.30), and high threat level (M = 2.10, SE = 0.22; t(435) = 2.99, p = .008, d = 0.59). The difference between medium and high threat levels was also significant, t(435) = 2.99, p < .001, d = 0.30 (the interaction is illustrated in Figure 7). The effects for non-infected faces were not significant (ps > .05).

All other interaction effects that included the COVID-19 threat were not significant (ps > .05).

Models Comparison

In the final step, we compared the performance of all three models (the baseline and the two moderating models) using χ² likelihood-ratio tests. The analysis revealed that the PVD Moderating Model exhibited significantly better performance than both the Baseline Model, χ²(32) = 99.23, p < .001, and the perceived threat of COVID-19 Moderating Model, χ²(16) = 29.58, p = .020. Additionally, the perceived threat of COVID-19 Moderating Model also outperformed the Baseline Model, χ²(16) = 69.65, p < .001. This suggests that incorporating both moderating variables significantly improved the model's ability to explain variability in participants’ readiness for social contact, with GA emerging as the strongest predictor.

Discussion

The primary goal of our project was to test the CPH (Fessler & Navarrete, 2003) by examining how the BIS (Ackerman et al., 2018) affects social behavior across pregnancy stages. We conducted both a cross-sectional and a longitudinal study assessing pregnant women's willingness to engage in social contact with individuals—healthy or displaying infection signs—from in- and outgroups.

Our results broadly support the predicted adaptive role of the BIS. Across both studies, pregnant participants were less willing to interact with visibly infected individuals and outgroup members. These findings align with prior research linking pathogen cues to increased interpersonal distance (Park, 2015), lower trust (Aarøe et al., 2016), negative intergroup attitudes (Fan et al., 2022; Zakrzewska et al., 2019), and reduced contact comfort (Bressan, 2021; van Leeuwen & Petersen, 2018). Germ aversion (Studies 1 and 2) and perceived COVID-19 threat (Study 2) both significantly predicted lower contact willingness.

However, our findings offer limited support for CPH predictions. We found no consistent main effects of pregnancy stage: contact avoidance was not systematically higher in the first trimester. Nor did we observe increased germ aversion or COVID-19 threat in early pregnancy (Fessler et al., 2005). Although Study 1 showed lower contact willingness in the first trimester, Study 2 revealed the opposite—lowest willingness in the third trimester and postpartum. This contradicts earlier studies emphasizing early-pregnancy sensitivity (Fessler et al., 2005; Navarrete et al., 2007; Żelaźniewicz & Pawłowski, 2015). These inconsistencies align with updated immunological models that propose dynamic immunocompetence shifts across pregnancy (Abu-Raya et al., 2020; Hové et al., 2020; Kaňková et al., 2022). Disgust sensitivity may instead track individual differences in immune status or external pathogen cues, including illness (Dlouhá et al., 2023) or environmental threat levels, such as during the COVID-19 pandemic (Kaňková et al., 2023). Although our results contradict the predictions of the CPH, they align with epidemiological data indicating increased infection susceptibility during pregnancy, with risk escalating as gestation advances. Specifically, Lindsay et al. (2006) reported progressively increasing odds ratios for influenza-like illness episodes from the first trimester to the postpartum period, and Neuzil et al. (1998) demonstrated a similar pattern for influenza-related serious morbidity. Thus, women in the third trimester and postpartum period appear to face heightened infection risk, and our findings suggest that behavioral adaptations in pathogen avoidance may correspond adaptively to this risk profile. However, these epidemiological studies did not examine the biological mechanisms underlying the observed susceptibility patterns, which limits our understanding of whether they reflect immunological suppression, physiological changes affecting respiratory function, or other pregnancy-related factors. An alternative functional explanation for the heightened pathogen avoidance observed in the third trimester and postpartum period centers on escalating maternal investment in the offspring. As pregnancy progresses, the cumulative energetic, physiological, and emotional costs to the mother increase substantially, rendering fetal or neonatal loss increasingly detrimental from an evolutionary perspective (see e.g., Butte & King, 2005; Jasienska, 2020). By late gestation, the fetus is highly viable, and postpartum, the newborn remains entirely dependent on maternal care. Thus, any infection jeopardizing maternal or infant survival imposes greater fitness costs than earlier in pregnancy. This escalating investment may serve as a proximate cue triggering upregulated behavioral prophylaxis—such as greater social distancing from ill individuals—independent of immunological suppression.

Importantly, the BIS appears functionally flexible: readiness to engage in social contact varied by both the condition of the target (infected vs. healthy) and participants’ germ aversion. Notably, we observed effects of germ aversion (GA) but not PI. This pattern is consistent with the conceptual distinction between these PVD subscales: GA reflects automatic, emotion-based avoidance of pathogen cues, whereas PI reflects more deliberative beliefs about one's susceptibility to disease (Duncan et al., 2009; Faulkner et al., 2004). Prior work has shown that GA often exerts stronger or unique effects on social attitudes and avoidance behaviors compared to PI (Faulkner et al., 2004; Park et al., 2003). Thus, the observed GA effects may reflect affectively-driven, context-sensitive pathogen avoidance, whereas PI may be less responsive to situational or physiological cues captured in our studies. In Study 1, aversion predicted avoidance in the first and third trimesters; in Study 2, from the second trimester onward. These results echo prior findings that PVDmodulates BIS activation (Ackerman et al., 2018), facilitating behaviors like prophylaxis (Shook et al., 2020), ethnocentrism (Navarrete & Fessler, 2006), and xenophobia (Faulkner et al., 2004). Nonetheless, in Study 1, germ aversion did not differ across trimesters, and in Study 2, the lowest willingness to contact infected others occurred late in pregnancy and postpartum, opposing CPH assumptions.

Perceived COVID-19 threat followed a similar moderating pattern. In both studies, higher perceived threat predicted lower willingness to interact with infected individuals. Yet, this effect was statistically significant only in the first trimester of Study 1. Notably, the COVID-19 threat increased postpartum. The absence of clear trimester-based effects again undermines support for CPH, instead pointing toward general BIS activation in response to situational disease salience. This pattern is consistent with findings from Sorokowska et al. (2024), who observed higher PVD in pregnant and postpartum women relative to childless peers. This suggests BIS activation may persist into parenthood as a protective mechanism for both mother and child.

However, higher PVD during pregnancy was unrelated to neonatal health outcomes such as birth weight or Apgar scores, implying that elevated BIS activity reflects precautionary adaptation rather than a response to immediate health risks. Theoretically, sustained BIS activation postpartum aligns with inclusive fitness theory (Hamilton, 1963): by protecting oneself and one's offspring, individuals promote their genetic fitness. Prior research has shown reduced disgust toward one's own child's feces (Case et al., 2006) or familiar elderly relatives (Cao et al., 2022), indicating that BIS may be modulated by kinship cues to balance disease avoidance with caregiving.

Limitations

These studies relied on self-reports, which may not capture unconscious or behavioral manifestations of pathogen avoidance. Nonetheless, our measures (germ aversion, infectability, COVID-19 threat) suggest BIS activation varies across pregnancy stages. Physical discomfort late in pregnancy may have also influenced willingness to engage in social contact (e.g., shaking hands, sitting close), although this measure has shown validity in past work (Szymkow et al., 2021a, 2021b). Additionally, factors such as fetal sex (Żelaźniewicz & Pawłowski, 2015), parity (Kaňková et al., 2023), medication, and method of insemination were not fully accounted for due to missing data.

Furthermore, contextual and demographic variables that may calibrate BIS responses were not fully included in our analyses. Recent work suggests that pathogen-avoidance mechanisms can be shaped by the degree to which individuals are able to avoid pathogen-relevant environments (Cepon-Robins et al., 2021). During the COVID-19 pandemic, opportunities for avoidance varied substantially—for example, between individuals working remotely and those exposed to interpersonal contact due to occupational demands (e.g., healthcare or education workers). Socioeconomic status, job type, or situational constraints may therefore influence the extent to which disgust sensitivity and PVD are expressed. Because our dataset did not include sufficiently detailed information on occupational exposure or avoidance constraints, we were unable to directly test these possibilities. Future studies should investigate how such contextual variation shapes pathogen avoidance across pregnancy, particularly in populations experiencing differing levels of environmental risk or constraints on behavioral avoidance.

Moreover, it is important to note that our findings may not directly generalize to more ethnically diverse populations, where intergroup contact is more frequent and ethnic outgroups may not be perceived as unfamiliar. Given that the data were collected in Poland, an ethnically homogeneous country, and that the sample was highly consistent in terms of ethnic background, the observed behavioral immune responses likely reflect heightened sensitivity to facial cues deviating from the prototypical local morphology. In more diverse contexts, repeated intergroup contact could attenuate such responses, and future research should examine whether similar patterns hold in populations with greater ethnic heterogeneity. Accordingly, the magnitude of outgroup-related sexual disgust or pathogen avoidance responses observed here may be partly amplified by the high ethnic homogeneity of both the country and the sample, consistent with Bressan's (2021) suggestion that individuals deviating from the typical local morphology may be categorized as outgroup members, eliciting stronger behavioral immune reactions. Also, as individuals higher in conservatism tend to show greater avoidance of potential pathogen sources (Tybur et al., 2016), and Poles generally exhibit a higher level of social conservatism compared to most Western European countries (World Values Survey, 2022), the results may be specific to such more tradition-oriented societies.

It should also be noted that the infection cues used in this study were limited to visual depictions of skin changes and rashes. While such cues are well-established elicitors of pathogen-related disgust, they may not capture the full spectrum of symptoms associated with contagious illnesses, particularly those transmitted via respiratory routes (e.g., coughing, nasal discharge, or feverish appearance). However, changes in facial coloration—such as paleness or flushing—can also signal infections of the upper respiratory tract, including influenza or other viral illnesses, and may partially overlap with the cues we presented. Therefore, future studies could employ a broader range of infection-relevant cues, including auditory or contextual indicators (e.g., coughing sounds, verbal information about contagiousness), to examine whether the observed effects generalize across different types of pathogen threats.

Conclusions

Our studies demonstrate that across pregnancy and postpartum, women show reduced willingness to engage with visibly infected individuals. Contrary to CPH, this effect was not confined to early pregnancy, suggesting that BIS activation responds more to perceived environmental threats than to trimester per se. These findings highlight the adaptive flexibility of the BIS in protecting both the mother and her offspring. Postpartum BIS activity may further function to safeguard dependent kin, consistent with evolutionary accounts of inclusive fitness. As shown in Sorokowska et al. (2024), elevated PVD extends beyond pregnancy, pointing to sustained behavioral prophylaxis during early motherhood.

Supplemental Material

sj-docx-1-evp-10.1177_14747049251411481 - Supplemental material for Reduced Readiness for Social Interaction as a Strategy for Pathogen Avoidance by Women in Various Stages of Pregnancy and Postpartum

Supplemental material, sj-docx-1-evp-10.1177_14747049251411481 for Reduced Readiness for Social Interaction as a Strategy for Pathogen Avoidance by Women in Various Stages of Pregnancy and Postpartum by Natalia Frankowska, Aleksandra Tołopiło, Michal Olszanowski, Šárka Kaňková, Mikołaj Kuczmarski and Aleksandra Szymkow in Evolutionary Psychology

Footnotes

ORCID iDs

Natalia Frankowska

Aleksandra Tołopiło

Michal Olszanowski

Šárka Kaňková

Mikołaj Kuczmarski

Aleksandra Szymkow

Ethical Considerations

The studies were approved by the Institutional Ethical Review Board at SWPS University, Faculty of Psychology in Sopot (WKE/S 2020/15/X/92).

Consent to Participate

The participants provided informed consent to take part in the study (by online survey), being fully aware of their right to withdraw at any time and of the information regarding the processing of their personal data.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the (Polish) Ministry of Education and Science in a Regional Excellence Initiative Program (project RID20/2020) awarded to Natalia Frankowska and the Czech Science Foundation Program (project GAČR 23-05519S) awarded to Šárka Kaňková.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The raw data and codes have been deposited at: .

Supplemental Material

Supplemental material for this article is available online.

Notes

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