Throughout most of human evolutionary history, women spent a large portion of their lives in states of increased vulnerability to threats due to pregnancy and infant care. The recurrent challenges posed by pregnancy may have shaped the development of self-protective strategies that help women combat those threats. Because having a strong social network can reduce threats from multiple sources, one major strategy may involve increased motivation to affiliate. Although past research has not directly assessed women's desire to affiliate, women demonstrate greater attunement to their social environment during pregnancy (Anderson and Rutherford, 2013; Ketterman et al., 2022). Moreover, women's increased social attunement begins shortly after ovulation, when pregnancy is still only a possibility, and is underpinned by increases in progesterone that begin in the luteal phase of the menstrual cycle (Maner and Miller, 2014). Thus, in the present research, we investigate how women's explicit desire to affiliate varies across the menstrual cycle, as well as whether that variation is associated with fluctuations in progesterone and estradiol.

During pregnancy, women face many threats and challenges. During the early months of pregnancy, women are especially vulnerable to illness because of immunological changes necessary for fetal development (Fessler et al., 2005). During later months of pregnancy, increased energetic demands (Jasienska, 2020) may limit women's ability to engage in goal-relevant behavior, including securing necessary resources. Goal-relevant behavior is also more difficult because women's locomotion is impaired (Sawa et al., 2015; Segal and Chu, 2015). Finally, parturition is dangerous. The risk of complications during labor, coupled with the human baby's size and positioning in the birth canal, make it especially dangerous for women to give birth without assistance from other people (Trevathan and McKenna, 1994). Such challenges were especially potent during most human history, prior to the advent of modern medical interventions.

Many of the challenges posed by pregnancy can be mitigated if women rely on a strong social support network. Having close others around lessens the threats posed by external dangers (e.g., hostile others, environment), inability to independently acquire resources, and giving birth. Indeed, during pregnancy, female animals—including women—demonstrate heightened tend-and-befriend tendencies, which include affiliating with others females in times of threat and stress (Taylor et al., 2000). Women also exhibit “nesting” behavior, including both household preparation and desire to selectively affiliate with close others (Anderson and Rutherford, 2013; Ketterman et al., 2022).

Notably, even prior to pregnancy, women demonstrate adaptive changes in behavior over the course of the menstrual cycle. The menstrual cycle can be distilled into three phases: follicular, peri-ovulatory, and luteal. The follicular phase, which begins on the first day of menstruation, is characterized by menstrual bleeding and the development of follicles. During the follicular phase, levels of estradiol start low and begin to rise while levels progesterone remain consistently low. The peri-ovulatory phase, which includes the day of ovulation and the preceding 4–5 days (Baird et al., 1995), is characterized by heightened likelihood of fertilization. During the peri-ovulatory phase, levels of estradiol rise further and peak, whereas levels of progesterone remain relatively low. The luteal phase, which begins after ovulation and lasts on average two weeks (Crawford et al., 2017), is characterized by physiological changes that begin to prepare women's bodies for possible pregnancy. During the luteal phase, levels of progesterone rise and peak, and levels of estradiol are usually moderately elevated (i.e., higher than follicular phase but lower than peri-ovulatory phase).

In parallel to the physiological preparations for possible pregnancy, social preparations for possible pregnancy also begin in the luteal phase (whether or not conception has occurred). In the luteal phase, for example, women are more preoccupied with what others think of them compared to the follicular phase (Wang et al., 2021). Women in the luteal phase also demonstrate heightened prosocial behavior. Compared to those in the follicular phase, women in the luteal phase behave more cooperatively toward strangers in an economic game (Wang et al., 2021), donate more money to charity (Stenstrom et al., 2018), and prefer spending money on gifts for close others rather than themselves (Stenstrom et al., 2018). Moreover, women increasingly prefer facial cues connoting kinship during the luteal phase compared to prior cycle phases (DeBruine et al., 2005). The latter two findings additionally suggest that women may be specifically investing in close others, who would be especially likely to provide social support.

Increases in social preparations during the luteal phase have been directly linked to progesterone. Indeed, although women generally demonstrate greater implicit affiliation motives than men (Drescher and Schultheiss, 2016), women's implicit affiliation motives are positively associated with increases in progesterone levels across the menstrual cycle (Schultheiss et al., 2003). Moreover, higher progesterone levels are associated with heightened attention to social stimuli (Maner and Miller, 2014), greater accuracy in identifying negative emotions (Maner and Miller, 2014; cf. Shirazi et al., 2020), and increased self-presentation concerns (Wang et al., 2021). More generally, when women's progesterone is high (vs. low), they demonstrate increased social monitoring such that they become especially sensitive to emotional displays that signal threat (Conway et al., 2007).

We examined whether women's desire to affiliate varied across menstrual cycle phases and was associated with within-woman fluctuations in progesterone and estradiol. At three times during the menstrual cycle—during the mid-follicular, peri-ovulatory (late follicular), and luteal phases—women reported their desire to affiliate and provided saliva samples that were assayed for progesterone and estradiol. We predicted that women would demonstrate greater desire to affiliate in the luteal phase, compared to the early follicular and peri-ovulatory phases. Furthermore, we predicted that women's desire to affiliate would be positively associated with within-woman fluctuations in progesterone (i.e., when women's progesterone levels during a testing session were higher than their average progesterone across sessions).

This study advances the current state of knowledge in multiple ways. First, although past research has hinted at possible variability in affiliative motives across the menstrual cycle, measures of affiliation have been only indirect, and have included assessments such as behavior on economic tasks (Ball et al., 2013; Stenstrom et al., 2018; Wang et al., 2021) and cognitive biases involving social stimuli (Conway et al., 2007; Maner and Miller, 2014; Schultheiss et al., 2003). The current work provides one of the first direct examinations of women's explicit desire to affiliate.

Second, although most previous studies have used general social stimuli (e.g., faces of strangers; Maner and Miller, 2014) to indirectly assess affiliative motives, we distinguished between global affiliative motivation (i.e., desire to be social and make new friends) and specific desire to affiliate with close others. Although we did not have strong predictions, we reasoned that, increases in social affiliative motives during the luteal phase may be limited to seeking out social support from existing close relationships with friends and family. Friends and family provide the most promising sources of social support and, although developing new social connections may be important, strangers nevertheless pose a threat. Because of these opposing forces, one might not expect to see a marked increase in general affiliation in the luteal phase relative to the follicular and peri-ovulatory phases.

Third, in addition to assessing the hypothesized positive link between progesterone and affiliation, we also explored whether within-woman fluctuations in estradiol would be negatively associated with desire to affiliate. At least two theoretical reasons suggest such a negative association. First, the peri-ovulatory phase (vs. other phases) and higher (vs. lower) levels of estradiol are associated with heightened intrasexual competition (Bakolas and Park, 2023; Durante et al., 2011; Fiacco et al., 2021; Fisher, 2004; Necka et al., 2016; cf. Stern et al., 2023). Consequently, during the peri-ovulatory phase and when estradiol is higher (vs. lower) across three assessments, women may view other women as competitors rather than sources of social support. Such associations may be especially strong for social targets (e.g., strangers or loose acquaintances) who may be a greater source of intrasexual competition than a source of social affiliation. This pattern would also be consistent with past research showing that women displayed lower trust in an economic game during the peri-ovulatory phase (when estradiol levels are especially high) relative to the follicular and luteal phases (Ball et al., 2013).