Introduction

The fraternal birth order effect refers to the finding that number of older brothers is uniquely predictive of male sexual orientation (Blanchard, 2004). Specifically, androphilic males (i.e., males who exhibit sexual attraction/arousal toward adult males) tend to have greater numbers of older brothers than gynephilic males (i.e., males who exhibit sexual attraction/arousal toward adult females). Evidence in support of the fraternal birth order effect is overwhelming. It has been documented in participants examined in recent years and in participants examined decades ago: in psychiatric patients and non-patient volunteers; in participants examined during childhood and adulthood; in transsexual participants and those who experience no dysphoria with their sexed bodies; in representative, national samples; in non-Caucasian citizens of the United States (i.e., Black, Hispanic, East Indian, and Asian); and, in samples collected from different Western nations, including England, Italy, The Netherlands, Canada, and the United States (for review, see Blanchard, 2004), by independent researchers (e.g., Bogaert, 2003; Camperio Ciani, Corna, & Capiluppi, 2004; King et al., 2005; Rahman, Clarke, & Morera, 2009; Rice, Harris, Lang, & Chaplin, 2008; Schwartz, Kim, Kolundziji, Rieger, & Sanders, 2009). In addition, this effect is specific to the influence of biological, as opposed to non-biologically related, older brothers regardless of whether males are raised with these brothers (Bogaert, 2006).

The existence of the fraternal birth order effect has prompted speculation regarding what influence biological older brothers have on the development of sexual orientation in their younger male siblings. The most prominent hypothesis is that this effect reflects the progressive immunization of some mothers to the male-specific antigens that are produced in response to the gestation of each successive male fetus. The production of maternal antibodies in response to the presence of these male-specific antigens is thought to influence the sexual differentiation of each successive male fetus’ brain and, by extension, those neural regions that regulate sexual orientation. This line of reasoning has been referred to as the maternal immune hypothesis (Blanchard, 2004; Blanchard & Bogaert, 1996; Blanchard & Klassen, 1997).

Despite the reliability with which the fraternal birth order effect has been observed, Blanchard (2004) cautioned that relying solely on data from Western populations presented limitations. Because male androphilia is expressed differently across cultures (Murray, 2000), extrapolating from patterns observed in Western populations to make statements regarding the development of male androphilia in non-Western populations may be imprudent. In contrast to Western cultures in which androphilic males tend to identify as “gay” or “homosexual” men, there are many non-Western cultures in which androphilic males tend to be transgendered and occupy “alternative” gender categories that are distinguished from “men” and “women.” Some contemporary examples include the xanith of Oman, the hijra of India, the kathoey of Thailand, the travesti of Brazil, the fakafefine of Tonga, and the fa’afafine of Samoa (Murray, 2000). Cultural differences in the expression of male androphilia may reflect unique cultural influences toward development, in which case attempts to compare the development of male androphilia in different cultural settings may not be warranted (e.g., Davenport, 1987; Johnson, Jackson, & Herdt, 2000).

Despite this cultural variability, cross-cultural universals in the psychosexual development of male androphiles appear to exist. For example, in Western cultures, male androphiles exhibit elevated gender-atypical behavior during childhood (Bailey & Zucker, 1995). Retrospective studies conducted in Independent Samoa, Brazil, Guatemala, Turkey, Thailand, and the Philippines have shown the same pattern of childhood gender-atypicality among male androphiles raised in these non-Western cultures (Bartlett & Vasey, 2006; Cardoso, 2005, 2009; Whitham & Zent, 1984). Such cross-cultural similarities in childhood behavior add weight to arguments that similar biological influences, which transcend cultural differences, play a role in the development of male androphilia. Further weight would be added to such arguments if it could be demonstrated that causal biological factors, such as those postulated by the maternal immune hypothesis, are likely to influence the development of male androphilia in non-Western cultures. Hence, establishing the existence of the fraternal birth order effect—a hypothesized outcome of maternal immune responses—in a non-Western culture would further substantiate arguments that similar biological influences underlie the development of male androphilia cross-culturally.

To date, no studies have demonstrated a fraternal birth order effect in a non-Western culture, but some have indicated that male androphiles in non-Western cultures tend to be later born among their siblings (Poasa, Blanchard, & Zucker, 2004; Tsoi, Kok, & Long, 1977; Vasey & VanderLaan, 2007; Zucker & Blanchard, 2003; Zucker, Blanchard, Kim, Pae, & Lee, 2007). Vasey and VanderLaan (2007) provided a systematic investigation of birth order and male sexual orientation in Samoan androphilic and gynephilic males. In Samoa, most androphilic males are referred to as members of an alternative gender category known as fa’afafine. Translated literally, fa’afafine means “in the manner of a woman.” These individuals self-identify as fa’afafine and not as men or women. Although the term fa’afafine implies that the members of this category are uniformly very feminine, they are, in fact, a heterogeneous group in terms of their gender role presentation (Schmidt, 2003; Vasey & Bartlett, 2007). In appearance and mannerisms, although most would be considered effeminate, they range from strikingly feminine to unremarkably masculine, although instances of the latter are rare.

Utilizing a sample of 83 fa’afafine as well as a control group of 114 Samoan gynephilic males, Vasey and VanderLaan (2007) found that fa’afafine tended to have greater numbers of older brothers, older sisters, and younger brothers. The finding that fa’afafine have more older brothers than their gynephilic counterparts is consistent with patterns observed in Western populations. However, none of the observed sibling category effects took precedence over another (i.e., the three sibling category effects documented did not significantly differ in magnitude), indicating no clearly unique contribution of older brothers and, thus, no genuine fraternal birth order effect.

Sibling category effects, apart from older brother effects, have been reported on occasion in studies of Western populations (Blanchard, 1997; Blanchard & Lippa, 2007; Bogaert, 1998; King et al., 2005). However, given that older brother effects are reported consistently whereas other sibling category effects seem to be a relatively rare occurrence in studies conducted in Western populations, one possible explanation is that these less often observed effects represent cases of Type I error. Alternatively, these additional sibling category effects may have indicated an association between male sexual orientation and the fecundity of kin. Elevated fecundity has been documented among the kin of androphilic males (Blanchard & Lippa, 2007; Camperio Ciani et al., 2004; Iemmola & Camperio Ciani, 2009; King et al., 2005; Rahman et al., 2008; Schwartz et al., 2009). Hence, the sibling category effects that are occasionally observed alongside older brother effects may be a consequence of elevated fecundity in the mothers of androphilic males.

This study replicated Vasey and VanderLaan (2007) using a larger, independent sample to determine whether the sibling category effects they observed were genuine or were likely to represent cases of Type I error. In addition, the replication sample was combined with the sample of fa’afafine and gynephilic males from Vasey and VanderLaan to create the largest data set concerning birth order and male androphilia ever acquired for a non-Western population. The primary advantage of doing so was that the size of the combined sample provided greater statistical power to test for possible differences in the magnitudes of different sibling category effects, thereby allowing an assessment of whether a genuine fraternal birth order effect existed in addition to a fecundity effect.

Method

Participants

All participants were recruited through a network sampling procedure on the two larger and more populated islands of Upolu and Savai’i. A network sampling procedure involves contacting initial participants who display qualities of interest (i.e., status as fa’afafine or gynephilic man), then obtaining referrals from them to additional participants who, in turn, provide further referrals, and so on. The rate of participation for all groups was greater than 90%.

To replicate the study by Vasey and VanderLaan (2007), new data were collected from 133 self-identified fa’afafine and 208 self-identified straight men that had not been interviewed previously. These data were collected during three field trips (March–June 2007; December 2007; July–September 2008). In order to obtain sufficiently large sample sizes to compare the magnitudes of different sibling category effects, we combined the data from the 133 fa’afafine and 208 gynephilic males in the replication sample with data from the sample of 83 fa’afafine and 114 gynephilic males interviewed in Vasey and VanderLaan. Thus, the combined sample consisted of 216 fa’afafine and 322 gynephilic males.

Procedure and Measures

All participants were interviewed using a standardized questionnaire that was available in English and Samoan, after being translated and back-translated by two fluent Samoan-English speakers. A Samoan-speaking research assistant was present to answer Samoan-speaking participants’ questions.

The questionnaire contained questions concerning basic biographic information regarding sexual orientation and age. Sexual orientation was assessed using Kinsey ratings (Kinsey, Pomeroy, & Martin, 1948). Specifically, participants were asked the following question: “Which statement best describes your sexual feelings during the last year?” Participants then selected one of the following seven possible responses: “sexual feelings only toward females” (Kinsey rating = 0), “most sexual feelings toward females, but an occasional fantasy about males” (Kinsey rating = 1), “most sexual feelings toward females, but some definite fantasy about males” (Kinsey rating = 2), “sexual feelings about equally divided between males and females with no strong preference for one or the other” (Kinsey rating = 3), “most sexual feelings toward males, but some definite fantasy about females” (Kinsey rating = 4), “most sexual feelings toward males, but an occasional fantasy about females” (Kinsey rating = 5), or “sexual feelings only toward males” (Kinsey rating = 6). Samoans, both inside and outside the fa’afafine community, recognize that fa’afafine are biological males that are socially distinct from men and women. Nevertheless, for the sake of consistency, participants were told, prior to answering questions pertaining to the Kinsey ratings, that the category “males” included straight men and/or fa’afafine whereas the category “females” included women.

With respect to participants in the replication sample, 129 (97%) fa’afafine described their sexual feelings as exclusively androphilic (Kinsey rating = 6), and the remaining 4 (3%) reported most sexual feelings toward males, but occasional fantasies about females (Kinsey rating = 5). For gynephilic males, 200 (96%) described their sexual feelings as exclusively gynephilic (Kinsey rating = 0), and the remaining 8 (4%) reported most sexual feelings toward females, but occasional fantasies about males (Kinsey rating = 1). Of the additional 83 fa’afafine interviewed in Vasey and VanderLaan (2007), all described their sexual feelings as exclusively androphilic (Kinsey rating = 6). Of the 114 gynephilic males, 104 (91.2%) described their sexual feelings as exclusively gynephilic (Kinsey rating = 0). Ten (8.8%) reported most sexual feelings toward females, but occasional fantasies about males (Kinsey rating = 1).

The age ranges of fa’afafine and gynephilic males in the replication sample were 18–53 and 18–67, respectively. We compared these fa’afafine and gynephilic males for age differences. Fa’afafine were significantly younger, on average, than the gynephilic males (fa’afafine, M ± SD = 27.83 years ± 7.98; gynephilic males, 29.78 ± 8.73), t(337) = 2.07, p = .04. (Note: age data were missing for two fa’afafine participants.) The age ranges of fa’afafine and gynephilic males in the combined sample were 18–60 and 18–67, respectively. For the combined sample, there was no statistically significant difference between these groups with respect to age (fa’afafine, M ± SD = 28.81 years ± 8.20; gynephilic males, 28.42 ± 8.23), t(527) < 1. (Note: age data were missing for eight fa’afafine participants and one gynephilic male participant.)

The questionnaire also included a section pertaining to birth order. Specifically, participants were asked to list all of the children their mothers gave birth to from first- to last-born. In addition to indicating their own birth order, participants indicated whether each sibling was male or female. We recorded four data points for each participant: number of older brothers, number of older sisters, number of younger brothers, and number of younger sisters. Participants’ birth orders were quantified using Slater’s Index (number of older siblings/total number of siblings), a metric that expresses birth order as a value between 0 (first-born) and 1 (last-born), and controls for family size (Slater, 1958). For each participant, we also computed two additional birth order indices, which were introduced by Jones and Blanchard (1998): (1) Fraternal Index (number of older brothers/total number of brothers), and (2) Sororal Index (number of older sisters/total number of sisters).

Results

Replication Sample

Table 1 presents descriptive statistics regarding the total number of siblings as well as the numbers of older brothers, older sisters, younger brothers, and younger sisters for fa’afafine and gynephilic males for the replication sample. Fa’afafine had a greater number of siblings, on average, than did gynephilic males, t(339) = 4.63, p < .001.

Table 1 Descriptive statistics for the total number of siblings as well as the numbers of older brothers, older sisters, younger brothers, and younger sisters of fa’afafine and gynephilic males
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Table 2 presents descriptive and inferential statistics pertaining to the Slater’s, Fraternal, and Sororal indices for fa’afafine and gynephilic males in the replication sample. Slater’s, Fraternal, and Sororal index values could not be computed for participants who did not have siblings, brothers, or sisters, respectively. Inferential statistics were performed to test for biases in the birth orders of fa’afafine and gynephilic males. For each index, the mean index value for each group was compared against a value of .5, the expected mean index value for samples drawn from a hypothetical stable population. Fa’afafine were significantly more likely to be later born according to all three indices. Gynephilic males were significantly more likely to have been early born according to Slater’s index, but did not differ significantly from the expected .5 value for the Fraternal and Sororal indices. Between-group comparisons of fa’afafine and gynephilic males revealed that fa’afafine were significantly more likely to be later born for all three indices.

Table 2 Descriptive and inferential statistics for fa’afafine and gynephilic males on the Slater’s, Fraternal, and Sororal indices
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A logistic regression analysis was conducted with sexual orientation (i.e., gynephilic versus androphilic) as the dichotomous criterion variable and number of older brothers, number of older sisters, number of younger brothers, and number of younger sisters as the predictor variables. The model accounted for 13.9% of the variance in sexual orientation. Table 3 presents the results of the logistic regression analysis. The results indicated that number of older brothers and number of older sisters were both statistically significant predictors of sexual orientation. The odds ratios derived from the logistic regression analysis for the effects of number of older brothers and number of older sisters were 1.36 and 1.14, respectively.

Table 3 Logistic regression of sexual orientation on numbers of siblings
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Expected sex ratios were obtained from the Samoan Statistical Service Division of the Ministry of Finance (2006) and indicated that a ratio of 109 male live births for every 100 female live births was appropriate for the cohort range of our sample. Table 4 presents the total and expected numbers of all male siblings, older male siblings, and younger male siblings for fa’afafine and gynephilic males. We assessed whether the total number of males in each category differed from the expected values based on the Samoan population parameters using the z approximation to the binomial test. The total number of male siblings, number of older brothers, and number of younger brothers did not differ significantly from the expected values for fa’afafine. The total number of male siblings was significantly different from the expected value for gynephilic males. Analyses revealed that, for gynephilic males, the number of older brothers, but not younger brothers, was significantly lower than the expected value.

Table 4 Comparisons of the total and expecteda numbers of all male siblings, older male siblings, and younger male siblings for fa’afafine and gynephilic males
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It is necessary to note that given the age disparity between our fa’afafine and gynephilic male samples, we also performed analyses in which we controlled for age. These analyses revealed that age had no impact on the statistical significance of the results reported here. Therefore, the analyses are presented here without controlling for age.

Combined Sample

Table 1 presents descriptive statistics regarding the total number of siblings as well as the numbers of older brothers, older sisters, younger brothers, and younger sisters for fa’afafine and gynephilic males for the combined sample. Fa’afafine had a greater number of siblings, on average, than did gynephilic males, two-tailed independent t-test with between-group equality of variances not assumed; Levene’s test for equality of variances, F = 10.11, p = .002; t(402.03) = 6.85, p < .001.

Table 2 presents descriptive and inferential statistics pertaining to the Slater’s, Fraternal, and Sororal indices for fa’afafine and gynephilic males in the combined sample. These analyses were performed in the same fashion as for the replication sample. Fa’afafine were significantly more likely to be later born according to all three indices. Gynephilic males did not differ significantly from the expected .5 value for all three indices. Between-group comparisons of fa’afafine and gynephilic males revealed that fa’afafine were significantly more likely to be later born for all three indices.

A logistic regression analysis was conducted with sexual orientation (i.e., gynephilic versus androphilic) as the dichotomous criterion variable and number of older brothers, number of older sisters, number of younger brothers, and number of younger sisters as the predictor variables. The model accounted for 13.5% of the variance in sexual orientation. Table 3 presents the results of the logistic regression analysis. The results indicated that number of older brothers and number of older sisters were both statistically significant predictors of sexual orientation. The odds ratios derived from the logistic regression analysis for the effects of number of older brothers and number of older sisters were 1.34 and 1.17, respectively.

We conducted further analyses to assess whether the older brother and older sister effects differed in magnitude. In doing so, we used Fisher’s r to z transformations to compare the partial correlations between sexual orientation and each of these statistically significant predictor variables, while controlling for all of the other sibling categories. The partial correlation between sexual orientation and number of older brothers was .275, and the partial correlation between sexual orientation and number of older sisters was .145. A two-tailed comparison of these partial correlations revealed that the older brother effect was significantly greater in magnitude, z = 3.16, p = .002.

Table 4 presents the total and expected numbers of all male siblings, older male siblings, and younger male siblings for fa’afafine and gynephilic males. We assessed whether the total number of males in each category differed from the expected values based on the Samoan population parameters using the z approximation to the binomial test. The total number of male siblings, number of older brothers, and number of younger brothers did not differ significantly from the expected values for fa’afafine. The total number of male siblings was significantly different from the expected value for gynephilic males. Subsequent analyses revealed that the observed numbers of older and younger brothers were both significantly lower than the expected values.

Discussion

The findings of this study were consistent with those of previous studies examining the relationships between male sexual orientation, birth order, and the fecundity of kin. To begin with, in both Western and non-Western cultures, androphilic males tend to be later born (Blanchard, 2004; Poasa et al., 2004; Tsoi et al., 1977; Vasey & VanderLaan, 2007; Zucker & Blanchard, 2003; Zucker et al., 2007). When we quantified birth order using Slater’s, Fraternal, and Sororal indices, fa’afafine were later born relative to gynephilic males as well as theoretical expectations based on the null model of a hypothetical stable population.

Studies conducted in Western cultures also point to an association between male androphilia and increased fecundity among kin (Blanchard & Lippa, 2007; Camperio Ciani et al., 2004; Iemmola & Camperio Ciani, 2009; King et al., 2005; Rahman et al., 2008; Schwartz et al., 2009). Vasey and VanderLaan (2007) reported that greater numbers of siblings on the part of fa’afafine were due to the existence of older brother, older sister, and younger brother effects. In the replication sample considered here, we observed independent older brother and older sister effects, suggesting that these two effects are genuine, and not the result of Type I error. However, there was no younger brother effect observed for the replication sample. In addition, the younger brother effect was not present in the combined sample even though this sample included the participants from Vasey and VanderLaan. As such, the absence of a younger brother effect in the present study raises the possibility that this effect represented a case of Type I error and was, therefore, not genuine.

It appears that there are various factors responsible for producing the older brother and older sister effects observed here. In Western samples, the older brother effect is due to a greater than expected number of older brothers among androphilic males, based on known population parameters for sex ratios, as well as a tendency for androphilic males to be later born (Blanchard, 2004). In contrast, the sex ratios of older siblings for fa’afafine did not differ from expected population values whereas gynephilic males had significantly fewer older brothers than expected in both the replication and combined samples. It seems likely, then, that the basis for the older brother effect reported here is threefold. First, gynephilic males have fewer older brothers than expected. Second, fa’afafine tend to be later born among their brothers compared to gynephilic males. Third, the mothers of fa’afafine tend to produce more children than the mothers of gynephilic males.

Clearly, the older brother effect is patterned differently in Samoa relative to the West. These differing patterns may arise due to population differences in fertility rates or attitudes that influence reproductive output, such as rules about the optimal number or sex of offspring (e.g., Blanchard & Lippa, 2007; Zucker et al., 2007). Regardless, in the context of the maternal immune hypothesis (Blanchard, 2004; Blanchard & Bogaert, 1996; Blanchard & Klassen, 1997), the consequence of producing greater numbers of children is that later-born sons will have a higher probability of being androphilic. Thus, although the sex ratios of older siblings appear to be patterned differently in Western populations relative to Samoa, the underlying mechanism that results in the developmental endpoint of male androphilia may be the same.

The tendency of fa’afafine to be later born and their mothers’ tendency to exhibit elevated fecundity are also necessary considerations to account for the observed older sister effect. The mothers of gynephilic males produced more daughters than expected whereas the mothers of fa’afafine produced more children and the sex ratio of these offspring did not deviate from the expected population value. As such, the only avenue by which an older sister effect could have emerged is through the elevated fecundity of fa’afafine’s mothers and the fact that fa’afafine are later born among their sisters. Interestingly, it has been proposed that Samoan parents decide a male child will be raised as a fa’afafine when there are insufficient numbers of girls in the family to carry out traditional female chores (Danielsson, Danielsson, & Pierson, 1978; Mageo, 1992). It is important to note, as Vasey and VanderLaan (2007) originally pointed out, that empirical evidence demonstrating that fa’afafine actually have more older sisters refutes this particular hypothesis regarding the etiology of fa’afafine.

Regardless of how the older brother and older sister effects arose, the present study found multiple, independent sibling category effects. In such an instance, it is difficult to discern whether the observed older brother effect represents a genuine fraternal birth order effect or is merely a consequence of a maternal fecundity effect. Certainly, the older brother and older sister effects, coupled with the overall sexual orientation difference in number of siblings, support the existence of a maternal fecundity effect. However, the fact that the older brother effect was greater in magnitude suggests that biological older brothers do, in fact, contribute to the development of male androphilia above and beyond any developmental influences that may be associated with biological older sisters. It appears, then, that number of older brothers is a unique predictor of male sexual orientation in Independent Samoa and, therefore, that a genuine fraternal birth order effect exists for fa’afafine. As such, the finding that the older brother effect was significantly greater than the older sister effect is the most valuable contribution the present study makes toward the literature concerning birth order and male sexual orientation in non-Western populations.

In addition to supporting the existence of both fraternal birth order and maternal fecundity effects, there was yet another consistency between the findings of the present study and those conducted in Western populations. The odds ratio of 1.33 associated with the older brother effect in Western populations indicates that each additional older brother increases the chances of developing male androphilia by approximately 33% (Cantor, Blanchard, Paterson, & Bogaert, 2002). In our combined sample, the odds ratio associated with the older brother effect was 1.34, indicating that each additional older brother increases the chances of developing male androphilia by approximately 34% in Samoa. These remarkably similar values suggest that the manner in which older brothers influence the development of male androphilia is constant across diverse populations.

Another aspect of our findings that deserves mention is in regards to the sibling sex ratios observed. As mentioned, in Western samples, androphilic males typically have an excess of brothers in relation to the expected sex ratio whereas gynephilic males do not (Blanchard, 2004). In contrast, our data from Samoa did not conform to this pattern. The sibling sex ratio for fa’afafine did not differ from the expected pattern. The lack of a higher than expected sibling sex ratio among the siblings of fa’afafine, coupled with their larger sibships, is consistent with mathematical models presented by Suarez and Przybeck (1980), which predict decreases in the sibling sex ratios of androphilic males as mean sibship sizes increase. The sibling sex ratio of gynephilic males did, however, significantly deviate from the expected population-based values, with gynephilic males having fewer brothers than expected. Interestingly, these Samoan data are consistent with patterns of relatively more strongly male-biased sibships in Western samples of highly feminine androphilic males such as homosexual transsexuals (Blanchard, 1997).

Vasey and VanderLaan (2007) also found that the sibling sex ratios of fa’afafine did not deviate from expected population-based values whereas gynephilic males had fewer brothers than expected. They highlighted that, given the Samoan population sex ratio is 109:100, it is difficult to reconcile why the families of gynephilic males, who presumably constitute the majority of the population, do not exhibit the expected sibling sex ratio whereas those of fa’afafine conform to the expected pattern. One possibility, they reasoned, was that their sample was somehow biased.

The possibility that sample bias is responsible seems questionable, however, given the consistency in the sibling sex ratio patterns observed in the present study and the study by Vasey and VanderLaan (2007). Also, because a network sampling procedure was employed to recruit participants, fa’afafine and gynephilic males were enlisted for the study in an identical manner and from the same social circles. Thus, if sample bias was somehow responsible for these sibling sex ratio patterns, then any hypotheses addressing the nature of the bias must take these two considerations into account. As an example of such a hypothesis, differences between Samoan fa’afafine and gynephilic males in emigration might create the necessary bias to produce the observed sibling sex ratio patterns. Specifically, if Samoan gynephilic males belonging to families with male-biased sibships were more likely to emigrate, then such gynephilic males would be relatively unavailable to include as participants. Thus, gynephilic males belonging to predominantly female-biased sibships would be relatively more available to sample, which could result in the sibling sex ratio patterns observed. Examining the sibships of Samoan-born fa’afafine and gynephilic males who have emigrated would aid in assessing the efficacy of this emigration hypothesis.

Vasey and VanderLaan (2007) provided an alternative explanation for the observed sibling sex ratio patterns. It is theoretically possible that a certain proportion of Samoan families are similar in composition to those of fa’afafine (i.e., greater number of children, expected offspring sex ratio). If so, this reproductive pattern would compensate for the effect of families that are similar in composition to those of gynephilic males (i.e., smaller number of children, lower offspring sex ratio), thereby creating the population-wide sex ratio observed in Samoa (i.e., 109:100).Footnote 1

Speculating further, such differences in number and sex ratio of offspring may be associated with whether individuals are related to fa’afafine. In support of this speculation, the existing empirical literature indicates that the kin of androphilic males exhibit unique reproductive patterns with respect to elevated fecundity (Blanchard & Lippa, 2007; Camperio Ciani et al., 2004; Iemmola & Camperio Ciani, 2009; King et al., 2005; Rahman et al., 2008; Schwartz et al., 2009; Vasey & VanderLaan, 2007). Also consistent with this explanation, women are capable of varying in their tendency to produce male or female offspring (James, 2000), and elevated fecundity in women is associated with elevated (i.e., more strongly male-biased) offspring sex ratios (James, 1987). It is also worth noting that fa’afafine may facilitate increased reproductive success among their relatives via the elevated avuncular tendencies they exhibit (Vasey, Pocock, & VanderLaan, 2007; Vasey & VanderLaan, 2008, 2009, in press). Whether maternal factors that increase the odds of androphilia in later-born males represent a maternal adaptation for producing avuncular sons or a by-product of elevated maternal reproduction cannot be discerned from the current literature and requires investigation.

Studies in Western cultures have demonstrated fraternal birth order as well as fecundity effects in relation to male sexual orientation. This study provided empirical support for the existence of both effects in a non-Western culture. The cross-cultural consistency with which these effects have been documented is consistent with the conclusion that culturally invariant processes underlie the development of androphilia in males. In addition, the existence of a genuine fraternal birth order effect in Samoa suggests the maternal immune hypothesis is applicable in non-Western cultures.