The relationship between fraternal birth order and childhood sex-atypical behavior among the Istmo Zapotec muxes.

Research on male androphilia (i.e., sexual attraction towards adult males) consistently finds that androphilic males tend to have more older biological brothers than males who are gynephilic (i.e., sexually attracted to adult females). This fraternal birth order effect (FBOE) has been well replicated among androphilic males who present publically in a male-typical (cisgender) and a female-typical (transgender) manner. There is some evidence that the FBOE is more pronounced among transgender androphilic males. However, no studies have directly compared both forms of male androphilia within the same culture. This study tested the FBOE, and its association with childhood sex-atypical behavior (CSAB), among the Istmo Zapotec of Oaxaca, Mexico, where both forms of male androphilia are referred to as a third gender, muxes. Our results indicated that both cisgender muxe nguiiu (n = 124) and transgender muxe gunaa (n = 120) were more likely to be later born among brothers than gynephilic men (n = 194). However, the number of older brothers did not differentiate between transgender and cisgender muxes, nor did it predict CSAB among muxes. These findings replicate the FBOE among both cisgender and transgender muxes but show no evidence that it is more pronounced among transgender androphilic males.