Eliot Slater created The Slaters Index which is used to represent the position of a persons birth order. To use this you divide the persons total amount of siblings with the number of the persons older siblings. Although, this index cannot be applied to men without any siblings for obvious reasons. According to the Toronto research groups research, their study showed that from their samples of gay men, the Slaters Index was slightly higher compared to those of straight men. The reason for gay men having a higher Slaters Index than the straight men in their study could have been due to the fact that they could have more older brothers, more older sisters, fewer younger brothers, or fewer younger sisters (LeVay, 206, p. 35-6). According to Bogaert and Blanchard, each older brother increases the chance that a man will be gay with the same set percentage of the previous value which is 33% (LeVay, 206, p. 39). However, some other studies show different results. For example, a study done by Schwartz and his colleagues found that the first two older brothers only have a small impact but grows with three or more older brothers.
Blanchard has emphasized that relying only on studies conducted on Western populations have limitations because male androphilia can be expres
sed differently depending on the individuals culture (Vanderlaan & Vasey, 2009). In non-Western cultures there are gender categories for androphilic males that are outside of simply men and women. One study done by Doug P. Vanderlaan and Paul L. Vasey investigated birth order and sexual orientation in androphilic and gynephilic males in Samoa. Androphilic males in Samoa are referred to as faafafine which directly translates into in the manner of a woman (Vanderlaan & Vasey, 2009). However, faafafines do not limit themselves to only presenting themselves in a feminine manner. They present themselves in a variety of ways and can range from being very feminine to very masculine. This particular study was done on 33 men who self-identified as faafafine and 208 self-identified straight men by using questionnaires asking questions about sexual orientation and age. The study showed that although patterned differently, the older brother effect was apparent in the Samoan population as well. The gynephilic males showed to have fewer older brothers than expected, faafafines tended to be born later among their brothers compared to the gynephilic males, and the mothers of faafafine tended to have more children than the mothers of gynephilic males (Vanderlaan & Vasey, 2009).