Recent studies on male homosexuals showed increased fecundity of maternal female relatives
of homosexual probands, compared to those of heterosexual controls. We have suggested
that these data could be explained by the transmission, in the maternal line, of an
X-linked genetic factor that promotes androphilic behavior in females and homosexuality
in males.
Our original studies were on relatives of male subjects who declared themselves to
be exclusively homosexual. However, the relationship between homosexuality and bisexuality,
including the possibility of shared genetic factors, is complex and largely unexplored.
To cast light on this issue, in the present study we examined whether relatives of
bisexuals show the same indirect fitness advantage as previously demonstrated for
homosexuals.
Subjects completed a questionnaire on their sexual orientation, sexual behavior, and
their own and their relatives' fecundity.
We studied 239 male subjects, comprising 88 who were exclusively or almost exclusively
heterosexual (pooled to comprise our "heterosexual" group), 86 who were bisexual,
and 65 exclusively or almost exclusively homosexual individuals (pooled in our "homosexual"
group). Bisexuals were here defined on the basis of self-identification, lifetime
sexual behavior, marital status, and fecundity.
We show that fecundity of female relatives of the maternal line does not differ between
bisexuals and homosexuals. As in the previous study on homosexuals, mothers of bisexuals
show significantly higher fecundity, as do females in the maternal line (cumulated
fecundity of mothers, maternal grandparents, and maternal aunts), compared to the
corresponding relatives of heterosexual controls.This study also shows that both bisexuals
and homosexuals were more frequently second and third born. However, only homosexuals
had an excess of older male siblings, compared to heterosexuals.
We present evidence of an X-chromosomal genetic factor that is associated with bisexuality
in men and promotes fecundity in female carriers.