In some research, gay men are more likely than lesbians to have characteristics that are not typical of heterosexuals. This finding indicates that biological influences may affect men and women somewhat differently. The timing of the onset of puberty is one example. The onset of puberty in the general population is typically 12 months later for boys than for girls. However, numerous studies have found that gay and bisexual men begin puberty earlier than heterosexual men, but the timing of puberty for lesbi­ans is the same as for straight women (Bogaert et al., 2002).

Some research has found a correlation between being homosexual and having older siblings for men but not for women. These studies reported that men with older broth­ers have a statistically significant increase in their chance of being homosexual, and each older brother increases the odds. No such relationship with older siblings of either sex has been found for lesbians. Researchers speculate that a maternal immune response to male fetuses occurs and increases with each pregnancy of a male fetus, and that this response influences prenatal sexual differentiation of the brain (Bogaert, 2005; Schagen

et al., 2011). However, most of the studies excluded volunteer subjects who, although they identified themselves as gay or straight, did not meet the studies’ criteria of con­sistent sexual fantasies and behavior. One study excluded 33% of the subjects (Jordan- Young, 2010). Further, other research that used a nationally representative sample of men and women five times larger than the sample size of other studies did not find a statistically significant correlation between male homosexuality and older brothers, call­ing into question those findings (Francis, 2008).

In conclusion, research suggests that a biological predisposition to homosexuality and bisexuality may exist for some individuals. However, in general, the causes of sexual orientation remain a matter of speculation. Rather than thinking in terms of a single cause for sexual orientation, it seems more appropriate to consider the continuum of sexual orientation as being influenced by dynamic interactions among various biologi­cal, environmental, and cultural factors, which are unique to each person and can vary over the life span. As researcher Lisa Diamond states, "Sexual and emotional feelings— like all complex patterns of human experience—develop as a result of dynamic inter­changes among innate, environmental, and cultural factors" (2008a, p. 250).