Saturday, August 24, 2013

Sexual Differentiation

Hormones play a significant role in the development of the fetus into a male or female. Androgen and other hormone levels produced in utero have a tremendous effect on the developing fetus. The implications of these effects may dispute long held beliefs about gender and sexual orientation, and whether an individual has the capacity to choose either of these identities. This goal of this paper is to write a brief summary of an article related to sexual differentiation. Further it describes two implications of the article's findings, and explicates whether the results of the study concurs or conflicts with the current body of literature. Finally, it will discuss one question that remains unanswered and is in need of further research.

Brief Summary of Chosen Article

The chosen study by Kangassalo, Pölkki, and Rantala (2011) suggested prenatal androgen levels influence sexual orientation in males and females and is evident in 2D:4D digit ratios. This study assessed how sexual orientation related to digit ratio as well as to the number and gender of participants’ older siblings (Kangassalo, Pölkki, & Rantala, 2011). Kangassalo et al. found that heterosexual men had lower digit ratios than non-heterosexual men. Additionally, non-heterosexual men had fewer younger siblings than heterosexual men. That this group of men also had more older sisters may mean that androgen is not the only influencing factor. Birth order may be as influential in the development of gender orientation as maternal androgen production in utero.

Aside from the androgen levels as an influencing factor, it is well established that homosexuality positively correlates to the number of older brothers and has been referred to as the fraternal birth order effect (Bogaert, 2002). This phenomenon occurs because of an immune reaction of the mother during pregnancy, which is believed to take place only when a woman is carrying a male fetus (Kangassalo et al., 2012). The immune reaction becomes stronger with subsequent pregnancies, and causes the mother to produce excessive antigens that affect the brain of the fetus; specifically, they constrain the development of typical male-oriented characteristics.

Implications of Study

Although birth order seems to influence gender orientation (Bogaert, 2002), it is unknown as to the exact nature of this influence. It may be that another maternal immune response occurs with subsequent female fetus pregnancies, or it may be an exclusively environmental factor, that having a specific number of older sisters has a particular gender-related effect on growth and development. Kangassalo et al. (2012) suggested a different environment in the womb might contribute to differences in subsequent pregnancies, in effect, altering the levels of androgen available to the fetus. More feasible, perhaps is that neither one is solely responsible, but that the mechanisms by which birth order and the number of older sisters are a confluence of factors that may be expressed in conjunction with other environmental factors and experiences.

It is well known that experience can alter neural development; perhaps experience can alter the production of hormones during post natal development. In any event, the implications of this study are that the mechanisms by which these factors influence sexual orientation remain a mystery. Another implication of these findings is the idea that female fetuses produce a different type of antigen that also affects an antigen response of the mother in subsequent pregnancies. This would explain why non-heterosexual individuals have a greater number of older sisters as well as an increased number of older brothers.

An Unanswered Question

Although not addressed in Kangassalo et al. (2012), other implications exist for these findings as well. If differences in digit ratios are, in fact, indicative or predictive of sexual orientation, the fact that these differences are established prenatally during the 13th or 14th week of pregnancy supports the idea that predispositions to heterosexuality and non-heterosexuality are created long before birth. However, the weight of this predisposition in conjunction with environmental factors, such as birth order, have yet to be quantified.

Agreeing or Contradictory Discoveries

The fundamental utilization of digit ratio to predict sexual orientation has garnered considerable research, some of which has been contradictory, although when accounting for cultural and racial diversity, much of the literature is in agreement with digit ratio utilization (Grimbos, Dawood, Burriss, Zucker, & Puts, 2010; Manning et al., 2000; Manning, Bundred, Newton & Flanagan, 2003). In this study, non-heterosexual men had more older brothers than the heterosexual men. This finding is consistent with Bogaert (2002), although in Kangassalo et al. (2011), in addition to more older brothers, the non-heterosexual men had more older sisters than did the non-heterosexual men. Because lower digit ratios signify lower androgen levels prenatally, the results of this study are in agreement with contemporary studies determining that non-heterosexual men experience higher androgen levels during prenatal development. These higher androgen levels in utero result in non-heterosexual orientation (Manning, 2002).


Bogaert, A.F. (2002). A review of recent research on fraternal birth order and sexual
orientation development. Canadian Journal of Human Sexuality, 11, 101–107. doi: 10.1016/j.yfrne.2011.02.004

Bogaert, A.F. (2003). Number of older brothers and sexual orientation: New tests and the
attraction/behavior distinction in two national probability samples. Journal of Personality and Social Psychology, 84, 644–652.

Grimbos, T., Dawood, K., Burriss, R.P., Zucker, K.J., and Puts, D.A. (2010). Sexual
orientation and the second to fourth finger length ratio: A meta-analysis in men and
women. Behavioral Neuroscience, 124, 278–28. doi: 10.1037/a0018764

Kangassalo, K., Pölkki, M., & Rantala, M. (2011). Prenatal influences on sexual orientation: digit ratio (2D:4D) and number of older siblings. Evolutionary Psychology: An International Journal Of Evolutionary Approaches To Psychology And Behavior, 9(4), 496-508.

Manning, J.T., Barley, L., Walton, J., Lewis-Jones, D.I., Trivers, R.L., Singh, D., ... Szwed, A. (2000). The 2nd:4th digit ratio, sexual dimorphism, population differences and reproductive success: Evidence for sexually antagonistic genes? Evolution and Human Behavior, 21, 163–183.

Manning, J.T., Bundred, P.E., Newton, D.J., and Flanagan, B.F. (2003). The second to fourth digit ratio and variation in the androgen receptor gene. Evolution and Human Behavior, 24, 399–405. doi: 10.1016/S1090-5138(03)00052-7,

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