Blanchard & Bogaert 1997

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Updated: Jan 16, 2020
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Research has shown that homosexual men have a later birth order than population norms or comparable groups of heterosexual men (Blanchard & Bogaert 1997). Some theories suggest the birth order effect among brothers, include conditioning-of-arousal to same-sex fantasy, and the maternal immune response. The maternal immune response is where a mother develops antibodies during a male pregnancy (Blanchard & Bogaert, 1996b: Blanchard & Klassen, 1997). There have been little to no studies that describe the fraternal birth order and sexual orientation in women and few results have been inconsistent. This paper will focus on the homosexual influences of both male and females and outline the fraternal birth order effect on women. By evaluating methods that are used in male studies, we can use those same methods to find favorable results in women who participate.


Studies done on sexual orientation find that men and women homosexuality are familial. This means that in twin and sibling study’s research has shown that there is a link to genetic influences and not simply environmental influences. However, there are different etiologies and it has been hypothesized that for male and female homosexuality, genetic factors of homosexuality are different. (Bailey and Bell, 1993). In this paper, we will explore factors that contribute to homosexuality in women, and the birth order effect done with male participants.

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Literature Review

Sexual orientation focuses on the onset influences of homosexual orientations and the difference between neurological structures and prenatal hormonal environments. Mothers who endure stress and interact with nicotine, influence sexual orientation of the fetus (Ellis & Cole-Harding, 2001). Blanchard (2001) states that genetics of male sexual orientation can be found in the fraternal birth order effect. The fraternal birth order effect says that with every older brother a male has, the odds of which male sibling developing homosexual orientation increases.

A study conducted by Blanchard & Bogaert recognized that that fraternal birth order effect between gay men and straight men reveals a link to sexual orientation with the number of older brothers. Height and weight were considered in this study along with birth weight. Boys with two or more older brothers had lower birth weights than those of boys with two or more older brothers. These results found support in the biological explanation for a maternal immune response in the fetal development of males.

There was no consistent evidence that birth order and sexual orientation are related in women (e.g., Bogaert, 1997). The fraternal birth order and handedness have been linked with men’s sexual orientation. The connection between handedness and older brothers exhibited higher number of older brothers with homosexuality in right-handed men.

The maternal immune response assumes that human sexual orientation is influenced by the prenatal hormone called testosterone in which affect the development of the brain in the fetus. It is argued that the woman’s immune system would be capable of “remembering” the number of male fetuses that she has carried and change the amount of testosterone to the following the fetus. Women who have given birth to sons are more vulnerable to react to certain male-specific proteins than those who have not given birth to sons. With each successful male pregnancy, a mother has an increased likelihood that the male specific antigen that develops the fetal brain will reduce. Therefore, there is an increased probability of homosexuality among later born sons.

A mother’s, immune system can sense a male fetus in the womb and may turn against the fetus and treat it as a threat. When there is an increase in the production of maternal fetoproteins a hormone responsible for the mother’s defense, clearly a mother’s immune system is threatened by the male fetus. In addition, with each ongoing male pregnancy, placenta weight increases (e.g., Vernier, 1975). The placenta increases in size in the anticipation or response to an immune system attack by the mother.

Boys who refer themselves as gender dysphoria and expressed same-sex sexual attraction in their teenage years and adulthood in relation to their birth sex, can be considered prehomosexual. Although, gender dysphoria is not a factor of sexual orientation, it is an element of transgender identity. However, the sex hormone is seen as a biological influence of sexual orientation and its development. Prenatal hormone differences are apparent in human sexual orientation. Same sex fantasies have been a factor to the birth order effect in men. This means that there is an increased likelihood of sex fantasy among brothers whose experiences create a strong learning conditioning effect on the younger of siblings. Birth order reflect later born sons to be more openminded and can be subject to experimenting sexually, including same-sex relations.

Research Proposal

The research proposal that I recommend is that of which has already been done. A series of questions regarding self-identification of sexual orientation, attraction, and behavior. Research will use the interview schedule created by ‘Alfred Kinsey (Gebhard & Johnson, 1979). We will collect data from women who complete sibship information in other words information on how many siblings one has and the number of brothers and sisters. Also, by assessing individuals of their self-identify sexual orientation, participants will be asked questions about their sexual orientation, sexual attraction, sexual behaviors and fantasies (Badgett & Goldberg, 2009; Kinsey, 1941; Kinsey, Pomeroy, & Martin, 1948; Vrangalova & Savin-Williams, 2012). However, inconsistencies can arise since not all participants interpret sexual orientation classed categories differently. So to ensure validity of a multidimensional assessment of sexual orientation, an independent t test will be executed to ensure that the presence of

noteworthy differences in sexual behavior, fantasy, and attraction for each sexual orientation identification. (Vrangalova & Savin-Williams, 2012). By redoing the research done on a sexual orientation continuum we will look at five sexual orientation groups. Heterosexual, mostly heterosexual, bisexual, mostly gay, and gay/lesbian.


The hypothesis question that will be answered by the research will be that of the birth order effect in women, and their claimed sexual orientation.


The hypothesis questions will be answered by a series of question regarding sexual orientation. Participants will provide answers for a series of questions beginning with the individuals birth order. By providing researchers with the number of older siblings, participants were asked to identify the number of older brothers, older sisters, younger brothers, and younger sisters. These questions will only include siblings that have the same biological mother. An example of questions included “How many older brothers do you have? Participants will also be asked “How would you classify your sexual orientation?” and select one of five options. Heterosexual, mostly heterosexual, bisexual, mostly gay/lesbian, gay/lesbian. Participants will be asked “How sexually attracted are you to men?” and selected on a scale of one to seven. One being not at all to seven very much. In addition

Expected Results

By calculating an independent samples t test, we will be able to compare mean ratios of individuals who identified as heterosexual and individuals who identify as gay. This procedure provides a more representative sample of a population’s sexual orientation by using a multidimensional assessment.

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Blanchard & Bogaert 1997. (2020, Jan 16). Retrieved from