12.6 Proximate environmental causes Part 1: Is sexual orientation influenced by the environment?

What do we mean by the environment?

Several studies have found correlations between same-sex sexual preferences and environmental conditions. In this case the “environment” can either be the many aspects of the environment a child grows up in after birth, or the pre-birth (intrauterine) environment, which is the conditions within the uterus during fetal development.

The post-birth environment

Studying the impacts of post-birth experiences is challenging for many reasons. It is very difficult to disentangle the impact of a tolerant environment on a homosexual person’s inclination to express their homosexuality. For example, there has been work suggesting that children of gay parents are more likely to grow up expressing same-sex sexual preferences. Is this because growing up in a gay family actually influences an individual’s sexuality, or because a family that is accepting of homosexuality creates a safe space for a homosexual or bisexual individual to express their sexuality?

Similarly, work in Denmark has shown that growing up in an urban environment is associated with the choice to marry a person of the same sex later in life. Diverse metropolitan areas are typically associated with greater tolerance towards gays and lesbians, so is it simply that this tolerance supports the expression of an existing characteristic, or is there something else about cities that causes homosexuality?

The pre-birth (intrauterine) environment

Somewhat more compelling is the work on the prenatal environment and homosexuality. According to many of these studies, differential exposure to prenatal hormones within the uterus, specifically the hormone testosterone, influences sexuality later in life.

Several studies have found evidence, through studying the development of certain body parts (e.g., fingers, ears), that lesbians were exposed to more testosterone in utero than were straight women. For example, finger (or “digit”) lengths, especially the ratio between the second (2D) and fourth (4D) fingers, seems to vary as a function of exposure to testosterone in the womb. The result is that, on average, the 2D:4D ratio is larger in women than men, and in lesbian women compared to straight women. Note that these differences are rarely noticeable without doing precise measurements of an individual’s finger lengths — you can’t guess someone’s sexual orientation by looking at their hands! This finding about testosterone exposure does not fully explain causes of homosexuality. It’s one loose but intriguing pattern.

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  Check Yourself

    Figure 12.7b) Comparison of 2D:4D ratio