Sexual orientation is “an enduring pattern of emotional, romantic, and/or sexual attractions to men, women, or both sexes.”1 In explaining what causes sexual orientation, it must be emphasized that no one can choose to be gay, bi-sexual, or heterosexual. Sexual orientation is determined by two main factors: nature and nurture. Nature refers to genes and biology, while nurture includes environmental and “social influences.”2
One nature factor in sexual orientation is genes. This has been confirmed in scientific studies that measure heritability: the “amount of phenotypic (observable) variation in a population that is attributable to individual genetic differences.”3 For example, if a group of individuals all receive good nutrition (share a similar environment), the differences in height will be due to genetic differences.4 Height is a highly heritable trait.
Heritability is measured as a statistic. It is “expressed as a proportion (such as .60)”, and “the maximum value it can have is 1.00.”5 If heritability is 1.00, “then all variation in a population is due to differences or variation between genotypes.”6 If heritability is 0.00, then “all variation in the population comes from differences in the environments experienced by individuals.”7 Heritability cannot be measured for an individual person, but “only to a particular group living in a particular environment” and “only to variations within a group.”8
Heritability is estimated through twin studies. A 2010 Swedish study, using data from the Swedish Twin Registry, “undertook the largest ever population based twin study to estimate the influence of genetic and environmental effects on same-sex sexual behavior.”9 For the male twins who had “any lifetime same-sex partner”, heritability estimates were 39%.10 For female twins, only “18-19% of same sex sexual behaviors were explained by genetic factors.”11 The study found that genes played a much greater role in same-sex sexual orientation for men than for women.
The second factor in sexual orientation is nurture. The Swedish study found that for male twins, unique environmental factors accounted for 61% of same-sex sexual behavior.12 For female twins, unique environmental factors accounted for 64-66% of same-sex sexual behavior, and 16-17% was due to shared environmental effects.13 For both male and female twins, nurture played a much greater role than nature in determining a person’s sexual orientation.
It is also the consensus of psychiatrists and psychologists that nurture is an important factor in sexual orientation. According to the Royal College of Psychiatrists, “sexual orientation is determined by a combination of biological and postnatal environmental factors.”14 Similarly, the American Psychological Association states, “There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation… No findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles.”15
Notes
- “Answers to Your Questions For a Better Understanding of Sexual Orientation & Homosexuality,” American Psychological Association,1, http://www.apa.org/topics/lgbt/orientation.pdf
- Mosby’s Medical Dictionary, 8th edition. s.v. “Nature vs nurture.” accessed February 11, 2017, http://medical-dictionary.thefreedictionary.com/Nature+vs+nurture
- Encyclopedia Britannica, s.v. “Heritability,” accessed December 12, 2016, https://www.britannica.com/science/heritability
- Carol Wade et al., Psychology, 3rd ed. (Toronto: Pearson Prentice Hall, 2010), 95.
- Wade, Psychology, 95.
- Encyclopedia Britannica, s.v. “Heritability,” accessed December 12, 2016, https://www.britannica.com/science/heritability
- Ibid.
- Wade, Psychology, 95-96.
- Niklas Långström et al., “Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden,” Archives of Sexual Behavior 39, no. 1 (February 2010): 76, https://www.ncbi.nlm.nih.gov/labs/articles/18536986/
- Långström, “Genetic and Environmental Effects on Same-sex Sexual Behavior,” 77.
- Ibid.
- Ibid.
- Ibid.
- “Royal College of Psychiatrists’ statement on sexual orientation,” April 2014, 2, http://www.rcpsych.ac.uk/pdf/PS02_2014.pdf
- “Answers to Your Questions For a Better Understanding of Sexual Orientation & Homosexuality,” 2, http://www.apa.org/topics/lgbt/orientation.pdf
Christopher John Lindsay 
How we choose to express our sexuality is, however, a matter of choice.
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Very informative and interesting! Impeccable and rigorous citations as well!! Great job, as always!!! –Paul
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It was actually really interesting, tough I still think there’s no “cause” to sexual orientation. But if different studies say so, I may can make my research too. However, there’s a thing I don’t agree with in the essay that is, at the begining, where you say
that sexual orientation is “an enduring pattern of emotional, romantic, and/or sexual attractions to men, women, or both sexes.”, because there aren’t not only male and female sexes, you have more than that like the androgynous (intersex). Then, I’d describe sexual orientation as “an enduring pattern of emotional, romantic, and/or sexual attractions to an especific sex or sexes”
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