Sexual orientation is not a binary outcome

The argument

Medical professionals can test the genetics of two embryos and tell expectant parents with 100% certainty that their babies will be identical twins. Being a twin is a binary outcome. However, when it comes to testing for qualities without a binary answer, things become more difficult. For example, we cannot test the genetics of a child and immediately determine how tall that child will be. Height is a non-binary outcome and depends on the interaction of different genes, childhood nutrition, illness, and environmental impacts. Same-sex attraction is not a binary outcome. Homosexuality is a sliding scale and is, like someone’s height, not something predetermined before we are born.

Context

With binary outcomes, it is possible to say with certainty that a foetus, if brought to term, will be born with a gene that will affect its development or behaviour. With non-binary outcomes, it is not possible. Sexual orientation is not a binary outcome and is therefore not something that is dictated before birth.

Premises

Counter-arguments

The argument's own analogy defeats its conclusion. Height is offered as the model of a non-binary trait that cannot be read from a genotype — and height is among the most strongly heritable human characteristics, substantially set before birth even though no test predicts it and nutrition and illness contribute. If sexual orientation resembles height, then it is not predetermined in the sense of being predictable from a genetic test, while still being shaped by factors present from birth, which is what the opposing position claims. The argument refutes genetic determinism and treats that as refuting innateness, and those are different claims. The available evidence points the way the analogy does. Twin studies find higher concordance among identical than fraternal twins; large genome-wide association studies have identified multiple loci of small individual effect, consistent with a polygenic trait rather than a single gene; and prenatal factors, including the repeatedly observed fraternal birth-order effect, have been documented. A complex, partly heritable trait distributed along a continuum is exactly what those findings describe, and a spectrum can be entirely innate — distribution is not evidence about origin. "Not predetermined" also does not deliver what this position needs, since asserting that people are not born gay requires an account of what does determine orientation, and none is offered here. On the reasoning as given, the sibling position that we do not know is much better supported than a positive denial.

Rejecting the premises

[Rejecting P1] The analogy defeats the conclusion: height is among the most strongly heritable human traits and is substantially set before birth even though no test predicts it, so a trait resembling height is shaped from birth rather than not predetermined. [Rejecting P2] A non-binary distribution is not evidence about origin, since a spectrum can be entirely innate — and the evidence describes a polygenic, partly prenatal trait: greater concordance among identical than fraternal twins, multiple loci of small effect, and the repeatedly observed fraternal birth-order effect. [Rejecting P3] Refuting genetic determinism does not establish that people are not born gay, which requires an account of what does determine orientation; with none offered, the reasoning supports the sibling position that we do not know.