Q. なぜ同性愛者が存在するのですか？- Why are there people attracted to the same sex?

A. 同性愛は遺伝的なものであるという説が知られていますが、他の説も存在しており、解明されていません。進化論の立場から言うと、同性愛者は子孫を残さないため、ダーウィンの言うように自然淘汰され、生き残ることができないはずです。しかし、国や時代を問わず、同性愛そのものは一般的に見られます。

その理由について、科学は全てを解明していませんが、遺伝子の発現を制御するエピマークが、胎児の性を制御する際に影響して、同性愛などを生じるのではないかとの研究が発表されています。このエピマークは、通常は胎児が男の子の場合に男性化を促し、女の子の場合には女性化を促しますが、父親から娘に（または母親から息子に）遺伝することがあり、その場合、胎児が同性愛者や性同一性障害、バイセクシャルなどのセクシャル・マイノリティーとなると考えられています。

いずれにせよ、同性愛や性同一性障害、バイセクシャルなどのセクシャル・マイノリティーも異性愛も、自分の意志で決めて選択した結果ではないというのが、広く認められた説であると言えます。

詳しくはWilliam R. Rice, Urban Friberg, and Sergey Gavrilets, “Homosexuality as a Consequence of Epigenetically Canalized Sexual Development,” The Quarterly Review of Biology, Vol. 87, No. 4 (December 2012)をご覧下さい。
>> http://www.jstor.org/stable/10.1086/668167

Q. Why are there people attracted to the same sex?

A. The answer to this question is still unclear. Theories that correlate same-sex attraction with genetics are widely known, but there are other theories as well. From an evolution theory standpoint, applying Darwin’s Theory of Natural Selection leads to a conclusion that since people attracted to the same sex do not produce offspring, this genetic trait should not be able to survive. However, there have always been people attracted to the same sex irrespective of the country or time period.

Science has not clarified everything regarding the reason(s) behind this phenomenon. Recent research argues that epi-marks, molecular changes that control genes, influence the sexuality of the fetus, and scientific studies have found that epi-marks cause fetuses to be attracted to the same sex. Epi-marks usually cause masculinization in male fetuses and feminization in female fetuses, but there are times when a male-specific epi-mark is passed from father to daughter (or a female-specific epi-mark from mother to son). As a result, the fetus can become attracted to the same sex or have a gender identity disorder. This biological process is considered to be at the root of creating sexual minorities.

It is widely accepted that homosexuality and gender identity disorders, similar to heterosexuality, are not chosen by one’s free will. Rather, sexual preferences and gender identity disorders are widely accepted to be biologically determined before birth.

For details, please refer to William R. Rice, Urban Friberg, and Sergey Gavrilets, “Homosexuality as a Consequence of Epigenetically Canalized Sexual Development,” The Quarterly Review of Biology, Vol. 87, No. 4 (December 2012).