The development of human sexual orientation remains a complex and multifaceted subject. It is often studied but its origins continue to elude us. In this preregistered study, our primary objective was to demonstrate the fraternal birth order effect (FBOE), which assumes a higher prevalence of older brothers in gay men than in their straight counterparts and which has also been recently recorded in lesbian women. Our second aim was to explore any potential impact of the FBOE on anal-erotic role orientation (AERO), both in gay and straight men. Our study sample included 693 gay men, 843 straight men, 265 lesbian women, and 331 straight women from Czechia and Slovakia. Employing a conventionally parameterized logistic regression model, we substantiated the FBOE among both gay men (OR = 1.35 for maternal older brothers) and lesbian women (OR = 1.71). These outcomes were confirmed by a more nuanced parameterization recently proposed by Blanchard (2022). Nonmaternal older brothers did not exhibit a significant influence on their younger brothers’ sexual orientation. Contrary to some earlier reports, however, our data did not establish the FBOE as exclusive to gay men with the receptive AERO. Furthermore, our observations indicated a lower offspring count for mothers of gay men compared to mothers of straight men. Emphasizing the positive FBOE outcomes, we acknowledge the need for caution regarding the various options that can be used to estimate the familial influences on sexual orientation.

人类性取向的发展仍然是一个复杂且多方面的主题。它经常被研究，但它的起源仍然让我们困惑。在这项预先注册的研究中，我们的主要目标是证明兄弟出生顺序效应（FBOE），该效应假设男同性恋者中哥哥的患病率高于异性恋者，而且最近在女同性恋者中也有记录。我们的第二个目标是探索 FBOE 对男同性恋者和异性恋者的肛交角色取向 (AERO) 的潜在影响。我们的研究样本包括来自捷克和斯洛伐克的 693 名男同性恋、843 名异性恋男性、265 名女同性恋和 331 名异性恋女性。采用传统的参数化逻辑回归模型，我们证实了男同性恋（对于母亲哥哥，OR = 1.35）和女同性恋（OR = 1.71）之间的 FBOE。 Blanchard (2022) 最近提出的更细致的参数化证实了这些结果。非母亲的哥哥对弟弟的性取向没有表现出显着的影响。然而，与一些早期的报告相反，我们的数据并没有表明 FBOE 是具有接受性 AERO 的男同性恋者的专属。此外，我们的观察表明，与异性恋男性的母亲相比，男同性恋者母亲的后代数量较低。在强调 FBOE 的积极结果的同时，我们承认需要谨慎对待可用于估计家庭对性取向影响的各种选择。