The Impact of Birth Order on Sexuality and the Maternal Immune Hypothesis

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The chapter delves into how birth order can impact the likelihood of individuals entering same-sex unions, particularly focusing on the increased probability for youngest men with older brothers. It sheds light on the maternal immune hypothesis and recent research findings that challenge existing notions.

It's National Siblings Day! To mark the occasion, guest host Selena Simmons-Duffin is exploring a detail very personal to her: How the number of older brothers a person has can influence their sexuality. Scientific research on sexuality has a dark history, with long-lasting harmful effects on queer communities. Much of the early research has also been debunked over time. But not this "fraternal birth order effect." The fact that a person's likelihood of being gay increases with each older brother has been found all over the world – from Turkey to North America, Brazil, the Netherlands and beyond. Today, Selena gets into all the details: What this effect is, how it's been studied and what it can (and can't) explain about sexuality.

Interested in reading more about the science surrounding some of our closest relatives? Check out more stories in NPR's series on The Science of Siblings.

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