Nearly everyone in middle school biology learned that if you’ve got XX chromosomes, you’re a female; if you’ve got XY, you’re a male. This tired simplification is great for teaching the importance of chromosomes but betrays the true nature of biological sex. The popular belief that your sex arises only from your chromosomal makeup is wrong. The truth is, your biological sex isn’t carved in stone, but a living system with the potential for change.
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Sadly, many self-styled skeptics, especially in the UK, subscribe to this oversimplification destined to introduce a child to a complex subject (what Jack Cohen & Ian Stewart dub "lies-to-children")as if science stopped there. It's the same attitude that they lambast in others on other subjects in their blogs and elsewhere. It's easy to demonstrate e.g. that homeopathy is bunk or that vaccines save lives. That doesn't make you a geneticist, endocrinologist, psychologist, social scientist. Or, indeed, a very nice person.