Like so many things, it comes down to a mixture of genetics and hormones. We know that at least some of the genes for hairiness are carried on the X chromosome – and the way these genes are expressed varies between different men and ethnic groups – but hairiness also correlates with high testosterone levels.
Male hairiness may in fact have evolved as a way of signalling testosterone levels to potential mates, because testosterone also makes men develop more muscle. So it’s possible that your boyfriend’s hairiness was one of the things that attracted you to him, as you subconsciously sought out a partner who could take down a woolly mammoth.
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