I have a Korean friend who had extremely thin hair as a child. Her father shaved her head, and then the hair grew in amazingly thick - she’s about 70 now, and it’s STILL thick. I think it’s the shaving and then growing back that does it.
I wish I'd known that when I was younger. My hair always been very fine, with no body whatsoever. It's why I've kept it short the majority of my life. I'm 77, and due to thyroid medications, my hair continually falls out, and has thinned even more as I've aged.