I misread someone else's comment about Biden's hair transplant.
I reckon Biden's hair transplant is more recent than 1991. That year his frontal hair was far less thick and full than now.
But even if his transplant was 20 years old, that wouldn't cause him to necessarily lose all his transplanted hair anyway. When one looks at the evolution of his hair through the years, it's clear that he maintained lots of crown hair 20 years after puberty (a period of higher Testosterone production in him than the last 2 decades). He was born in 1942, so his puberty happened in about 1957. 20 years later in 1977 he still had a fairly good hair cut.
This is a post transplant picture. Hair loss was obviously aggressive at the back. I'm not sure when this was taken but my search parameters on Google include '1980'.
Correction: I meant "pre-transplant" when I posted "post-transplant"
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I guess your next move would be to find pictures right after the transplant or during the 1990s showing the side or back of his head.
I reckon, he had hair transplanted to that area too. Remember, his frontal scalp is NOT as aggressive as his vertex back.
If there was a transplant at the back, then you don't have to wait twenty years for your answers.