I doubt his drug use made him go bald, what about all the legendary rockers who kept their hair...
either he got unlucky and became NW7 but what Fred said is common knowledge, he got badly burned during that commercial and that is where his addiction to painkillers came from.
His brothers all seem to be NW2.5 at worst so it doesn't seem like MJ had the genetics for male pattern baldness, if anything his family was blessed in that department.
Imagine if he hadn't gotten burned during that advert, his life might have been drastically different. He might have kept his afro-style hair and maybe not tried to be white as much?
Imagine if your self image is a young carefree boy (no matter what his issues were) - it must have been soul destroying to have to look in the mirror and apply that wig. it must be bad enough for a person with a normal psyche. (one of the reasons i refuse to wear one)
His brothers all seem to be NW2.5 at worst so it doesn't seem like MJ had the genetics for male pattern baldness, if anything his family was blessed in that department.
I don't mean to be pedantic, but... check out big brother Jermaine's "hairline":
And little brother Randy has shaved his head completely, though from looking at other pics of him, he may be trying to cover up grays rather than baldness.
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The rest of the Jackson brothers seem to have pretty intact hairlines, though, so your point still stands. Who's to say how Michael's would have turned out.