Huh. It's just occurred to me that Superman was an illegal alien... I'm not all that well versed in comic book lore though; did he ever actually get naturalized?
The Nationality Act of 1940 basically says if someone presumably under 5 is found in the US, theyre a citizen unless theyre proven to be from another country. In a couple stories he's granted honorary citizenship.
Of course it depends on the storyline. In one, Superman renounces his American citizenship while Clark Kent kept it.
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The flying and susceptibility to kryptonite is a dead giveaway.
I’d guess quite a lot of babies were born outside of hospitals back then. Getting a birth certificate for a baby probably wasn’t terribly hard.