(I think the Supreme Court in its wisdom decided then that a black man was only three-fifths of a man)
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Actually, this was not Supreme Court decision at all. This is in the Constitution.
When they were deciding how to apportion representatives, the slave states had fewer free white males than the northern states, but if slaves were counted, then there were more people in the South. So they pragmatically declared that 3/5 of a male slave equalled one whole non-slave, and the two halves of the Union were once more equal to one another in Congress.
They figured that a free White male would vote in the interests of his wife and minor children (and those who didn't have the property to be qualified to vote), and a Southern slaveholder would vote in the interests of his slaves. Slaves were most often referred to as "servants", and were seen as big dumb children who were not qualified to act on their own behalf. In return for their salvation, this was seen as a fair deal. "For what should it benefit a man if he gaineth the world and loseth his immortal soul?"
Indians counted as a full human being if they paid taxes, but those who were untaxed didn't count for squat.
The idea of declaring that a (male) slave was equivalent of 3/5th of a White freeman for the purposes of representation is generally seen as symbolic of American pragmatism.
Of course, in the Constitution, we were all seen as big dumb children, and not qualified to elect our leader without adult supervision, and the Electoral College was the result of that. That is how we came top get a big, dumb child as president as opposed to the much smarter Al Gore, who got more votes.
Turns out the people were wiser than the flipping Electoral College, which needs serious abolishing.