The One-drop rule in America started because West Africans were brought over as slaves to work on plantations, and it wasn't uncommon for White Southerners to have sexual relations with Black slaves.The One-drop rule became common to identify the individuals of such pairings and "keep them in their place", so to speak - the fact that they had European ancestry wasn't enough to make them "White" in society, because they had obvious Black admixture and were born into slavery. Even after slavery was abolished in 1865, the one-drop rule was still prevalent; having any African ancestry made one "Black". Mestizos in America have a completely different history than Black Americans. But I wouldn't rule out the One-drop rule for them either: although they have European ancestry, they certainly were never considered "White". For example, on the US Census Bureau forms, there is a box specifically for "Non-Hispanic Whites" implying that having any Hispanic blood makes one Non-White.