Wikipedia’s article on the demographics of Mexico uses census data from 1793 and 1921. It states that in 1793, indigenous Mexicans made up 66.1% of the population. But by 1921, they were only 29.1% of the population, while Mestizos were almost 60%. Additionally, I remember reading that an 1820 census found that 60% of Mexicans spoke an indigenous language, which I interpret as meaning at least 60% of Mexico was indigenous at that time.
I also know that Spanish census data isn’t completely accurate due to its tendency to over- and underestimate the indigenous population.
What caused the decline? Was it a rapid growth in the non-indigenous population or a tendency for indigenous people to identify as Mestizo?
This is a complex question that can only be answered by a complex answer lol.
You see the topic of race in Mexico is very complex, given that in colonial times the people with more power were peninsulars as they were called (that came from the peninsula of spain and portugal) if you were born in Mexico but you had peninsular blood you were called criollo and native indians well, they were called indians or their respective ethnic name.
So laws were made that the people with most power were peninsular, every virrey (king of the new spain) was peninsular, criollos on the other hand had less privileges and indians were basically all slaves, so the independence from Spain was won by criollos who convinced indians to fight alongside them.
It was until the mexican revolution that indians on the south and people from the north (mestizos mostly but had more european blood than indian) fighted for their rights of owning the land they worked and were fed up from the government (there were other more complex reasons of political dispute, but this is the heroic bronze history tale that we mexicans learn at school).
The revolution left the country in shambles with a small population, so the government decided on two things: it will ban any planned parenthood propaganda and ban anticonceptives, encourage people to have children in high quantities and it removed race from the national census to unify people this is one reasonsbut it also was because due to years of miscenegation, most of the population by the 1930s had a racial admixture, only the high noble class of criollos or indians from remote towns were full blooded, you could say that was the main reason.
This miscenegation of spaniards or other races with indiands and mestizos just blurred the race line more in Mexico, however there are notable differences, the south has mostly indian genes, while the north is probably 60% european genes and the other 40% is indian and other in the general population, the center of the country is probably more 50% 50%.
So today, probably 10% of the population is full blooded indian, another 10% is full or mostly european and like 65% or more is mestizo, but is hard to know since everyone will tell you my race is mexican LOL, which is a misconception due to all what i just commented.