Why do Mestizos usually say they are mostly Native than Spanish when they are not?
In theory, Criollo status could also be attained by people of mixed origin who had the equivalent of a great grandparent with Amerindian ancestry. Such cases might include the offspring of a Castizo (3/4 Spanish, 1/4 Indian) parent and one Peninsular or Criollo parent. This one-eighth rule, also in theory, did not apply to African admixture.
A person's legal racial classification in colonial Spanish America was closely tied to social status, wealth, culture and language use. Wealthy people paid to change or obscure their actual ancestry. Many indigenous people left their traditional villages and sought to be counted as mestizos to avoid tribute payments to the Spanish. Many indigenous people, and sometimes those with partial African descent, were classified as mestizo if they spoke Spanish and lived as mestizos.Often, but only early on, the term mestizo was associated with llegitimacy; The term also has a pejorative use about something that is not "pure". However, it evolved in the ensuing centuries. According to historians Michael C. Meyer and William L. Sherman, early in the 16th century Spanish colonial usage of the term, mestizo "was almost synonymous with bastard" (illegitimate child).
Because the term had taken on a myriad of meaning the designation "Mestizo" was removed from census counts in Mexico and is no longer in use. In May 2009, Mexico's National Institute of Genomic Medicine issued a report on a genomic study that involved 300 Mestizos from the states of Guerrero, Sonora, Veracruz, Yucatán, Zacatecas, and Guanajuato. The study found that the Mestizo population of these Mexican states were on average 55% of indigenous ancestry followed by 41.8 % European, 1.8% African, and 1.2% East Asian ancestry. The study also noted that whereas Mestizo individuals from the southern state of Guerrero were on average 66% of indigenous ancestry, those from the northern state of Sonora were about 61.6% of European ancestry. The study found that there was an increase in indigenous ancestry as one traveled towards the Central and the more Southerly states of the country, while the indigenous ancestry declined as one traveled to the Northern states in the country, such as Sonora.
According to another study presented by the American Society of Human Genetics Mexicans were found to be 58.96% European, 36.05% Amerindian, and 5.03% African. Sonora shows the highest European contribution (70.63%) and Guerrero the lowest (51.98%). In Guerrero one also observes the highest Asian contribution (37.17%). African contribution ranges from 2.8% in Sonora to 11.13% in Veracruz. 80% of the Mexican population was classed as mestizo (Amerindian-Spanish).