Ryan Enos performs a valuable service:

In 2008, the Pew Hispanic Center, calculated that there might be 11.2 million illegal immigrants in the United States. That is about 4% of the population. ...poll respondents have a completely distorted estimate of the number of illegal immigrants in their state. The average response when asked what percent of their state is illegal immigrants was just over 17% - that is, the average American thinks almost 1 in 5 people are illegal immigrants.  

He goes on:

Americans of all political ideologies show some tendency to, not only over-estimate the number of illegal immigrants, but also to have trouble separating Hispanics generally from illegal immigrants particularly. Let me explain: in addition to asking about the percentage of illegal immigrants in their state, I also asked what percentage of their state is Hispanic. I think it is safe to assume that when most people hear the term “illegal immigrant”, they almost exclusively assume that the illegal immigrant is from Latin America - meaning that when they say 17% of their state is illegal immigrants that they are saying that at least 17% of their state is Hispanic.

If I am correct about this, a rough way to estimate the percentage of Hispanics that a respondent thinks are illegal immigrants is simply to compare their answer to the questions about the percentage Hispanic to their answer about the percentage illegal immigrants. Among all white respondents, the average answer to the question about illegal immigrants was almost 2/3rds the size of their answer to the question about Hispanics – meaning, if they thought about it – they are likely indicating that they think that 2/3rds of all Hispanics in their state are illegal immigrants. Among “very conservative” whites the answer to the Hispanic question was, astoundingly, almost 3/4ths the size of the answer about illegal immigrants – implying that 3/4ths of Hispanics are illegal immigrants.

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