It seems you're looking for facts regarding immigration? Unfortunately all I have about that is anecdotal. Living in Arizona, I've worked with many Mexican, South and Central American immigrants, and I've heard many times that increased border control has only resulted in them bringing their whole families and staying in the U.S. because it's too difficult to cross every year as migrant workers used to.

For other issues, I highly recommend Freakonomics, the book (http://freakonomicsbook.com/thebook/index.html) or the blog (http://freakonomics.blogs.nytimes.com/). You can probably get the book or audiobook at your local library. It's enjoyable, random and thorough.

As to that e-mail, you could point out the classic defense that the same line of reasoning would lead one to believe that ice cream causes drowning, since ice cream sales and death by drowning both increase in the summer. Democrats typically gain greater support in areas with higher population density, which -the e-mail doesn't mention- have higher levels of education, but also more violent crime.

So clearly, education causes crime, eh?