How Many Americans Are Married To Their Cousins?

FiveThirtyEight is a website that analyzes opinion polls.  They have an interesting article on cousin marriage.

"An estimated 0.2 percent of marriages in the United States are between individuals who are second cousins or closer — that means there are about 250,000 people in America in those relationships. I know you asked about first cousins, but all the research I’ve found uses second cousins as the benchmark of consanguinity (more commonly known as intermarriage).

You’ll notice that if you take a global perspective, consanguinity is not rare at all. Of the 70 countries studied, only 18 have consanguineous relationships as less than 1 percent of all marriages. In five countries, more than 50 percent of all marriages are between people who are second cousins or closer, and in Burkina Faso, it’s estimated that two of every three marriages are consanguineous.  High global prevalence is in itself a reason to study consanguinity — about 10.4 percent of the world’s population is married to a second cousin or closer, or is the product of such a union: That’s about 750 million people.

Marriage between cousins wasn’t always prohibited in America. From the 1860s, states began legislating on various aspects of marriage, including mental-capacity restrictions, statutory ages and anti-miscegenation laws. Bans on cousin marriage were introduced at the same time as part of a post-Civil War acceptance of the need for oversight, even in personal matters. Today, the practice is highly stigmatized, partly because of health concerns about children born to first cousins."

According to Wikipedia:

"Data on cousin marriage in the United States is sparse. It was estimated in 1960 that 0.2% of all marriages between Roman Catholics were between first or second cousins, but no more recent nationwide studies have been performed.  It is unknown what proportion of that number were first cousins, which is the group facing marriage bans. To contextualize the group's size, the total proportion of interracial marriages in 1960, the last census year before the end of anti-miscegenation statutes, was 0.4%, and the proportion of black-white marriages was 0.13%.  While recent studies have cast serious doubt on whether cousin marriage is as dangerous as is popularly assumed, professors Diane B. Paul and Hamish G. Spencer speculate that legal bans persist in part due to "the ease with which a handful of highly motivated activists—or even one individual—can be effective in the decentralized American system, especially when feelings do not run high on the other side of an issue."

As far as I can tell it is legal to marry your second cousin (same great-grandparents) in every state.