There are more cousins getting married than you think

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). A brief reminder here, Daryn, if, like me, you get confused about who is a second cousin: It’s someone who shares a great-grandparent with you.

The first-cousin marriages you’re asking about are just one type of consanguineous relationship (that’s pronounced kon-sang-gwin-ee-uh s in case you were struggling, as I was). To assess consanguinity, researchers give relationships an inbreeding coefficient — the higher the number, the closer the two individuals are related. First cousins have an inbreeding coefficient of 0.0625. Anything at or above 0.0156, the coefficient for second cousins, is considered consanguineous; that includes relationships between people and their nephews and nieces.

But the estimate that 0.2 percent of U.S. marriages are between people who are second cousins or closer needs to be treated with plenty of caution. For one thing, 25 states ban marriage between first cousins, and another seven states have restrictions on it (for example, in Arizona first-cousin marriage is allowed only if both people are 65 or older, or if one is unable to reproduce). Those laws might make some individuals reluctant to say they are in a consanguineous relationship and result in some undercounting of relationships.

What’s more, the 0.2 percent estimate is based on studies that were conducted in the United States between 1941 and 1981. To find out whether the share has changed since then, I emailed Alan Bittles, a professor at the Centre for Human Genetics at Edith Cowan University, which, like you, is in Western Australia. Bittles has spent almost 40 years researching consanguinity and has published more than 100 papers on the topic.