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. [...]
Since then, Bittles has attempted to piece together a map on the global prevalence of consanguinity. His 2001 findings are summarized in the map below (you can also view and download the numbers here). They include Australia, where it’s estimated that 0.5 percent of all marriages are consanguineous.
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.
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