Inbreeding is the source of jokes about British royalty and is associated with increased birth defects among offspring. The practice is so reviled that 31 U.S. states ban marriage between first cousins or allow it only if the couple has undergone genetic counseling or at least one partner is sterile or no longer fertile because of age.

But those laws "seem ill-advised" and "should be repealed," a geneticist and medical historian write in today's PLoS Biology. "Neither the scientific nor social assumptions that informed them are any longer defensible."

The US "cousin marriage" prohibition stretches back to the 1858, when Kansas barred such marriages; Texas was the most recent state to pass a ban, in 2005, write Diane Paul, a political scientist emeritus at the University of Massachusetts, Boston, and Hamish Spencer, head of zoology at the University of Otago in New Zealand. (European countries didn’t ban the practice because there, "the rich and noble were marrying" their cousins, Spencer tells us. "In America it was immigrants and the rural poor — a much easier target of legislation than your monarch.")

First cousins share about an eighth, or 12.5 percent, of their genes, according to a 2002 study in the Journal of Genetic Counseling. Because of that overlap, there's a 1.7 percent to 2.8 higher risk of intellectual disability and genetic disorders, including seizures and metabolic errors among children whose parents are first cousins than among the general population, says Robin Bennett, a certified genetic counselor and lead author of that research.

That elevated risk is "comparable to a 40-year-old woman having children and we consider that perfectly acceptable," Spencer tells ScientificAmerican.com. "I can't imagine a law saying they're not allowed to have children."

The father of evolution, Charles Darwin, married his first cousin, Emma Wedgwood, as did Albert Einstein when he walked down the aisle with cousin Elsa. But while marriage between first cousins occurs often in some parts of the world, and was not uncommon among immigrants and the rural poor during early American history, the practice is rare in the West, Spencer says.

"It's not an issue because most people aren’t interested in their first cousin," Spencer admits. "But it does affect some individuals and it doesn’t seem particularly fair."

It's worth noting that sex between more distant cousins may actually offer reproductive advantages. Pairings between third and fourth cousins result in more offspring and grandkids than more conventional couplings between folks who aren’t related, the Icelandic biotech company deCODE genetics reported in February.

Image of DNA by iStockphoto/Luis M. Molina