What do famous Americans such as author Edgar Allan Poe, Wild West outlaw Jesse James and theoretical physicist Albert Einstein have in common?

They all reportedly married their first cousins.

The legality of cousin marriage in the United States varies from state to state. The practice is illegal in 25 states. A first cousin is the child of either parent's brother or sister.

In some societies around the world, marrying a first cousin is often preferable, not only to keep property or money within the family, but in some cases to keep a "good catch" from going off with a stranger.

But the practice is generally viewed as taboo in the United States.

Opposition to first-cousin marriage in the U.S. dates back to the Puritans, among the earliest European settlers in America, who opposed such unions as far back as the 17th century, according to the book "Consanguinity in Context" by medical geneticist Alan Bittles.

Marriages are considered "consanguineous" when couples are either second cousins or more closely related.

The first actual laws against first-cousin marriage appeared during the Civil War era, with Kansas banning the practice in 1858, followed by Nevada, North Dakota, South Dakota, Washington, New Hampshire, Ohio and Wyoming in the 1860s.

While first-cousin marriages were once favored by the upper classes in the U.S., such alliances declined sharply in the mid-to-late 19th century, possibly because advances in transportation and communication offered perspective brides and grooms greater access to a wider pool of marital prospects.

Also, as families grew smaller, so did the number of marriageable cousins. And women became more independent during that period, so their marital options increased.

One of the earliest people to influence American public opinion on the issue was the Rev. Charles Brooks of Massachusetts. Brooks delivered a paper at a meeting of the American Association for the Advancement of Science (AAAS) in 1855 that asserted first-cousin marriage led to birth defects among the children of such unions.

Alexander Graham Bell, best known for inventing the telephone, also waded into the debate. He suggested introducing legislation to ban consanguineous marriages in families with deaf-mute members so that the condition would not be inherited by children of such marriages.

A seven-year Columbia University study published in 2018 found that children whose parents are first cousins have a 4% to 7% probability of birth defects, compared with 3% to 4% when the parents are distant relatives who marry.

From 1650 to 1850, the average person was fourth cousins with their spouse, according to the study. By 1950, the average person was married to their seventh cousin. The researchers believe that today, many couples are 10th to 12th cousins.

The data on consanguineous marriage in the U.S. is "scant and incomplete," according to Bittles. CousinCouples.com, a website for people who are romantically involved with their cousin, estimates that about one out of every 1,000 U.S. marriages is between first cousins.

However, Bittles finds that number to be unrealistically low.

"The recent large-scale migration to the USA of couples from countries where consanguineous marriage is traditional may not reveal their premarital relationship," he told VOA via email. "In terms of numbers, this particularly applies to immigrants from Arab countries ... where 20-plus percent of marriages are consanguineous, and South Asian countries such as Pakistan and Afghanistan where more than 50% of marriages may be consanguineous."

Some states allow first-cousin marriages only if the couple can't have children because they are too old or one of the parties is found to be infertile.

When you look past first cousins, there are a number of prominent Americans who married more distant cousins. Presidents Thomas Jefferson and John Adams both said "I do" to their third cousins. President Franklin Roosevelt was married to his fifth cousin, once removed. And the first wife of Rudy Giuliani, former mayor of New York and President Donald Trump's lawyer, was his second cousin once removed.

Worldwide, only a handful of countries prohibit first cousin marriages.

"Besides the USA, they comprise the People’s Republic of China and Taiwan, the Republic of Korea and the Democratic People’s Republic of Korea, and the Philippines," Bittles says. "Even in the People’s Republic of China, the ban on first-cousin marriages is not enforced in officially recognized ethnic minorities where consanguineous marriage has been traditional."

Bittles expects the number of cousin marriages in the U.S. to diminish over time as family sizes decline and there are fewer cousins available to marry, and as the children of migrants internalize negative mainstream U.S. views on marrying your cousin.