There’s a whole lot of marrying going on in Jane Austen’s novels. Among the major characters of her six major novels, at least nineteen couples tie the knot.

One wedding was so singular that it could have been halted in certain quarters, then and now. The marriage in Mansfield Park between Fanny Price and Edmund Bertram, who are first cousins, would have been illegal for much of England’s history and would still have been illegal under Catholic canon. Even today, marriage of first cousins is illegal in half the jurisdictions of the United States, though it is legal in other Western nations—and quite common in other parts of the world.

As one might suspect, English law on cousin marriage diverged from Catholic doctrine as the result of Henry VIII. His tendency to tire of a wife—and his need to sire a male heir—put him regularly in need of a new marriage. This regularly put him afoul of church doctrine.

Just as he manipulated canon law to have his first marriage to Catherine of Aragon annulled so that he might marry Anne Boleyn, he later had the marriage laws altered so that he could marry Catherine Howard. Under the old law, his marriage to Howard would have been incestuous because she was Anne’s first cousin. The law applied whether a person was cousin by blood or marriage. Where before no one closer than fourth cousins could marry, the Marriage Act of 1540 made marriage legal for first through third cousins.

A ban on incestuous marriages probably preceded civilization, as people recognized that inbreeding caused deformities and other birth defects. In ancient times, no one knew what degree of separation would prevent problems, so tradition (often via religion) became very cautious.

Modern genetics largely contradict the fear of defects among the children of first cousins. Unless both carry a specific genetic problem, the risk for cousin couples is only 1.7 to 2.8 percent higher than with other couples. Conversely, cousin couples suffer fewer miscarriages. It has been posited but not proven that similar blood chemistry may account for the lower miscarriage rate.

By the 1800s, cousin marriage was not unusual. The most famous of Austen’s time was that between the Prince Regent and Caroline. Similar blood chemistry didn’t help much in that horrific mismatch!—a mismatch that Austen comments on in her letters when she sides with the princess.

Closer to home, Jane’s brother Henry married Eliza, their first cousin, whose exotic charm created sibling competition between Henry and James as to which cousin would win her heart. A first-cousin marriage occurred in the family’s next generation, too, when Francis, the oldest son of Jane’s brother Frank, married Fanny, the daughter of Frank and Jane’s brother Charles.

By that generation, it was estimated that about one in fifty marriages for ordinary people involved cousins vis-à-vis about one in twenty for the aristocracy and other swells; the higher number among the wealthy likely related to the desire to keep family property together. The estimate came from George Darwin, the son of Charles Darwin and Emma Wedgwood of the pottery dynasty—first cousins! Whenever his children became ill, Charles worried that they were weak from inbreeding.

Cousin marriage appears twice in Austen’s novels. In Pride and Prejudice, Lady Catherine proves the economic rationale for cousin marriage—that of building family fortunes—in her determination to join her daughter Anne to her nephew. Darcy has the good sense to reject his listless relative for the spirited if poor Liz Bennet.

And of course Fanny and Edmund marry at the end of Mansfield Park. Whatever the church tradition, which still discouraged cousin marriage, no eyebrows shot up. Interestingly, the subject is raised before Fanny is ever invited into the family, when Mrs. Norris declares that Sir Thomas need not worry about a match between one of his sons and their cousin: “do not you know that, of all things upon earth, that is the least likely to happen, brought up as they would be, always together like brothers and sisters? It is morally impossible. I never knew an instance of it.”

By the novel’s conclusion, many years later, Sir Thomas gives not a thought to the match being between cousins, recognizing only Fanny’s many virtues.

Austen makes a point of mentioning “married cousins” on the last page, but only in the context of their joy in a relationship “as secure as earthly happiness can be.” It’s as if the familiarity that came from cousinage—their growing up together in the same house—bode well for a companionable life. At the very least, in this family of affairs, divorce, elopements, and general scandal, Fanny’s moral worth transcends any consanguineous concerns.

Next time: A look at another form of consanguinity.