As Prince William prepares to take his oath to Kate Middleton on Friday, the ceremony will represent not only a new chapter in his life, but in the history of the monarchy. After all, he’s the first heir to the throne granted the right to freely choose his own mate on the basis of love, an ideal the rest of the Western world embraced in the late 18th century. The last time a British monarch wanted to marry the woman he loved, in the 1930s, he had to give up his throne. Even in the late 1970s, Prince Charles was compelled to suspend his long-time affair with the woman he loved so he could search for a young bride with “noble blood” who could quickly bear him “an heir and a spare.”

But when Prince William decided to abandon the medieval tradition of marrying to bolster the family’s political networks and his heirs’ bloodlines, he jumped right into the 21st century with both feet. Not only did he choose a commoner – from a multimillionaire family, true, but the daughter of a flight attendant and the great-great-granddaughter of a coal miner – he also disregarded all the conventions about proper gender roles and sexual behavior that governed marriages of royalty and commoners alike until the last third of the 20th century. Kate is a thoroughly modern Middleton, William’s intellectual equal and his live-in companion for the past year.

For thousands of years, kings and queens married their children off to seal social and military alliances and to enhance their descendants’ claim to the throne. They insisted that their boys marry virgins to ensure, as the ancient Greeks put it, that no seed from a lesser family would find its way into the furrows plowed by their son. Children’s romantic preferences played no part. Marriage among nobility was about advancing the family’s political, economic, and reproductive interests, not finding true love.

Commoners also downplayed the importance of love. The middle classes saw marriage as a means of arranging business partnerships and gaining advantageous in-laws rather than as uniting two soul mates. Peasants and artisans pressured their children to seek spouses who brought needed skills and capital to the family enterprise.

In the late 18th century, new ideas about individual rights led growing numbers of Europeans and Americans to support a new model of marriage, based on love and personal choice. But their definitions of love and criteria for choosing a mate were still nowhere near contemporary values. Love was thought to result from the union of two diametrically opposed gender stereotypes, not the equal partnership of two unique individuals. The ideal husband was older, wiser, and more competent in the public sphere than the woman. The ideal wife deferred to her husband’s greater experience and knowledge and was expected to maintain at least the fiction of chastity on her wedding day. This model held sway through the first 70 years of the 20th century.

By the 1980s, a more modern, egalitarian version of the love match was taking root. Women began delaying marriage to complete their education or gain work experience, and the age difference between husbands and wives narrowed. Fewer men listed chastity and deference as qualities they wanted in a wife, and more men said they wanted an intellectual equal. This was fortunate, because the feminist movement made women less willing to “play dumb” to catch or keep a man. In choosing a mate, men and women increasingly sought shared interests rather than adherence to rigid gender roles. Divorce rates, which had peaked in the late 1970s in both the United States and Great Britain as women gained the freedom to leave unsatisfactory marriages, began to fall again, especially for educated couples who subscribed to egalitarian conceptions of marital partnership.

The marriage of Prince William’s parents in 1981 was an awkward amalgam of medieval royal traditions and 1950s gender conventions. Prince Charles obviously expected a traditional marriage of convenience, while Lady Diana Spencer, like any good 1950s bride, believed that true love was hers now that her prince had come. She was just modern enough to cause a stir in certain circles by not including the word “obey” in her wedding vows. But there was nothing else contemporary about what Diana brought to the match, and thus there was little chance that the love she longed for would bloom.

For one thing, Diana was barely 20 when she wed. This would have been about average for a bride in the late 1950s, but by the 1980s, marrying so young had become a decided risk factor for divorce. She never graduated from high school – another powerful risk factor for contemporary marriages. And, like half of all U.S. couples in the 1950s, but unlike most of their contemporaries, Charles and Diana married after a courtship of only six months. Charles was sexually experienced, while Diana, her uncle assured the press at the time of her engagement, was “a bona fide virgin.”

Prince William and Kate Middleton, by contrast, are right in step with 21st century marital trends and are at much less risk for divorce than William’s parents. Their decision to marry is the product of love, and love marriages are generally less stable than arranged ones because love can always end. But William and Kate have many characteristics that tend to produce lasting love rather than infatuation followed by disillusionment. Like most couples who marry today, they are closer to age 30 than to 20. The age gap is between them is negligible; In fact, Kate is the older of the pair by five months. The two met in college but only began dating two years later, and aside from a brief split in 2007, they have been a couple for eight years. They are evenly matched in academic achievement: both are college graduates who earned similar – and very respectable – grades. And, like almost three-quarters of couples who wed today in the United Kingdom and the United States, they were already living together when they announced their decision to marry.

Skeptics about the match point out that couples who cohabit before marriage are, on average, slightly more likely to divorce than couples who do not. But in the United States, at least, this risk disappears for individuals who have lived only with the person they eventually marry and who make a deliberate decision to wed, rather than sliding into marriage because it’s the easiest next step. If there is any couple in England today that is not going to be allowed to slide into things, it’s Kate and William. That may well be an irritant to a modern pair that seems prone to dispense with many traditional formalities of royal life, but in this case it’s something that works in their favor.

The best thing this couple has going for them is William’s break with tradition in choosing a woman who is his educational and intellectual equal – and an older woman, to boot. For every year that a woman postpones marriage, up until her early 30s, her chance of divorce drops. Divorce rates for college-educated couples are nearly three times lower than for dropouts like Diana, and couples who consider each other intellectual equals report higher marital satisfaction. When schoolmates told Kate she was lucky to be going out with a catch like Prince William, she reportedly responded, “He’s lucky to be going out with me!” Despite the fact that the royal family is reportedly not looking forward to hobnobbing with commoner in-laws, they may be the ones who luck out.

Stephanie Coontz is Director of Research and Public Education at the Council on Contemporary Families. Her most recent book, on women and marriage in the 1960s, is A Strange Stirring: The Feminine Mystique and American Women at the Dawn of the 1960s (Basic, 2011).

*Photo courtesy of UK_repsome.