Marriage rates have indeed plummeted among young adults, to the point that a demographer cited by Regnerus estimates that one-third of people currently in their early 20s will never get married. But another new book about modern relationships, Eli Finkel’s The All-or-Nothing Marriage, contends that while “the institution of marriage in America is struggling ... the best marriages today are better than the best marriages of earlier eras; indeed, they are the best marriages that the world has ever known.”

Because marriage for many is no longer a gateway to adulthood, but rather an optional “capstone,” it’s held to a higher standard. Regnerus asserts that modern mating dynamics make it hard for people to find a relationship that seems worth committing to; Finkel argues that when marriages manage to live up to today’s lofty expectations, they can be extremely fulfilling. One may be more optimistic than the other, but both show how increasing romantic freedom has changed romance itself.

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Regnerus’s description of sexual economics relies on a stark division of gender roles: Men provide the demand and women are the supply. There is a long history of what he calls the “exchange relationship,” in which women control men’s access to sex. In order to get it, men bring to the table resources, commitment, and fidelity.

In previous eras, this exchange was effective at producing marriages (though it also went hand-in-hand with strict sexual mores and women’s subjugation). But now that sex before marriage and sex outside of relationships is common, safe, and less stigmatized, men don’t have to work as hard for it, according to Regnerus. So they ghost and flake and dither about committing to one person. Many women don’t need what resources men have to offer, anyway; they have their own. But men have more power in the mating market in this model, which leads to women also embracing, or at least going along with, cheap sex and some of the rude behavior that comes with it.

Regnerus doesn’t talk much about LGBT relationships, except to say that these market dynamics might make women more likely to “experiment with same-sex relationships,” to circumvent the problem of noncommittal men. He also writes that because there is no gatekeeper in gay men’s relationships, they are less likely to be sexually monogamous.

When it comes to heterosexual relationships, Regnerus sums up his theory like this: “It’s not that love is dead, but the sexual incentives for men to sacrifice and commit have largely dissolved, spelling a more confusing and circuitous path to commitment and marriage than earlier eras.”

This all smacks strongly of gender essentialism. Regnerus’s underlying premise is sound: Many studies have found that, on average, men want sex more than women, and women value having sex in the context of commitment more than men do (though of course individuals differ). Still, throughout the book, Regnerus takes this theory pretty far. He sounds a bit like your proverbial grandma cautioning that a man will never buy the cow if he’s getting the milk for free.