The most immediate cause of fewer children is simpler than the debates around fertility make it sound: fewer people form durable partnerships while they are young enough to have the families they want. A child normally arrives after several earlier events—a meeting, a commitment, and a shared home. Tack on the time to “find yourself”, and each event has become later and less certain.
In the United States, the median age at first marriage in 2025 was 30.8 for men and 28.4 for women, up from 23.5 and 21.1 in 1975. The change gave individuals more freedom to leave bad relationships, more time to mature, and less pressure to marry the first plausible person nearby. It also removed years from the period in which a couple can have children without age-related difficulty. A wedding delayed seven years removes seven years of childbearing, makes the first birth later, and the third or fourth less likely.
Marriage is also less common. Married couples made up 47 percent of American households in 2025, down from 66 percent fifty years earlier. Cohabitation has replaced part of the decline, and childbearing outside marriage has become ordinary enough that marriage statistics alone mislead. Roughly half of first births reported in the 2015–2019 National Survey of Family Growth occurred before marriage; about half of those premarital births occurred within cohabiting unions.
Cohabitation has not been a demographic substitute. It includes couples moving toward marriage, couples testing a relationship, and couples who have made a commitment without legal marriage. Those arrangements have different levels of stability. A household that may dissolve next year is a harder place to welcome a child, even if a first child has already arrived.
The market for a spouse
The phrase “marriage market” is a cold way to describes a human problem. A person can want marriage and children very much and still fail to find someone suitable who wants the same life at the same time.
Education has changed the pool of desirable mates. Women now earn more college degrees than men, while couples still tend to pair with someone of similar education. Income and steady employment matter too, particularly for men. A national study of sexual activity found that young men with lower income or part-time or no employment were more likely to report no sexual activity. That fits a wider pattern in which weak employment makes men less confident about commitment and less attractive as husbands, while women have less economic reason to accept an unreliable partner.
Expectations have risen alongside independence. A spouse is now expected to be a romantic companion, emotional confidant, good parent, and fair housemate for decades. These are decent hopes. Taken together they can create a search for a person who does not exist, or a reluctance to commit before every uncertainty has cleared. It never does.
Geography adds more friction. Educated young adults cluster in expensive cities; declining towns may have too few women or too few economically established men. Sex ratios become uneven across regions and religious communities. The relevant pool is the much smaller set a person might meet, admire, and plausibly marry.
Dating apps widen that pool and sometimes produce marriages that could not have happened otherwise. In 2022, 30 percent of American adults had used a dating site or app, and a little over half of users described the experience positively. Yet an apparently endless catalogue can also keep choice provisional. The apps are good at presenting options. They do not create courage, trust, shared purpose, or the willingness to stop looking.
Stability and the next child
Divorce does not explain the original postwar fertility decline by itself, but marraige instability still changes fertility. Separation can end a couple’s childbearing years together, consume time before another partnership forms, and make both adults more cautious about another child. The effect falls especially hard on a second or third birth, which requires confidence that the household will endure.
Single adulthood has also become a longer normal stage of life. Some people prefer it. Others remain single through a series of small postponements while still imagining marriage later. The difference can be invisible at thirty and painful at forty. Survey intentions preserve the family a person hopes to have after the years available to form it have begun to narrow.
I think marriage is a promise between two people, and its goods exceed demography. But that is why it matters for demography. Children are easier to welcome when two adults have promised to remain, their families recognize the promise, and the surrounding culture expects them to carry it through every ordinary difficulty.
Public policy cannot manufacture that promise. A culture can nevertheless make commitment easier to reach, honor adults who keep it, and stop pretending that partnership formation is a private hobby unrelated to its future. Before the empty school or shrinking workforce comes the household that never formed.
Citations
- U.S. Census Bureau, “Census Bureau Releases New Estimates on America’s Families and Living Arrangements”, November 2025.
- Kimberly Daniels and others, Fertility of Men and Women Aged 15–49 in the United States: National Survey of Family Growth, 2015–2019, National Center for Health Statistics, 2023.
- Peter Ueda and others, “Trends in Frequency of Sexual Activity and Number of Sexual Partners Among Adults Aged 18 to 44 Years in the US, 2000–2018”, JAMA Network Open 3, no. 6, 2020.
- Colleen McClain and others, “From Looking for Love to Swiping the Field: Online Dating in the U.S.”, Pew Research Center, 2023.