An article in this month’s issue of Parents magazine explores the new “norm” of unmarried childbearing—the increasing number of younger Americans who are choosing to have and raise children in cohabiting unions instead of marriage. The article features a few happily unmarried couples raising children, most of whom echo a popular Millennial view of marriage as essentially unnecessary to parenting.

“Traditional marriage is beautiful and wonderful, but it’s not important for me because a wedding is what you do when you start your life with someone,” said cohabiting mom Allison, who is raising two kids with her boyfriend of four years. “With two kids, a dog and a cat, we’re already living it.” Jennifer, a single mom who recently ended a nine-year cohabiting relationship with the father of her three year-old-son, said prior to their split, she and her ex were “secure in our relationship, and no wedding, piece of jewelry, or common last name was going to make us feel any more so.”

While some cohabiting adults seem happy enough to live together without marriage, what about their children? It is an important question considering that about one in four American children today are born to cohabiting parents. According to Child Trends, the number of cohabiting couples with children under 18 has nearly tripled since the late 1990s—increasing from 1.2 million in 1996 to 3.1 million in 2014. The National Center for Health Statistics reports that the majority of recent non-marital births (58 percent) are to unmarried women living with their child’s father.

On the surface, the trend away from divorced or unwed mothers raising kids on their own, toward more children living with both of their parents, seems like a positive one for children raised outside of marriage. However, when it comes to child well-being, cohabiting unions more closely resemble single motherhood than marriage. As eighteen noted family scholars stated in a 2011 report from the National Marriage Project, “cohabitation is not the functional equivalent of marriage,” and it is “the largely unrecognized threat to the quality and stability of children’s lives today.”

For children, the differences between cohabiting and married parents extend far beyond the lack of a marriage license. Compared to children of married parents, those with cohabiting parents are more likely to experience the breakup of their families, be exposed to “complex” family forms, live in poverty, suffer abuse, and have negative psychological and educational outcomes.

Unstable Unions: One of the major sources of inequality between cohabiting and married parenthood is that cohabiting couples tend to split up at higher rates than married couples. According to the 2013 National Marriage Project report, Knot Yet, children of cohabiting parents in their twenties are three times more likely to experience the dissolution of their family than children born to married parents. The Fragile Families and Child Wellbeing Study (FFCW), meanwhile, finds that “nearly half of parents who are cohabiting at the time of their child’s birth break up within five years, compared to only 20 percent of married parents.”

Complex Families: Because of the fragile nature of cohabiting unions, children born to cohabiting parents are also more likely to transition in and out of new—and often confusing—family forms after their parents split up. According to the FFCW study, nearly 40 percent of unmarried mothers will cohabit with a new partner after their relationship with their child’s father ends, and 14 percent will have another child with a new partner. As Sara McLanahan and Christopher Jencks explain in a recent article, the instability and complexity of cohabiting unions “have important consequences for children’s home environment and the quality of the parenting they receive. Both the departure of a father and the arrival of a mother’s new partner disrupt family routines and are stressful for most children, regardless of whether the father is married to their mother or merely cohabiting with her.”

Child Poverty: Children raised in cohabiting unions are significantly more likely to experience poverty than those whose parents are married. In fact, cohabiting parents are second only to single mothers in terms of child poverty rates. According to a study by the National Center for Family and Marriage Research at Bowling Green State University, children in married-couple households have a poverty rate of 11 percent, compared to a 47 percent poverty rate for children in cohabiting opposite-sex couple households, and a 48 percent child poverty rate in single-mother households.

One reason for the higher poverty rates among children in cohabiting unions has to do with pre-existing differences between cohabiting and married parents. According to a Child Trends analysis, cohabiting parents tend to have less education, lower incomes, and less secure employment than married parents. Also, because cohabiting unions are more likely to dissolve than marriages, children in cohabiting unions are at a greater risk of spending time in a single-parent family, which significantly increases their poverty risk.

Child Abuse: While children living with their unmarried biological mother and her live-in boyfriend face a higher risk of suffering child abuse than kids in any other type of family, children who live with their own cohabiting parents are more likely to be abused than children of married parents. Data from the Fourth National Incidence Study of Child Abuse and Neglect shows that children living with biological cohabiting parents are over four times as likely to be physically, sexually, and emotionally abused as those living with their own married parents.

Negative Life Outcomes: On average, children living with cohabiting biological parents fare worse on several social, psychological, and educational outcomes than children born to married parents, even after controlling for factors like race, household income, and parental education. According to the National Marriage Project, children in cohabiting families are more likely to use drugs, suffer from depression, and drop out of school than children from married-parent families. While some of the negative effects of cohabitation on children can be partly explained by their parents’ lack of resources, according to W. Bradford Wilcox, “cohabitation has an independent negative impact on children.”

While cohabiting parenthood may look like marriage in that it provides children with both a mom and a dad, it is a more fragile and less safe family union than marriage that robs children of a wide range of social, psychological, and educational benefits. As Wilcox has written here, “No other institution reliably connects two parents, and their money, talent, and time, to their children in the way that marriage does.”

Despite the popularity and convenience of cohabitation, marriage is still the best setting to have and raise children. Now more than ever, we need to do a better job of communicating that truth to the next generation.