One of the main findings was about how couples handle relationship milestones, like moving in together. Every relationship goes through milestones, or transitions, that mark how serious the relationship is getting. Going on a first date is one; a first kiss is another. Other milestones might include the “define the relationship” talk—the moment a couple says they are actually a couple—sex, engagement, marriage, and children.

In the past, these milestones tended to follow a straightforward order that began with courtship, passed the milestones of marriage, cohabitation, and sex, and ended with children. The structure and rigidity of courtship meant that couples had less freedom, but also that each milestone was ritualized with most couples following the same script. Men didn’t just propose to women, for instance; they first attained permission from the father of the bride-to-be. Couples moved through these milestones deliberately, in part due to societal expectations and in part because they knew that each step had life-altering consequences.

Now marriage comes at the end of whatever sequence people want to follow. About nine in 10 couples have sex before marriage, half of all women cohabit before marriage, and four in 10 babies are born to unwed moms.

The freedom to choose any relationship sequence has benefits, but it may also come at a cost long-term. Couples today seem less likely to move through major relationship milestones in a deliberate, thoughtful way. Rather, the new data show that they tend to slide through those milestones. Think of the college couple whose relationship began as a random hookup, the couple who moved in together so that they could pay less rent, or the couple who chose to elope on a whim rather than have a formal wedding. These are couples who, often without realizing it, slid through relationship transitions that could have been planned out, discussed, and debated.

The data show that couples who slid through their relationship transitions ultimately had poorer marital quality than those who made intentional decisions about major milestones. How couples make choices matters. For example, right after moving in with their partners, individuals were asked how they started cohabiting. On a five-point scale, they indicated whether they slid into the move or made a decision about it together. Those who rated their move as a decision had happier marriages later on. Those who moved in together without a mutual commitment to marriage first had lower marriage quality down the line. Twenty-eight percent of people who slid into cohabitation were in happy marriages while 42 percent of those who decided to live together were in high-quality marriages.

Another example is hooking up and its subsequent slide into a relationship. One-third of those who married said their relationship with their eventual spouse began as a hookup, and they, too, were unhappier in their marriages later on.