You don't know if people will divorce or not until 1) they get a divorce or 2) one of them dies instead. As a result, we can only reliably calculate this percentage for the cohort of couples who married 50+ years ago. We have many reasons to assume that couples marrying now, or who married ten years ago, will not divorce at the same rate as those who married 50 years ago. As another poster says, we can estimate based on historical comparisons and other data. The NY Times article referenced in the link shows some of that: http://www.nytimes.com/2014/12/04/upshot/how-we-know-the-div.... But we can't measure it directly. And we may get surprised - the number of people currently divorcing in their 50s and older is at a historic high, which wouldn't have been predicted by comparisons to previous cohorts.