Why did increasing cohabitation lead to new population categories in the United States?

The rise in cohabiting unmarried couples during the 20th century necessitated new statistical methods to accurately track shifting social demographics in the United States.1

As the number of unmarried couples living together grew significantly, the U.S. Bureau of the Census required a formal way to categorize these households for data collection purposes. By 1976, when the number of such arrangements in the United States approached 1,000,000, the Bureau introduced the statistical category known as POSSLQ, which stands for "persons of the opposite sex sharing living quarters".1

This classification was a direct response to the increasing prevalence of cohabitation—defined as living together as or as if a married couple—which had become a notable trend in Western societies. This shift also contributed to a rise in extramarital fertility, reflecting broader changes in traditional marriage and family patterns.12