From the study:
... we analyze 1785 ancient individuals from the last 45,000 years, a substantial fraction of the published global human aDNA record. ...
... we find that matings among first-cousins or closer relatives, though widely practiced today in numerous societies, are generally infrequently observed in aDNA data. ...
In contrast to the high abundance of long ROH typical of close kin unions in the present-day individuals, long ROH was uncommon in the ancient individuals, including up to the Middle Ages. Additional data from these regions and others with high levels of long ROH today, such as North Africa as well as Central, South, and West Asia, will help resolve with more precision the origin and spread of these well-studied kinship-based mating systems. Overall, our results show how an ROH-based method can be used to inform understanding of shifts in cultural marriage/mating practices.
From the story:
... a new genetic study finds only three percent of prehistoric people were the offspring of cousins. For comparison, researchers say that number is actually ten percent today.
