Want lots of children and grandchildren? Try marrying your third or fourth cousin, or so suggests a new study.

Mates who share the same great-great grandparents have more children than those who are not related, according to the study, which drew on research from the deCODE Genetics genealogical database in Iceland and is published in the Feb. 8 issue of the journal Science.

Lead author Kari Stefansson of the University of Iceland in Reykjavik and colleagues found that third cousins on average had one more child and two more grandchildren than distantly related couples, according to an analysis of Icelandic families in the last two centuries.

Between 1800 and 1824, Icelandic third-cousin couples had on average 4.04 children and 9.17 grandchildren, compared with 3.34 and 7.31 for couples composed of eighth or higher cousins, the study found. The results were similar for the years 1924 through 1949.

But not all kissing cousins reap these same benefits, researchers said. Married first or second cousins had fewer children than third or fourth cousins, according to the research.