In other words, you thought that you were having fewer kids than your great-grandparents because you're living in a city instead of on a farm.But you're really having fewer kids because you're not married to your third cousin. Helgason A, Pálsson S, Guðbjartsson DF, Kristjánsson Þ, Stefánsson K. An association between the kinship and fertility of human couples. If you do genealogy, and your ancestors were in the USA west of the Atlantic seaboard before the Civil War, odds are you'll find at least one set of first cousins marrying in your tree. There was a brief span of time in the US (1870 - 1930 or so), after doctors started washing their hands and before birth control was popular, when women routinely had 8 - 12 children, most of whom lived to adulthood.

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Just to make my example easy, say the couple had 10 children who had 10 children each, for 100 grandchildren.

That means each grandchild would have 9 siblings, 90 first cousins on their father's side and 90 first cousins on their mother's side.

A number of people ask me that question after reading my page about cousins, which explains first cousins twice removed and second cousins once removed.

Short answer: sure, unless your family has a history of genetic problems.

A lot of my Monroe County relatives married first cousins.

Even more married a bit further "out" - second, first once removed, third, etc.

OK, so that might not be so easy, but if the demographic data are good enough it should be possible.

Another potential problem is outlined in the article: age at first birth: One caveat: More closely related couples may just start making babies earlier than others.

If you take it up a notch, to three generations, each great-grandchild could have 900 second cousins on each side of their family.

Second cousins share great-grandparents, not grandparents. You have four sets of great-grandparents, and (4 x 900) makes 3,600 second cousins in addition to the 180 first cousins.

The team found that women born between 18 and who partnered with a third cousin had an average of about four children and nine grandchildren, while those related to their mates as eighth cousins or more distantly had three children and seven grandchildren.