Evolution is not kind to older dads, a new research suggests. University of Göttingen, Germany researchers found that older fathers end up having fewer grandchildren. Apparently, mutations that appear in old age are transferred to the offspring. To improve the fitness of the gene pool, evolutionary processes limit the number of children these offspring can have of their own.

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Previously, geneticists showed that older fathers have genetic mutations. Most of these are harmless, but some can be harmful. The first hints were reported in the 1930s by J. B. S. Haldane who noticed a peculiar inheritance pattern in families with long histories of haemophilia. He noticed that the mutation that causes the blood-clotting disorder was much likelier to be found in the X chromosomes that fathers passed to daughters. Haldane then proposed that children inherit more mutations from their fathers than from their mothers.

Much later, in 2012, researchers found the age at which a father sires children determines how many mutations those offspring inherit. By starting families in their thirties, forties and beyond, men could be increasing the chances that their children will develop autism, schizophrenia and other diseases often linked to new mutations. “

The older we are as fathers, the more likely we will pass on our mutations,” says lead author Kári Stefánsson, chief executive of deCODE Genetics in Reykjavik. “The more mutations we pass on, the more likely that one of them is going to be deleterious.”

So why dads and not mothers? Sperm is constantly renewed from dividing precursor cells, which acquire new mutations with each division. Women, on the other hand, are born with their lifelong basket of eggs.

German researchers now suggest that evolution is weeding out these mutations by inhibiting child bearing, they report in  bioRxiv.org.

They analyzed the census records from 17th and 18th century Germany, Canada and Sweden, as well as data from 20th century Sweden national population registry. About 1.3 million people were included in the analysis, in total. Researchers found that both in preindustrial and modern times children born to older fathers had fewer kids that survived past the age of 5. For every decade that a father aged from the baseline measurement, his children had between 5% (20th century Sweden) and 13% (preindustrial Germany) fewer children of their own. This effect seems to be magnified when millions of children are involved. Over many breeding cycles, potentially harmful mutations are kept at bay in the gene pool.

“We can use this understanding to predict the effect of increasingly delayed reproduction on offspring genetic load, mortality and fertility,” the researchers conclude in their paper.

The findings might concern many older fathers, but you should not worry that much. On average U.S. married men have their first child by age 25, while the average age of single men as first-time fathers is 22, according to the U.S. Department of Health and Human Services.  That’s a lot later than a hundred years ago, or only three generations. Men are also living longer, so the same mechanisms that are inhibiting child bearing might adapt. As such, the researchers emphasize that prospective fathers shouldn’t be dissuaded from having kids by the findings.