Just like moms, American dads are getting older, according to a new study. Sadly, there’s no data suggesting their jokes are getting funnier, though.

Between 1972 and 2015, the mean age of US fathers crept up 3.5 years, from 27.4 to 30.9. The percentage of dads in their 40s more than doubled, from 4.1 to 8.9 percent. Likewise, fathers 50 and above jumped from 0.5 to 0.9 percent. And the uptick in greying pops appeared across all races, education levels, and regions.

The study, led by Stanford researchers and published this week in Human Reproduction, offers the most extensive look at paternal data yet, harvesting dad data from nearly 169 million birth records over the four decades. And the researcher's finding of across-the-board aging papas is a mixed bag, they write.

As men age, semen quality declines. Researchers estimate that the germline—the cellular lineage of sperm—picks up two mutations per year. Many studies have linked older fathers to increased risks of “autism, psychiatric illness, neurologic disease such as neurofibromatosis, pediatric cancer and chromosomal abnormalities” in children.

That said, older dads appear to be better at being dads than their younger counterparts, according to earlier research. Dads aged 35 to 44 are more likely to live with their children, which suggests being there and more involved with their offspring. And having a father around at birth links to improved infant health and lower infant mortality.

The data doesn’t explain why exactly dads are getting older, but the same trend has been seen in mothers and in other countries. Between 1970 and 2000, the mean age of American moms jumped by 2.6 years, from 24.6 to 27.2. And the mean age of mothers at the birth of their first child jumped 4.9 years between 1970 and 2014, from 21.4 to 26.3. (The current study didn’t look at first-time dad ages, but earlier work has suggested it’s also increasing. It jumped from 25.3 in 1988 to 27.4 in 2010. And American families are on average having fewer children now than they were in the 1970s).

Researchers attribute the jumps in parental ages to women having more access to the workforce and higher education, plus improved contraception and fertility treatments.

While every demographic category assessed in the new study showed an upward trend in dad ages, the oldest dads were Japanese, coming in with an average of 36. White and black fathers were the youngest. Also, the lower the education level, the lower the mean father age. Dads with a college degree had an average age of 33.3. Northeastern and Western regions had the highest mean paternal ages.

The study has limitations, the authors note. For one thing, the data set wasn’t complete as it was missing some data points for certain periods and demographics. To try to make up for that, the researchers had to turn to modeling. Still, even when they plugged in the lowest age recorded in the data (11) to substitute for all the missing data, they still saw upward trends.

Overall, the researchers conclude:

Our findings substantiate the need for further research on the health and social implications of older fathers, as paternal age is indeed rising within the USA.

Human Reproduction, 2017. DOI: 10.1093/humrep/dex267 (About DOIs).