It can be challenging to characterize paternity trends because slightly over 10 percent of U.S. birth records are missing the father's information.
"Urologists are kind of gatekeepers for men’s reproductive health. This gives us a glimpse into the basic demographics of paternal populations, and that's important," says Eisenberg.
A Shifting Norm
Women have been waiting longer and longer to enter motherhood, though fathers and mothers are both stretching out their prefamily years. The average age gap between first-time mothers and fathers is now 2.3 years.
"The gap between the father's age and mother's age has declined, suggesting that maybe there’s a bit more parity now between parents," says Eisenberg. "I think that's a good thing."
In May, the CDC released data that marked an important milestone in modern U.S. society: For the first time, more women are entering motherhood in their 30s than their 20s. In 2016, the birth rate among women ages 30 to 34 narrowly surpassed that of women 25 to 29.
The Stanford University analysis reported variations by race, education and even geography. While mean paternal age has increased across all races and ethnicities, Asian-Americans were the oldest dads, while African- and Hispanic-Americans were on the younger side of the spectrum. In 1972, the mean age of Japanese-American fathers was 30.7 on average. In 2015, it was just over 36 -- the steepest increase of all.
Fathers with college degrees are waiting the longest to have kids. Men who live in the Northeast are the oldest overall.
Reproductive Risks From Waiting For Fatherhood
It can be difficult to characterize who an "older" father is. Some studies include men over 35, others start the clock at age 40.
But a preponderance of studies do agree that the longer a man waits to have children, the higher the risk of his offspring developing autism or schizophrenia, or being born with a birth defect.
"Men who are 55 have a higher risk than those who are 35. The incidence is two to three times in men who are over 40 years old -- this seems to be the consensus right now," says Dr. Magdalena Janecka, who studies genetic and developmental psychiatry at the Icahn School of Medicine at Mount Sinai, in New York.
There are two widely studied, perhaps overlapping, hypotheses for this. One is that as men age, their sperm develop genetic mutations. A 2004 study published in Nature found that the number of DNA mutations in sperm is three times higher in men aged 36–57 than in men under age 35. (The study is behind a paywall but the results are summarized here).
The second hypothesis, which Janecka says has stronger grounding in research, is that men who get started as fathers later in life are a self-selecting group who may have their own social development challenges. Those traits then get passed on to their offspring.
"They themselves may suffer from some autism traits; they may be more withdrawn, for instance," says Janecka. "Those men would be more likely to have a child with autism at any time in life."
Both Janecka and Eisenberg point out that the autism risk, though well-documented, is still very small and statistically rare.
"People do ask me about risk to offspring, being an older father. I tell them the risks are at a population level, not at an individual level," says Eisenberg.
But the numbers, now that he has them, do speak for themselves.