A great many things have changed for humanity since the first of our species appeared on Earth. But throughout human history, one thing has apparently remained the same: men have always approached parenthood much later in life than women, new research suggests.
A study conducted by scientists at Indiana University in Bloomington in the US tracked genetic mutations in modern human DNA sequences to estimate when men and women conceived babies over the past 250,000 years, since our species emerged.
To do that, they created a model based on data tracking the age of conception and DNA mutations over three generations of 1,500 Icelanders and their parents. They first applied this model to a sample of 2,500 modern people living around the world, and then dated back the emergence of different mutations to create a timeline of motherhood and fatherhood through the ages.
What they found is that through the millennia, dads were considerably older, on average, than women: men became fathers at 30.7 years old, while women became mothers at 23.2 years old. That means that throughout history, men discovered the joys and pains of parenthood a staggering seven years later than women (whether they took – and now take – an active role in parenting is debatable, of course).
The findings were published in Science Advances.
Previous studies had also found that men have typically become parents later than women, but their estimates were limited to the past 40,000 years. Finding out at what age people had children in the distant past is not an easy feat, but mutations spontaneously occurring between generations offer scientists a way to track back that data.
These mutations are not shared between parents and children, but tend to occur when DNA becomes damaged before conception. According to recent research, older parents might be passing more mutations than younger parents to their children.
Biological clock ticking
But why is it that men became fathers so much later than women became mothers?
“The longer generation times for men can be generally explained by the fact that men are biologically able to have children later in life than women, bringing up the average age of fatherhood”, lead researcher Richard Wang said in the journal Nature.
But the answer is not that simple, because the infamous ‘biological clock’ which women are told starts ticking as they approach the age of 30 is not the only factor to consider.
The average age at which people have children - both women and men - is influenced by many different factors, Wang and his colleagues wrote, including environmental and cultural factors.
The finding could also point towards social factors such as pressure on men in patriarchal societies to build status before becoming fathers, Mikkel Schierup, a population geneticist at Aarhus University in Denmark, told Nature.
Other non-biological factors have led to changes in the age women around the world have children nowadays, for example. According to the World Population Review, women on average have their first child around the age of 28. In Western countries and other nations where women are able to pursue a career, some women are having their first child in their 40s.
The team at Indiana University acknowledged this trend, with Wang and his colleagues writing that they have reported “a substantial increase in female generation times in the recent past”.
But as the average age of mothers is changing, it’s also likely that that of fathers is gradually rising too. Whether dads will always be, on average, older than mums, is something only time will tell.