And what do men hear about having kids as they grow older? Beyond maybe a parent wishing for a grandchild, usually not that much at all.
You might assume that biology explains why women face so much pressure to have kids as they age, but men don't. After 35, the thinking goes, it gets harder for a woman to get pregnant, and, if she does conceive, there’s a greater risk the baby will have health problems. According to that thinking, men are mostly untouched by this process, often able to father kids until a ripe old age.
Except it turns out this isn’t really true. Researchers are increasingly arguing that men have a biological clock, too, even if most people don’t hear it ticking.
Research has shown that age affects a man's ability to have a child in a similar way to a woman's, though the timeline isn't the same. It’s harder for older men to father children, and their offspring are more likely to have health problems, too. In fact, new research suggests that many problematic genetic conditions may be more closely linked to the age of the father than the mother.
But few people today seem to be aware of how a man's age can affect a pregnancy and the health of the child. The missing knowledge could be useful for a young couple thinking about when to have kids, or a childless older couple feeling the pressure of aging. It could be a strong reason for many couples to rethink their plans about career and family, and for many people to reexamine their ideas about work, gender and their health.
How biological clocks started ticking in the first place
Both men and women in the U.S. today are having kids at an older age than they ever have before. The number of children born to men and women in their teens and 20s has been steadily declining, while more and more kids are born to parents in their 30s and 40s, as the data below from the Centers for Disease Control and Prevention show.
These trends have a strong link with economic class, education and race: The more educated a woman or a man is, the more likely he or she is to have children later. White, non-Hispanic women are also likely to have children at an older age than black and Hispanic mothers.
Yet the trend toward having kids later in life seems to be putting far more stress on women than men. According to a poll by Pew Research Center, 51 percent of women with children under age 18 said being a working mother makes it harder to advance in a job or a career, while only 16 percent of working fathers felt a similar impediment.
“A lot of women are postponing childbirth until they are 30, but if they can’t get pregnant at 30 they start panicking,” says Liberty Barnes, a sociologist who studies medicine and gender. “They have this very short window, and they feel this pressure, whereas I don’t think men are feeling any pressure.”
This dilemma between career and children was what gave rise to the idea of the ticking female "biological clock" as we know it.
When the term "biological clock" first surfaced in the 1950s, it referred to cyclical rhythms in the human body, like patterns of sleeping and activity, says Jenna Healey, a PhD candidate at Yale who is studying the history of reproductive technologies.
In the decades after World War II, most Americans were getting married and having kids in their late teens and 20s. But as birth control became widely available and women entered the workforce in greater numbers in the 1970s, the phrase "biological clock" began to be used to describe a new conundrum for society: the waning reproductive possibilities for older women who had postponed childbirth to focus on their careers.
Healey says the first reference she has found in the media to a woman's biological clock was a column in The Washington Post in 1978, in which columnist Richard Cohen discusses the anxiety of several career women, who he has folded together into one "composite woman."
"Sometimes, the Composite Woman is married and sometimes she is not. Sometimes, horribly, there is no man in the horizon. What there is always, though, is a feeling the clock is ticking," the column says.
The idea of a woman's "biological clock" quickly spread through popular culture, popping up in Cosmopolitan articles and Cathy cartoons. "By the early '80s it’s a very widespread cultural expression," Healey says. In 1982, an influential study in the New England Journal of Medicine warned that a woman's ability to have a baby dropped off after the age of 30 (though later studies suggested the age could be higher). In 2011, the American Society for Reproductive Medicine even funded a controversial ad campaign to remind people that "women in their twenties and early thirties are most likely to conceive."
Today, the term is commonly used to refer to women's reproductive aging, but the idea of a "biological clock" for men is less well accepted. While various studies over the past century have shown links between advanced paternal age and reproductive difficulties, it's only in the last decade that some people have begun to argue that the concept of a biological clock ought to apply to men, too.
One of the main people to popularize the idea is Harry Fisch, a New York-based doctor of reproductive medicine who wrote a book called "The Male Biological Clock" in 2005, which describes how male fertility and testosterone levels tend to decline as a man ages and what can be done about it.
Fisch says a lot of people found the idea that men even have a biological clock to be deeply offensive. “I took so much crap for writing that book, it was unbelievable," he said. "Holy cow, when it first came out, a lot of people were upset that I was debunking the machismo image of men as they age.”
Yet Fisch and others say the science is clear. As they age, both women and men experience declining fertility, declining hormone levels and increasing risk of health complications for the child -- the very combination that most people think of when they talk about a ticking "biological clock."
The science of the male 'biological clock'
Studies indicate that a man's age can affect his fertility in three main ways. The older the father, the harder it may be for a couple to conceive a baby. Older fathers are also more likely to see pregnancies result in miscarriages. And the older age of the father can potentially trigger health problems in a child, too.
Unlike women, who are born with a finite number of eggs, men continue to produce sperm throughout their life, and some can father children into their 60s and beyond — an age where women’s clocks have totally stopped ticking. George Lucas, Steve Martin and Rod Stewart all famously fathered children in their late 60s. But for most men, testosterone declines as they age, which can lead to decreased libido and erectile dysfunction. And as they get older, men also see a decline in the quantity and genetic quality of sperm.
“The aging male, at least from a reproductive perspective, is not as good when he’s older as when he’s younger,” says Alexander Pastuszak, an assistant professor of urology at the Baylor College of Medicine in Houston and a specialist in male fertility and reproductive problems.
These changes mirror similar changes in women, with a notable difference: Most seem to occur a few years later in men, and happen more gradually.
There's a lot of disagreement about when exactly in a man's and a woman's life these changes occur; various studies and experts cite different ages and time periods. But many argue that, for women, fertility declines gradually in the mid- to late 30s, and then sharply in the 40s. For men, the change appears to happen more in their 40s and 50s.
One study of 2,000 couples in France found that while women’s reproductive capacity tends to decline around 35, men experience a similar, though more gradual, decline after the age of 40. Several studies suggest that men over 35 are twice as likely to be infertile as those younger than 25. Another study of about 2,000 couples in the United Kingdom showed that, after controlling for a woman’s age and other factors, men who were 45 and older took five times longer to conceive than those who were 25 and younger.
The charts below, from the U.K. study in the journal “Fertility and Sterility,” show how the advancing age of women (on the left) and men (on the right) tend to lengthen the amount of time it takes to get pregnant. For the women and men in their 20s and early 30s that the study examined, nearly 80 percent got pregnant within a year. But those rates fell to only 36 percent for women over 40, and only 25 percent for men over 50. (You can click on the graphic to enlarge it.)
Some doctors argue against the term "the male biological clock," mostly because they don't like the metaphor. Larry Lipshultz, a doctor of urology at Baylor College of Medicine in Houston, says the term conveys a false sense of finality.
"There’s just no consensus on when men should be considered of an advanced age reproductively," he says. "But the concept is certainly one that we should embrace, in that men are not immune to the effects of aging on their reproductive system."
For men, some of these changes are treatable or avoidable. Men who are overweight are far more likely to experience low testosterone levels. And male fertility problems are often linked with other health problems, including cancer, hypertension, heart disease or kidney disease, Pastuszak says.
But other changes in a man's reproductive health are just an unavoidable consequence of the march of time. Numerous studies now show that the genetic quality of a man’s sperm degrades as he ages, which may lead to infertility, miscarriages, and an increased risk of autism, schizophrenia and certain cancers for the child. This is why the American Society for Reproductive Medicine now recommends that sperm donors "ideally be less than 40 years of age to minimize the potential hazards of aging."
The key is that, while women are born with a finite number of eggs, men form new sperm throughout their lives, with the existing sperm replicating its DNA and then splitting into two, over and over again. That might seem like a fountain of youth for men, but it’s actually more like a game of telephone — each time the process is repeated, there’s the chance that the DNA will change a little.
For this reason, the number of genetic mutations in a man’s sperm appear to increase gradually and steadily with age. According to Icelandic researcher Kári Stefánsson, one of the authors of a 2012 study on the subject, the result is that there is a much larger number of genetic conditions that are associated with the age of the father, including early onset cancers, schizophrenia and autism, than with the age of the mother.
Compared with a 20-year-old father, a 40-year-old father brings twice the number of new mutations to a child and is two to three times more likely to conceive a child that will eventually develop schizophrenia or autism, Stefánsson says. But the research shows no connection between the age of the mother and these mutations.
“To me, this was a remarkable observation because when I was in medical school, we were always warned about the age of the mother," Stefánsson says. "You could argue that in our work, there is buried a little bit of redemption for the old mother that we are always criticizing for the risk of having children, because what is truly dangerous is the high age of the father."
Many experts say these findings should be interpreted with some caution, because scientists still don’t really understand how genetic mutations ultimately result in specific conditions, like schizophrenia and autism. For example, some mutations in the sperm's genes can actually be repaired by the egg before being passed on to the child.
And others, including Stefánsson, say these findings shouldn’t dissuade older couples who want to have a child from trying to do so, because the chance of having a child with a genetic disorder is still very low. The study suggests that the chance of having a child with these disorders might double from around 1 percent to 2 percent with a man's older age, not a terribly large increase in risk "when you look at it in the context of everything that we have to go through, from the time that we are conceived until we die," Stefánsson says.
Yet Stefánsson argues the findings are substantial enough to influence public health, and how people think about paternal age more broadly. "For society, this is very important."
The second sex, at least in reproduction
The disparity between how we think about men's and women's fertility isn't just a matter of science. It also speaks volumes about our assumptions about reproduction, families and gender.
One reason is that doctors and scientists have put much more time and money into studying women's reproductive health than men's. Barnes says that, by her calculations, in 2013 there were five reproductive endocrinologists -- who mostly treat female patients -- for every one urologist specializing in male fertility in the United States. According to Lipshultz, male infertility research is easily a decade behind female infertility research in the U.S.
This is partially because women’s role in reproduction -- the cycle of menstruation, pregnancy and delivery -- is more immediately obvious than the role that men play. It's also because, for much of human history, giving birth was an incredibly dangerous activity. In the U.S., about 18 women today die in pregnancy or childbirth for every 100,000 live births, down from about 600 women per 100,000 live births a century ago.
But it’s also in part because of deeply entrenched cultural attitudes toward reproduction. Barnes, the sociologist of medicine and gender, argues there’s long been an assumption that fertility, pregnancy and childbirth are women’s issues — both in popular imagination and in the medical community.
While women are educated to think about their role in reproduction, men, generally, are not. Until recently, doctors recommended that women go to the gynecologist every year — providing ample opportunities for a woman to talk about reproduction with a doctor. Many men, in contrast, don't even know what "the male version of a gynecologist" is. (In the U.S., it's a urologist or andrologist.)
Lipshultz says the idea that paternal age affects fertility and pregnancy has not yet filtered out into clinical practice, and that he has never seen a man referred to discuss advanced paternal age. Yet any woman who will be over the age of 35 at the time of delivery will have her medical file stamped with the term "advanced maternal age," and most will undergo genetic screening for fetal abnormalities. A man's age, in contrast, is almost never considered.
“As a sociologist, I would say, of course this is linked to gender beliefs,” Barnes says.
Lessons from the ticking clock
Experts say that these findings about paternal age and fertility shouldn’t dissuade older couples who want to have a child from trying to do so. However, they could be a consideration for young people who are just thinking about when they want to have families.
“If you’re 40 and you’re trying to have a child, you do the best you can,” says Fisch, the author of the book on the male biological clock. “But I have a daughter who is 22, and my recommendation to my kids is, why don’t you try to have a family early? And that’s for both men and women. But I certainly would love it if my kids had kids before they’re 30, and that’s from a fertility doctor’s point of view.”
Alexey Kondrashov, a geneticist at the University of Michigan, agrees. He says the findings about older men and genetic mutations are not enough to discourage older parents from trying to have children. However, they might be enough to encourage young men to freeze their sperm in case they want to have kids later in life.
“18-year-olds don’t think about these things, but if I was 18 years old and capable of thinking of things like that, I would think about it carefully,” he said.
As the idea of the paternal biological clock becomes more widespread, it could also be an important guiding principle for how our society functions more broadly — for example, encouraging more doctors to talk to men about their fertility, and turning stressful workplaces into family-friendly ones. It could also be an argument for supportive policies, like paid family leave or child care, that might help men and women have kids at a younger age, says Rene Almeling, a sociologist at Yale University who studies medicine and gender.
"If we really start to think of reproductive responsibility as something that is not just applicable to women ... I think it raises a whole host of questions about other contentious debates we have in this country," Almeling says.