It's fairly well-established that the children of older mothers are more likely to have genetic defects than those of their younger counterparts. Now we have some of the first solid evidence that older dads could carry a risk as well.
The link between the age of a baby's mother and the risk of birth defects, more properly known as congenital disorders, is straightforward enough. A 30-year-old woman has only a 1 in 1,000 chance of giving birth to a baby with Down syndrome. By 35, the risk has more than doubled to 1 in 400, and it skyrockets after that to 1 in 100 by the age of 40 and 1 in 10 by 50. The reason behind this is the increase in chromosomal abnormalities in the mother's eggs, which naturally accumulate over time.
It's a lot harder to find definitive proof of that a similar link between mental health problems and the age of the father, though a ton of studies have been tried to demonstrate just such a relationship. Various studies have suggested links between older fathers and incidence in their children of Alzheimer's, autism, bipolar disorder, schizophrenia, type-1 diabetes, Down syndrome, lower intelligence, and multiple sclerosis, among a whole bunch of others.
That's a dramatic list, but for every study that proposes one of these links, another study comes along and argues no such causal relationship actually exists. It's a tricky issue — and, considering parents on average are waiting longer and longer to start families, an increasingly important one — but we're rather frustratingly still stuck having to sort through tons of conflicting evidence, with no clear answer in sight. That said, I should stress right now that, whatever the risk, we're still only talking about risks here, not certainties, and plenty of older mothers and fathers have perfectly healthy children, including the 66-year-old Pablo Picasso and his young son in the photo up top.
Here's the latest evidence to support a link between paternal age and increased risk of congenital disorders. The Malaysian Mental Health Survey (MMHS) recently concluded that children had only a 9% risk of mental health disorders - including obsessive-compulsive disorder, depression, anxiety, and various phobias - when the father was 19 or younger, and that slowly rose all the way to 42% if the father was 50 years or older and the mother was at least 11 years younger than the father.
It's that last bit that appears to be an unexpected driver of these disorders. Whatever the actual age of the father, being at least eleven years older than the mother meant at least a 24% increase in the risk of these mental health issues in their children. So now it's not just absolute age of the father...it's relative age as well? It probably isn't immediately obvious how that would even work. One would guess there are sociological or environmental factors at play to account for that phenomenon, but we can't say for sure.
We're much more certain about why the absolute age of the father matters. Much like with maternal congenital disorders, it's to do with the gradual accumulation of mutations in the father's sperm. If anything, it's surprising that we're having such trouble finding evidence of paternal genetic defects compared to those from the mother.
After all, women are born with all the eggs they will carry already in their ovaries, and the eggs only undergo about 23 genetic mutations by the time women reach menopause. Sperm, on the other hand, divides every 16 days, and the sperm of a 50-year-old male has undergone roughly 800 cell divisions. That's way more opportunities for mutations to creep in, but we're not seeing the effects of these mutations clearly in the data.
What we can say with something vaguely approaching certainty is that older men are at increased risk of conferring mental illnesses on their children. By the age of 50, the existence of that danger is relatively clear, but the increased risk may start as early as 35. Just what those specific risks are or how much of a danger they actually pose is a very, very open question. But it would seem that males do have their very own biological clocks, and they're most definitely ticking.