Very few people want to be childless — just 8 per cent of women aged 18-35, according to new polling. But many of us end up that way: the latest data from the ONS show that 18 per cent of women now reach the age of 45 without having any children; for younger cohorts of women, the number is likely to be higher, especially among those who attend university. The same polling found that the average number of children that young women desire was 2.3; in reality, assuming current birth rates stay where they are, they will end up with just 1.6. And this number is raised slightly by higher birth rates to foreign-born women: a young woman who was born in Britain can, all else being equal, expect fewer children.

These figures were presented on Monday by Miriam Cates MP and others at a panel in Westminster, to a packed audience of journalists and think tankers. Low birth rates, it seems, have recently passed a tipping point to become a mainstream issue in conservative politics.

Brits, along with people in almost all the world’s wealthy countries, stopped having enough babies to replace themselves back in the 1970s. So far, we have mostly been insulated from the consequences of this change by immigration, which keeps the number of workers topped up. But in the longer term, dwindling birth rates present a very serious challenge. It is not just a problem for our public finances, as an increasingly small number of working age adults are asked to support more and more dependents. It is potentially an existential cultural threat, as our economy and the maintenance of infrastructure depend fundamentally on the confidence that future generations will be there to keep them going.

To many, concern about this issue feels distasteful. When having a child is such a personal decision, it seems highly illiberal for governments to declare any interest in it. And to draw attention to the gap between the average desired and realised family sizes can be seen as a patronising disrespect for people’s choice to delay starting families or to remain childless.

But it may be a mistake to think of low fertility in this way. Arguably, unwanted childlessness has a lot in common with another ailment of modernity: obesity. (This might sound “stigmatising” but that’s not my intention — please hear me out!). Nobody wants to be obese. But daily micro-choices can add up to long-term weight gain that is extremely difficult to reverse. It’s nonsensical to say that if people arrive at this point through thousands of their own daily decisions, it is “not respecting their choice to become obese” if we have a national strategy to combat obesity. Instead, we wonder what it is about our modern environment — whether it’s the types of food available to us, or the low level of activity built into our daily lives — that leads so many of us to an outcome that is not what we would have chosen.

Similarly, unwanted childlessness can result from a sequence of smaller decisions, such as the choice not to prioritise finding a long-term relationship, or to pursue careers that are uncertain and poorly paid. Each of these might make sense in the short term, but the cumulative effect can be that people don’t try to start families until relatively late, which means they end up with fewer children than they would have liked, or none at all.

So-called “career women” tend to come in for a lot of flak here, but anecdotally, it is men who are much more likely to engage in wishful thinking that the family thing will sort itself out at some point in the future. Men often believe that the “biological clock” doesn’t apply to them, but this isn’t true — not only because male fertility declines with age, but more meaningfully, because their fertility is tied to that of a female partner. If a man wants to start thinking about becoming a father for the first time in his 40s, realistically this would mean partnering up with a woman a decade younger than him —- but the average marriage has an age gap of just two to three years, a figure that has stayed remarkably steady over time. Perhaps it is partly for this reason that childlessness is more common in men than women, with a quarter of men reaching the age of 42 without children, compared to one in five women.

There are, of course, important differences between obesity and unwanted childlessness. Unlike being overweight, plenty of people do actively choose not to have children, and there is nothing wrong with that choice. But just as the modern environment causes the average person, for a variety of reasons, to become fatter than they’d like, it also causes us to on average have fewer children than we’d like. The unaffordability of housing and childcare certainly plays a role. But even countries where housing is more affordable or where childcare is very heavily subsidised have fertility that is below replacement levels, and below the levels that young women say they want. Even Hungary, which offers very generous cash support to parents, has a fertility rate of just 1.6 children per woman.

According to one study, the most common reason that people give for unplanned childlessness is that they never found the right partner. So what is it about modern life that makes it difficult for people to form long-term relationships?

Perhaps even more fundamentally, thinking seriously about the inevitable compromises needed to have a child at a specific time, rather than at some non-specific point in the future, requires us to confront our own mortality, which is frightening. Maintaining a relationship often requires years of serious compromise before that even begins. In an age of individualism and modern conveniences, many of us are in denial about the fact that we will not always be young, and that there won’t always be a “later” in which to make plans.

Combine this with the widespread belief that having children means giving up on fun … and the results are hardly surprising

Combine this with the widespread belief that having children means giving up on fun, excitement, and self development (meaning these things must be completed to your satisfaction before starting a family), and the results are hardly surprising. One particularly memorable slide presented at the panel on monday showed an advertisement for egg freezing, photographed on the London underground: “Freeze now. Live today. Try later.” Could the idea that “life” is what happens before you have children be stated any more blatantly?

This is one of the most important problems of modern times, and we cannot allow it to remain as politicised as it has been. There are as many possible approaches to it as there are political views in the population at large. Naturally, plenty of conservatives and authoritarians are going to be attracted to this issue, and propose solutions that are in line with their worldview. On Monday, one young man suggested — to a widespread intake of breath from the rest of the audience — that the answer is to instil the idea that the only legitimate sexual activity is that which takes place “within a marriage that is open to life”. But some other suggestions, like a “universal baby income”, are radically progressive. As the reality of the demographic crunch begins to bite, governments around the world will be casting about for solutions. If liberals want their proposals to be the ones that are implemented, they will need to stop being squeamish and join the conversation.