Fertility rates are falling across the globe – even in places, such as sub-Saharan Africa, where they remain high. This is good for women, families, societies and the environment. So why do we keep hearing that the world needs babies, with angst in the media about maternity wards closing in Italy and ghost cities in China?

The short-range answer is that, even though this slowdown was predicted as part of the now 250-year-old demographic transition – whose signature is the tumbling of both fertility and mortality rates – occasional happenings, such as the publication of US census data or China’s decision to relax its two-child policy, force it back into our consciousness, arousing fears about family lines rubbed out and diminishing superpowers being uninvited from the top table.

The longer range answer is that our notion of a healthy, vibrant society is still rooted in the past. The inevitable byproduct of the demographic transition is that populations age, in a chronological sense, but life expectancy, and particularly healthy life expectancy, have increased dramatically over the last half-century, and the societal definition of “old” has not kept up (though artistic experiments such as casting 82-year-old Sir Ian McKellen as Hamlet might help to challenge age-related stereotypes).

In the 19th century, a country needed youth to operate its factories, consume what they churned out and constitute a fighting force in times of war. That became less true over the 20th century, and in the 21st it bears very little relation to reality. More and more of the jobs that require stamina and strength – including fighting – are done by machines, while a nation’s products are consumed globally.

Gross domestic product (GDP) might influence a nation’s geopolitical standing and a large GDP fills government coffers, but there’s no evidence that young workers are any more productive than older ones today. Twenty-somethings and 50-somethings have different kinds of intelligence, says gerontologist Sarah Harper of the University of Oxford, but both play a part in entrepreneurship. And if you care about human wellbeing you should pay more attention to GDP per person than per country.

Demographer Ron Lee of the University of California, Berkeley, and others have shown that GDP per person, and hence living standards, are highest when fertility falls just below replacement level (around 2.1 births per woman) – to 1.6 or even less. When fertility is either much higher or much lower than that, quality of life falls off again. Lee would be worried if he was South Korea at 0.8 births per woman, or China at an estimated 1.3, he says, but England and Wales (1.6), the European Union (1.5) and the US (1.6) are all hovering around that sweet spot.

That doesn’t mean we don’t have to adapt to the new reality. We do, in part because the way many countries distribute resources is also rooted in the 19th century and is unsustainable. More people need to work longer, for example. Although creativity doesn’t fall off with age, skills change, and we need to replenish those that are lost from the workforce. And when elderly people do finally stop being productive, we need to find new ways and new workers to care for them.

Immigration – which tends to bring in young adults – is a critical component of that adaptation, smoothing the demographic transition for richer countries while redistributing capital to poorer ones where fertility rates remain relatively high. The evidence is overwhelming that, in general, immigration is good for societies – economically, but also socially. Closing doors to it is, in this sense, self-destructive.

So there’s work to do, but in a world in the grip of a climate crisis, to which we’ve added 7 of the nearly 8 billion humans in just a couple of centuries – and to which we will almost certainly add another 3 billion before our numbers start falling again – it’s absurd to say that what’s lacking is babies.

In fact, trying to force people to have either more or fewer babies turns out to be rather pointless. Despite China’s decades-long one-child policy, its fertility decline hasn’t been dramatically steeper than elsewhere in East Asia. France’s valiant efforts to encourage large families with financial incentives haven’t made much difference either, compared with the rest of Europe. As child mortality drops, and women’s health and education improve, fertility falls. Parents choose to invest more time, money and love in fewer children. You can sway their decisions slightly by making life harder or easier for families – through childcare provision, say, or parental leave allowances – but the demographic transition is unstoppable.

The climate crisis is an interesting case, because saving the planet is often the reason people give for deciding to have fewer or no children – rather than, say, improving living standards for their fellow humans more directly – but it’s not clear what impact such decisions will have on the climate. We know the climate crisis and human population growth are linked, but not exactly how. Six births per woman is demonstrably bad for the environment, but for anything up to two the evidence is far more ambiguous. Rather than deny yourself children if you want them, Harper says, better to have one or two and raise them as environmentally conscious consumers. Policies limiting carbon emissions and plastic waste would be far more effective and timely tools for undoing or at least mitigating the damage we’ve done to the planet.

Covid-19 has temporarily exacerbated the longer-term decline in fertility rates. This, too, was predictable. Familiarity breeds contempt and children, quipped Mark Twain, but only the first part holds in a pandemic. As more data comes in on this, expect more headlines about once-great nations abstaining themselves into oblivion. Don’t believe them. We do need solutions, but they don’t have to look like babies.

• Laura Spinney is a science journalist and the author of Pale Rider: The Spanish Flu of 1918 and How it Changed the World