A candlelight vigil for the victims of the Christchurch mosque attacks in New Zealand, in Islamabad, Pakistan, on March 18. (Akhtar Soomro/Reuters)

Leslie Root is a PhD candidate in demography at the University of California at Berkeley.

“It’s the birth rates.

It’s the birth rates.

It’s the birth rates.”

This is how the accused New Zealand mosque shooter’s manifesto begins. The manifesto itself is titled “The Great Replacement,” a reference to a far-right theory that immigration and differences in birthrates will cause the gradual replacement of white people by non-white people.

As a demographer, I’m very familiar with this kind of argument. The modern academic discipline of demography was founded at a time when immigration, low birthrates among the wealthy and white, and what these trends meant for the “health” of the future population were major concerns for liberal reformers and intellectuals alike. The field’s focus gradually shifted. Nazism starkly illustrated the horrors that were a logical endpoint of eugenic privileging of population composition over individual human life. Explosive global population growth in the Cold War era made concern over low birthrates passe.

But the way contemporary demographers and the mainstream outlets that cover these issues talk about population still contributes to Great Replacement narratives. Other demographers agree: In the wake of Friday’s attack, population researchers on Twitter have begun coordinating to discuss what needs to change. We might usually think of our research as ideologically neutral. But the truth is that if we want to avoid giving scientific cover to murderous racists, we need to rethink how we talk about fertility rates and the changing racial and ethnic compositions of populations.

“Sub-replacement fertility” is a technical term that means a total fertility rate of less than about 2.1 children per woman, the number needed for each generation to exactly “replace” itself numerically. It’s convenient to summarize fertility in a single number, but the total fertility rate is a complicated metric. It fluctuates from year to year based on whether women are delaying parenthood. And the size of the reproductive-age population can change the relationship between the total fertility rate and the rate of population growth, so that a number below 2.1 does not always indicate a shrinking population.

Nonetheless, the fact that most developed countries are experiencing sub-replacement fertility is widely covered and presented as a major problem for countries such as China and states such as Vermont. And given that these appear to be long-term trends, demographers need to be much more pointed about the real consequences of negative population growth, why populations shrink and how to adapt to these trends.

Essentially, low fertility and population decline lead to an aging population and a larger share of old people relative to working-age people. This makes it difficult to fund pension systems and can slow economic growth.

But the way we talk about birthrates tends to downplay the mundane economic issues at hand, focusing instead on flashier imagery: a fertility “crash,” “plummeting” birthrates or a “demographic cliff.” In reality, fertility rates are relatively stable in much of the developed world, and we’re far from either the very low birthrates or small population sizes that would actually pose existential threats to our futures. Given that, it’s important to understand and to emphasize that low fertility is an economic, not a civilization-level, problem, and to avoid making it out to be a crisis.

The second problem is the way we talk about future population composition. The prime example of this in the United States is coverage of the coming “majority-minority” era, when people of color will outnumber white, non-Hispanic Americans. The majority-minority narrative uses a highly exclusionary version of whiteness that pits white non-Hispanics against everyone else. This formulation also ignores the fact that, even under this exclusionary definition, whites will still constitute the single largest racial group of Americans for many decades.

But the bigger problem is that this framing subtly reinforces the idea that raw numeric dominance of whites over people of color is normal or desirable. And, like sensationalist coverage of birthrates, it provides fuel for the survival-of-the-fittest mentality at the root of conspiracy theories such as the Great Replacement and white genocide.

The larger key to transforming the way we talk about population is accepting that no trend, past or future, lasts forever. The United States looked one way during the 20th-century era of very low immigration; soon, it will look another way. Eternal demographic decline would eventually be catastrophic. But so would eternal growth, and today’s birthrates don’t lock us into either scenario. We are in a relatively new era of population dynamics. The small families, long lives and geographic mobility we know today are less than two centuries old. It’s simply false that only way forward is to keep things the way we imagine they’ve always been.

Changing the way we talk about population will not change the minds of racist terrorists; I doubt that if I had fact-checked his manifesto, the mosque shooter would have reconsidered committing mass murder. But those of us who study population owe it to the world, and to ourselves as a discipline, to talk about it in ways that don’t lend false scientific and rhetorical legitimacy to their ideas.

Read more:

The Post’s View: America needs more workers. Trump’s war on immigration won’t help.

Christine Emba: The recession is long gone. Where are the babies?