“If North Korean soldiers walk across the border” A chilling warning from Musk, is it true… calculated demographically - 경향신문

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Elon Musk, CEO of Tesla, on January 20 last year, makes a gesture reminiscent of a Nazi-style salute at an event celebrating the inauguration of U.S. President Donald Trump. AFP Yonhap News

“There is no need to invade. North Korean soldiers can simply walk across the border.”

This remark by Elon Musk, CEO of Tesla, about South Korea's ultra-low fertility sounds like political rhetoric mixing exaggeration and fear, yet it leaves an uncomfortable question. At what point does a society with a rapidly shrinking population face a threat to national sustainability. Is Musk's warning merely provocative, or does it have some demographic basis. We verified the figure he cited, “South Korea population 1/127 after three generations,” using demographic arithmetic.

First, Musk's phrase “after three generations” needs to be converted into a demographic time unit. In demography, one generation is typically 25 to 30 years. Applying this, three generations are about 75 to 90 years, in other words pointing to South Korean society at the end of this century or around 2100. That is, his statement is not about an immediate crisis but a warning about what demographic structure arrives in the long term if the current fertility pattern persists.

Then how does the population decline. If we simplify population change, it can be expressed as the formula ‘next generation population = current population × (total fertility rate ÷ replacement level)’. The total fertility rate (TFR) is the average number of children a woman bears in her lifetime, and the replacement level is the fertility needed to maintain population size, accounting for mortality and sex ratio (about 2.1). South Korea's recent TFR is 0.75. Plugging this into the formula yields 0.75 ÷ 2.1 = 0.357. This means the population shrinks to about 35.7% each generation.

After three generations: Will it really be ‘1/127’?

So how much would the population actually shrink after three generations. Apply the per-generation retention ratio calculated above (about 0.357) three times in a row.

That yields the calculation that only about 4.5% of the current population remains. If we assume South Korea has 50 million people, then in 75 to 90 years it would be 50 million × 0.045, or about 2.25 million.

This is less drastic than “1/127 (about 0.8%)” mentioned by Musk, but his core message that the population would shrink to a certain percentage of today is not far off statistically. Under the extreme assumption that the fertility rate does not rebound from the current level, this arithmetic suggests the possibility that by the end of the century South Korea could shrink into a country with a population in the low millions.

The reason the numbers fall so sharply is that population decline does not proceed in a ‘straight line’ but as an ‘exponential function’. Simply put, it decreases by the same ratio each generation. Although each drop may look similar, the same ratio is multiplied to a smaller base each time, so the pace of decline snowballs over time.

Applying the per-generation retention ratio calculated for South Korea (about 35.7%) makes the change more intuitive. After one generation, the population falls from 100 to 35.7, after two generations from 35.7 to 12.7, and after three generations from 12.7 to 4.5. On the surface it may look like it is “shrinking little by little” each time, but the magnitude of decline accelerates as generations accumulate.

This is exactly the menace of ‘exponential decline’ that demographers point to. When fertility stays far below the replacement level (2.1) for a long period, the population does not decrease gently but traces a trajectory of sharp collapse after a certain point. Even if Musk's warning sounds exaggerated, mathematically the calculation is that “it is only a matter of time; the decline is inevitable.”

Would results change if the birth rate rises a little?

Then would this sharp downward trend change if the fertility rate rises even a little. Plugging in different assumptions using the same method shows a stark contrast.

First, assume the TFR recovers to 1.2. The generation-retention ratio becomes 1.2 divided by the replacement level 2.1, or about 0.571. Applying this across three generations converges to the cube of 0.571, or about 0.186. Based on the current population of 50 million, about 9.3 million would remain after 75 years. The population declines, but not at a pace that warrants the term collapse.

If the TFR recovers to around 1.5, the decline becomes more moderate. The generation-retention ratio is 1.5 divided by 2.1, about 0.714, and applying this over three generations yields 0.364. Using today's population, about 18 million would remain after 75 years. By contrast, if the TFR reaches the replacement level of 2.1, the retention ratio becomes 1, and in the long term the population size is maintained.

Ultimately, the message in the numbers is clear. Even rising from the current 0.75 to around 1.2 would drastically slow the speed of the population cliff. The problem is that South Korea remains stuck in the world's lowest “0.7 range”. The issue is not simply low fertility but that an excessively low level is becoming structurally entrenched, which darkens long-term population prospects.

Regarding the counterargument, “Is North Korea not also experiencing low birthrates?”

It is true that North Korea is also experiencing lower fertility and population aging. However, the speed and structure differ considerably from South Korea. According to recent estimates from the United Nations (UN) and the World Bank, the total fertility rate in South Korea remains in the 0.7 range, whereas North Korea is assessed at about 1.8 to 1.9. As for population size, the South has about 51 million people and the North about 25 to 26 million, so the South is roughly twice as large, but in terms of long-term decline rates, South Korea is shrinking its demographic base much faster.

Musk's phrase that “there is no need to invade, the population will disappear anyway” is certainly a political analogy. However, looking only at demographic trends, the fact that South Korea has entered a phase of population decline much more steeply than North Korea is also confirmed by statistics.

Elon Musk's remark is closer to a warning that mixes provocative expression with political rhetoric. Even so, the core figures are not entirely without demographic basis. If a TFR of about 0.75 were to hold for 75 to 90 years, a scenario in which the population of South Korea shrinks to about 3 to 5% of its current size would be well within the realm of mathematical possibility.