“Net immigration (the number of people who migrate to the United States minus the number who leave) is projected to become an increasingly important source of population growth in the coming years, as declining fertility rates cause the annual number of deaths to exceed the annual number of births starting in 2030,” the CBO writes. “Without immigration, the population would begin to shrink in 2030.”

From that point on, every additional person added to the U.S. population will come from immigration, a demographic milestone once associated with aging countries like Italy and Japan. 

The shift is striking not only for what it says about America’s rapidly aging society, but also for how soon it is expected to arrive. Just a year ago, many demographic forecasts—including the CBO’s own forecast—placed this crossover well into the late 2030s or even the 2040s. The updated outlook from CBO moves the timeline forward by nearly a decade.

This rapid acceleration, the CBO said, is driven by the “double squeeze” of declining fertility and an aging populace, combined with recent policy shifts on immigration. CBO analysts have drastically lowered their expectations for the total fertiility rate, now projecting it to settle at just 1.53 births per woman — well below the 2.1 “replacement rate” needed for a stable population. At the same time, the massive “Baby Boomer” generation is reaching ages with higher mortality rates, causing annual deaths to climb.

The timeline further compressed following the passage of the 2025 Reconciliation Act, which increased funding for more ICE agents and immigration judges to process cases faster, resulting in approximately 50,000 immigrants in detention daily through 2029, CBO said. The office calculated that these provisions will result in roughly 320,000 fewer people in the U.S. population by 2035 than previously estimated.

The new projections show that U.S. population growth will steadily decelerate over the next three decades until it finally hits zero in 2056. For most of the 20th century, the population grew at close to 1% a year: a flat population would represent a historic break from that norm. 

The economic consequences of this shift are hard to overstate. While the number of retirees swells, the pool of workers funding the social safety net — and caring for the aging population —  is narrowing. Americans aged 65 and older are the fastest-growing segment of the population, pushing the “old-age dependency ratio” sharply higher. In 1960, there were about five workers for every retiree. Today, that ratio is closer to three-to-one. By the mid-2050s, the CBO projects it will fall to roughly two workers per retiree. The contraction will have “significant implications” on the federal budget, including outsized effects on Social Security and Medicare, placing pressure on those trust funds which rely on a robust base of payroll taxes that a stagnant population cannot easily provide.

Further, because national GDP is essentially the product of the number of workers multiplied by their individual productivity, the loss of labor force growth means the American economy will have to rely almost entirely on technological breakthroughs and AI to drive future gains. This may be happening ahead of schedule, as continued weak employment growth in December showed a “jobless expansion,” in the words of KPMG chief economist Diane Swonk, as Fortune previously reported.