American media have largely ignored the issue of population decline for the simple reason that it hasn’t happened here yet. Unlike Europe, the United States has long been the beneficiary of robust immigration. This has helped us not only by directly bolstering the number of people calling the United States home but also by propping up the birthrate, since immigrant women tend to produce far more children than the native-born do.

But both those advantages look to diminish in years to come. A report issued last month by the Pew Research Center found that immigrant births fell from 102 per 1,000 women in 2007 to 87.8 per 1,000 in 2012. That helped bring the overall U.S. birthrate to a mere 64 per 1,000 women—not enough to sustain our current population.

Moreover, the poor, highly fertile countries that once churned out immigrants by the boatload are now experiencing birthrate declines of their own. From 1960 to 2009, Mexico’s fertility rate tumbled from 7.3 live births per woman to 2.4, India’s dropped from six to 2.5, and Brazil’s fell from 6.15 to 1.9. Even in sub-Saharan Africa, where the average birthrate remains a relatively blistering 4.66, fertility is projected to fall below replacement level by the 2070s. This change in developing countries will affect not only the U.S. population, of course, but eventually the world’s.

Why is this happening? Scientists who study population dynamics point to a phenomenon called “demographic transition.”