The United States population growth rate in 2019 was the slowest in a century due to declining births, increasing deaths, and immigration restrictions, according to a U.S. Census Bureau report released on Monday. The nation’s population grew from 2018 to 2019 by almost 1.5 million people, which is the slowest rate since 1917 to 1918 during World War I, according to William Frey, a senior fellow at The Brookings Institution. An aging population of Baby Boomers are responsible for the natural increase rate—the difference between the number of births and deaths—hitting below 1 million. “With the aging of the population, as the Baby Boomers move into their 70s and 80s, there are going to be higher numbers of deaths,” Frey said. Migration to the U.S. reached 595,000 people from 2018 to 2019, in a major decrease from one million in 2016, according to population estimates. Frey said the drop is due to Trump’s restrictions on immigration combined with the U.S. posing to have fewer economic opportunities.