(CNN) — Life expectancy fell across the majority of high-income countries, signaling a collective and simultaneous decline among affluent nations for the first time in decades, a new study finds.

After more than a century of steady progress, new research warns that the world’s life expectancy boom is slowing, largely because improvements in early-life mortality have already been achieved.

Since the early 20th century, sustained declines in mortality have driven steady increases in life expectancy, particularly in high-income countries. Several studies have reported that this ...