(NEXSTAR) – For the first time in Gallup poll history, fewer than 50 percent of Americans said they belonged to a house of worship. Forty-seven percent of Americans said they belonged to a church, synagogue or mosque in 2020, Gallup announced Monday, the lowest number in the poll’s eight-decade trend.

The figure is down three percentage points from 2018 and a whopping 23 points from 1999.

When Gallup first began the poll, in 1937, U.S. church membership was 73 percent and remained near 70 percent before steadily declining near the turn of the century.

What accounts for the drop?

According to Gallup, “The decline in church membership is primarily a function of the increasing number of Americans who express no religious preference. Over the past two decades, the percentage of Americans who do not identify with any religion has grown from 8% in 1998-2000 to 13% in 2008-2010 and 21% over the past three years.”

The rest of the decline can be attributed to those who do have a religious preference but don’t attend church, Gallup said.

Part of the reason for the steady drop is generational changes. Younger people tend not to belong to a place of worship compared to older generations, so “those in older generations who were likely to be church members [are] being replaced in the U.S. adult population with people in younger generations who are less likely to belong.”

Currently, 31 percent of millennials say they have no religious affiliation, up from 22 percent ten years ago, while 33 percent of Generation Z have no religious preference.