For the first time in nearly two decades, only half of U.S. households donated to a charity, according to a study released Tuesday. The findings confirm a trend worrying experts: Donations to charitable causes are reaching record highs, but the giving is done by a smaller and smaller slice of the population.

The study, published every other year by Indiana University’s Lilly Family School of Philanthropy, comes from a survey that has been tracking the giving patterns of more than 9,000 households since 2000, when 66% of U.S. households donated to a charitable organization. That number dropped to 49.6% in 2018, the latest year with comprehensive figures from those households.

Experts say many factors are contributing to the decline. The percentage of Americans who give to religious causes has decreased in tandem with attendance at worship services as the number of Americans not affiliated with any religion grows. Separately, the share of Americans who give to secular causes began to drop following the economic turbulence of the Great Recession, but it hasn’t bounced back. It reached a new low — 42% — in 2018, the study said.