WASHINGTON - The number of grade-school American children who spend time at home alone has plunged by almost 40 percent since 1997, a result of both federal aid for after-class programs and parents revamping work schedules, census data show.

Only one in nine kids ages 5 to 14 spends after-school hours in a home without parents, according to a census report. That compares with about 1 in 5 left unsupervised in 1997. Some 4.5 million children were alone for an average of 6.5 hours every week in 2011, the latest year for which figures are available.

The decline in so-called latchkey children, a phenomenon first described during World War II, could be stalled in the face of threatened federal spending reductions, advocates say.

"The funding keeps getting cut," said Jodi Grant, executive director of the Afterschool Alliance, an organization established in 2000 by the U.S. Department of Education and private foundations. "There's just no end in sight."

Legislation in 1998 directed $1.2 billion in federal money to states for the creation of after-school programs. The law, which funds a network of school activities as part of the 21st Century Community Learning Centers, is targeted for cuts under the budget-reduction program known as sequestration. One advocacy group estimates it could shrink available child-care slots by 3.4 percent, or 56,000 positions.

The drop in the ranks of latchkey kids may also be attributed to different approaches to parenting over the decades, said Martha Erickson, former director of the University of Minnesota's Children, Youth and Family Consortium.

A growing fear of "stranger danger" and other perceived threats to children's safety make parents less likely to leave their kids unsupervised, she said.

Those who pay exceptionally close attention to their children - dubbed helicopter parents - are also increasing, and they're less likely to allow their kids to be left alone after school, Erickson said.

The percentage of latchkey children with single working parents dropped by 42 percent from 1997 to 2011, a surprising finding, said Lynda Laughlin, a Census Bureau family demographer and author of the report. About 14 percent of their children aren't supervised after school, down from 24 percent in 1997.

The decrease in latchkey kids has occurred as out-of-pocket costs for child care have soared in the last 25 years. The typical American family paid $143 a week for some amount of child care in 2011, a 70 percent jump over the inflation-adjusted 1985 average payment of $84. Median household income has risen only 7.3 percent to $50,054 during that time.