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 one in five 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. They believe that “children need an abundance of adult-directed, enriching activities, so unstructured time is a waste of time,” she 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.

“Our speculation is that working parents, particularly working single parents, have been able to find jobs that better mirror the schoolday,” Laughlin told C-SPAN in May. “So they’re able to take advantage of when their child’s enrolled in school, that they have a job that matches that schedule.”

The experience of latchkey kids is a mixed blessing, advocacy groups say. Children left unsupervised after school hours have been found to have more emotional and social problems, though they also develop a greater sense of self-reliance at an earlier age, according to the William Gladden Foundation, a Florida-based nonprofit organization for troubled young people.

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.

The child care tab was steepest for mothers with college degrees, who paid an average of $178 weekly, according to the census report. Moms who dropped out of high school paid an average of $111 weekly. The typical married mom paid $157 weekly, while single moms ran up an average $112 child care tab.

About one in six parents told the Census Bureau they rely on multiple arrangements to provide after-school care for kids. About 15 percent cited enrichment activities as the source of their child care. Slightly more than one in four had grandparents or other relatives take care of children.

The percentage of unsupervised kids had been increasing as more mothers entered the workforce in the 1970s and ’80s. By 1995, the government said, the ranks of latchkey kids made up about 18 percent of the grade-school population.

Since the 1998 law setting up the after-school programs was passed, with an initial budget of $40 million, the number of children served at more than 11,000 learning centers has grown to 1.7 million, according to a report by the Afterschool Alliance. A 2011 study by Learning Point Associates, a Washington-based education consulting firm, found 72 percent of children who attend the centers improve their homework and class participation. More than one-third had better grades in English and math.