THURSDAY, Sept. 12 (HealthDay News) -- Cases of physical and sexual abuse of U.S. children seem to have declined over the past 20 years, but cases of neglect appear unchanged, health officials reported Thursday.

Child neglect accounts for about 75 percent of all cases of abuse, while physical abuse accounts for 15 percent and sexual abuse 10 percent, according to the study from the Institute of Medicine.

Boys and girls face about the same risk of neglect and abuse. In 80 percent of cases it's parents who are the neglectors and abusers -- and of these, 87 percent are biological parents. More than half the time the perpetrators are women, according to the report.

"These consequences not only affect individuals, but also their families and the broader society," said report co-author Anne Petersen, a research professor at the University of Michigan's Center for Human Growth and Development. "For example, the annual burden from child abuse and neglect is $80.3 billion."

Of this total, $33.3 billion is direct costs, such as hospitalization, childhood mental health care, the child welfare system and law enforcement. There's also $46.9 billion in indirect costs, including special education, early intervention and homelessness, according to the report -- an update of a similar report released in 1993.

In 2011, state child protective services agencies dealt with nearly 677,000 children who were victims of physical abuse, sexual abuse, psychological abuse and other types of neglect -- about nine out of every 1,000 children in the country, according to the report.

And the report authors said those number might underestimate the true extent of the problem.

The reasons for the decline in physical and sexual abuse of children aren't known, the authors said.

Yet over the same 20-year period, incidents of psychological and emotional child abuse seem to have increased. The statistics, however, don't make it absolutely clear whether neglect has increased, decreased or stayed the same, the report acknowledged.

One doctor was surprised to hear that the report found a decline in cases of physical and sexual abuse.

"We are seeing a lot of physical and sexual abuse," said Dr. Rodney Baker, director of pediatric emergency services at Miami Children's Hospital. "So it's hard for me to say it's declining -- that wasn't my impression."

"Neglect could also be on the increase," he added.

Baker agreed that the extent of the problem really isn't known, adding that more needs to be learned to determine what to do about it.

"We don't really have a good way to measure how much abuse and neglect there is or what to do about it -- it still remains a mystery," he said.

This lack of knowledge is why the report calls for a national plan to coordinate research on child abuse and neglect, as well as creation of a national surveillance system to more accurately identify cases.

The report also calls for more research on the causes and effects of abuse and neglect, as well as what can be done about the problem.

The Institute of Medicine is an independent, nonprofit organization that provides unbiased advice to decision-makers and the public.

More information

To learn more about the problem of child abuse and neglect, visit the U.S. Department of Health and Human Services.