Being “fat” is the most common reason children are bullied, and something needs to be done about it.

That is the predominant view of thousands of adults from four different countries who, when asked why children are bullied, said the most common reason was not race, religion, physical disability or sexual orientation, but weight. Nearly three-quarters of respondents said that schools and anti-bullying policies need to address the issue, with many calling it a “serious” or “very serious” problem.

Yet most state anti-bullying laws don’t protect overweight children, said Rebecca Puhl, deputy director of the Rudd Center for Food Policy and Obesity at the University of Connecticut in Hartford and the lead author of the report, the first cross-national study investigating weight-based bullying, published in Pediatric Obesity.

There are no federal laws that guarantee equal treatment of people who are overweight or obese.

“It is actually legal to discriminate on the basis of weight, and that sends a message that bias, unfair treatment or bullying of overweight children is tolerable,” Dr. Puhl, a professor of human development and family studies at UConn, said.

As obesity rates have risen, she said, so much emphasis has been placed on taking personal responsibility for body weight and changing behaviors “that there is a perception that these youth are somehow to blame for their weight and in some way deserve this treatment.”

“There’s also a widespread misperception that stigma may not be such a bad thing, and that maybe criticism will get people motivated to lose weight,” Dr. Puhl said. In fact, she said, the opposite is true: People who are picked on because of their weight often engage in unhealthy behaviors. Students who are teased for being fat in gym class, for example, often start skipping P.E. to avoid being bullied.

For the new study, researchers surveyed the views of 2,866 adults in the United States, Canada, Iceland and Australia. These four countries have similar rates of childhood and adult obesity, as well as similar cultural attitudes that laud thinness and being physically active, Dr. Puhl said.

At least 70 percent of participants in all of the countries perceived weight-based bullying to be a common problem, with 69 percent characterizing it as a “serious” or even “very serious” problem. While about half of respondents listed “being fat” as the most common reason children are picked on, fewer than 21 percent in any country listed race, ethnicity or nationality as the most common reason. Fewer than 15 percent listed sexual orientation, fewer than 12 percent listed physical disability and fewer than 6 percent listed religion or academic ability.

About three-quarters of participants across countries said schools should make efforts to raise awareness about weight-based bullying and implement policies that protect overweight kids, and supported bolstering anti-bullying laws to address weight-based bullying.

Support for a more active government role was weaker among Americans, however, with only half saying the government should play a more active role and only 47 percent supporting a federal law to prohibit weight-related bullying.

The new survey is not the first to report such findings: A 2011 National Education Association study found that 23 percent of teachers reported weight-based bullying to be a concern in their schools, with lower percentages reporting discrimination based on gender, physical disability or perceived sexual orientation.

“The politically correct movement doesn’t seem to have touched body weight,” said Deborah Carr, chair of the sociology department at Rutgers University in New Brunswick, N.J. “Weight stigma is the most acute among upper middle class educated people, which is the population that cherishes the lean physique the most.”

Indeed, as obesity rates have increased in recent years, perceived weight and height discrimination have also risen, research shows.

With about one third of American children and adolescents weighing in at levels considered to be overweight or obese, public health officials are concerned about discrimination they will face in the workplace, in education and in health care settings, as well as among peers and by their own family members.

Obese teenage girls get less financial support for college from their parents than girls who aren’t obese, and obese workers earn less than non-obese workers, according to a Centers for Disease Control and Prevention report on weight bias authored by Dr. Reginald L. Washington, the chief medical officer of Rocky Mountain Hospital for Children in Denver.

The C.D.C. report reviewed current research and found bias among physicians, educators, family members and peers. A study of more than 400 doctors, for example, found that one in three listed obesity as a condition they responded negatively to, ranking it just behind drug addiction, mental illness and alcoholism. The C.D.C. also cited research showing that families often pick on overweight family members; nearly half of overweight girls report being teased about their weight by family members.

While some health experts acknowledge that individual genetic and metabolic differences mean that some people are more prone to gaining weight than others, the most widely disseminated public health message is that anyone can achieve a desirable weight by eating less and exercising regularly.

Dr. Carr said public health experts walk a fine line “between wanting someone to have a healthy body image and feel good about themselves at any size — and wanting them to watch their weight.”