(CNN) -- Bullying isn't just about physical violence or emotional pain -- it can impact kids' educations, too.
Kids bullied their entire school career have declining test scores, a growing dislike of school and failing confidence in their abilities, say the authors of a study published Monday in the Journal of Education Psychology.
Researchers tracked several hundred children in the United States from kindergarten through 12th grade, and found nearly a quarter experienced chronic bullying through their school years.
"The good news is that it goes down. The longer kids stay in school, the less likely it is that they will be victimized," said Gary W. Ladd, professor of psychology at Arizona State University, who led the study.
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Once the kids start high school, the aggression tends to taper off.
Ladd wondered about the bullied kids who said "they didn't like school and didn't want to go there."
"The majority of research done on bullying and victimization addresses children's psychological and health adjustment," Ladd said.
But when it comes to understanding their school achievement, there wasn't much out there, leading him and his colleagues to investigate.
Vulnerability to victimization
In 1992, Ladd and his colleagues enlisted 383 kindergarteners -- 190 boys, 193 girls -- into their study. The participants attended various public schools, mostly in Illinois. The team then frequently assessed each child's feelings of victimization, enthusiasm for school, academic esteem and performance via teacher evaluations and standardized test scores.
Among the assessments were annual surveys in which the children described their experiences with bullying, addressing whether they had been hit, picked on, or verbally abused by other kids. Frequency was measured on a scale of one, meaning "almost never," to five, meaning "almost always." The study is part of a larger investigation of children's social, psychological and academic adjustment funded by the National Institutes of Health.
Nearly one-quarter of the children came from families with low annual incomes ($20,000 or less) and 39% from middle to high incomes (more than $50,000), with the remaining students, about a third, from the middle level in between these two extremes. Approximately 77% of the children were white, 18% African-American, while the remainder had Hispanic, biracial or other ethnic backgrounds.
Though the study began largely in Illinois, by the fifth year, participants had spread to 24 states, which Ludd noted as evidence of the "mobility of the American population, especially in the rust belt area."
The new study found that both the prevalence and frequency of victimization declined over the years of schooling, but they also identified subtypes revealing differences in both the bullying and its effects.
Nearly a third of the kids, 32%, experienced little or no bullying. Meanwhile, about a quarter of the kids, 26%, suffered decreasing bullying over time. The academic scores of this group were similar to those of the little or no bullying group, suggesting that kids could recover when bullying lessened over time.
"There are some kids who seem like they escape or they are able to become less victimized as they move through school," said Ladd. "I'm sorry that we don't know why. I think that's one of our next questions."
Nearly a quarter of the kids (24%) suffered chronic levels of bullying during their school years. These kids had lower academic achievement, a greater dislike of school and less confidence in their academic abilities.
About a fifth (18%) experienced moderate bullying, which increased later in their school years. Their results were similar to those who had been chronically bullied.
Overall, boys were much more likely to suffer chronic or increasing bullying than girls. And, in every age group, even though bullying in general declines over time, more boys than girls were bullied.
"So some children appear to start school well, but become more vulnerable to victimization as they move along," said Ladd. "Of course, that group worries us more than the group escaping from victimization because if you're going to prevent these problems, it's important to find kids early and do something about it."
The researchers said the study was unique in its focus on how bullying impacts academic performance, and for how long it studied the students. Typically, said Ladd, studies of bullying are short-term, using data from the beginning to the end of a single school year. Even the longest studies follow kids for five or six years, he said.
Wondering what a bully will do next
For a student who experiences bullying, it's hard to escape into school work.
"The children who are frequently bullied are not only not liking school and not wanting to be there, but are finding it hard to participate in the classroom," said Ladd, adding that there are a lot of factors that probably discourage their engagement.
"Most of the classrooms that we worked in had some kind of group activities, collaborative activities with other children," he said. "It might be especially hard for kids who are victimized to participate in those groups if their bully is sitting there with them."
"One of the things kids talked about was that it was harder for them to pay attention when they were sitting in the classroom thinking about what the bully was going to do to them next or what they were going to do to them after school or things of that nature, so we also wondered about whether or not this was a major distraction for children."
Cyber-bullying wasn't an issue when the study began and so was not tracked. However, Ladd commented that, based on other studies and what his colleagues say, "the kids who are most abused by their peers in school are getting it on social media, too."
"A lot of children who are bullied don't talk about it at home, don't tell their parents. They're embarrassed to admit that they're being treated that way," said Ladd.
To be aware of this, the researchers add that parents might need to visit their child's teachers or talk to school personnel to understand more about their child's life.
'Bullies are often suffering, too'
According to Michelle K. Demaray, a psychology professor at Northern Illinois University, the long study period was a strength, but also contributed to a flaw in the research: By 12th grade, 23% of the participants had left the study.
"As with any longitudinal study, a disadvantage was the attrition or drop-out rate," said Demaray.
But the researchers themselves acknowledged this and adjusted their calculations accordingly. They also compared the "kids who dropped out to those who didn't and only one major difference was found -- boys were more likely to drop out," said Demaray. The comparison showed no racial or family income differences.
Like Ladd, Demaray hopes to learn more about the group of kids who become increasingly victimized over time.
"The more we learn about this group, the more we can figure out potential prevention and intervention efforts targeting them and maybe reduce their chances of becoming victimized later in school," said Demaray.
Ryan M. Hill, an instructor in the department of psychiatry and behavioral sciences at UTHealth McGovern Medical School, found the study to be "exceptionally well done."
"The research has been clear for years that the impacts of bullying are profound and varied," said Hill, noting that bullying influences depression, suicide-related behaviors, and a range of other mental health outcomes.
"Bullying doesn't just impact a handful of students," Hill said. Adding up the numbers, he noted that "more than 40% of children" have their academic performance impacted by bullying.
Though his own research focus is on the prevention of depression and suicide, Hill said it is also important to "not forget about the bullies."
"The research is clear that bullies are often suffering, too," said Hill. "Both bully victims and perpetrators are in need of services and preventing bullying means also preventing children from becoming the bullies."
Adding that verbal, physical, and cyber bullying all have impacts on children's well-being, Hill concluded: "Each new piece of evidence about the effects of bullying emphasizes the need to develop policies and interventions to prevent and address bullying in our schools and communities."
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