(CNN)-- Victims of bullying may suffer mental and physical consequences even after bullying occurs, research shows.
A new study in the journal Pediatrics finds that bullying is associated with poor physical and mental health among children, particularly among those who were bullied in the past and are being currently bullied.
The effects were strongest among children who were bullied continuously, in more than one grade, particularly in terms of psychological health, said lead author Laura Bogart, associate professor of pediatrics at Boston Children's Hospital. Psychological measures included negative emotions such as anger and depression.
"We were able to show that these effects of bullying snowballed and compounded over time," Bogart said.
Researchers used a large sample of students at public schools in three metropolitan areas: Los Angeles, Houston and Birmingham, Alabama. A total of 4,297 students and their parents participated in all three phases of the study. More than 60% had household incomes of less than $50,000 a year, and less than half of parents had a high school degree or lower, suggesting a high proportion of participants from low socioeconomic status.
The children and their parents responded to computer-assisted personal interviews in English or Spanish. The first round took place in fifth grade; the next came two years later, when nearly all the kids were in seventh grade, and then three years after that, when almost everyone was in 10th grade.
Questions to assess bullying and victimization included, "How often did kids kick or push you in a mean way during the past 12 months?" If a child responded to of the six questions related to victimization with "about once a week" or "a few times a week," he or she was considered to have experienced bullying.
The children also answered questions about bullying in the context of both the past and present. Researchers evaluated the children on mental and physical health parameters, including depression and self-worth.
Researchers found particularly striking differences in mental health when comparing children who had been bullied with those who had not. Among fifth-graders, about 4% of kids who had not been bullied showed low psychological health, far less than the 31% of kids who had been bullied.
In later years, researchers found a strong relationship between low psychological health and bullying, especially among children who said they were being bullied at that time, or both at that time and in the past.
The study found that about 45% of children in 10th grade who were bullied both in the past and the present had low psychological health, compared with 31% of those currently bullied, 12% of kids bullied only in the past and about 7% who had not been bullied.
Physical health had a similar relationship to bullying, although the relationship between bullying and physical health was not as strong as with mental health.
Those who experienced past and present bullying also tended to have worse symptoms of depression than other children surveyed. The worst depression symptoms were shown by 30% of 10th-graders bullied in the past and present, compared with 19% of those bullied only currently, 13% of those bullied in the past only and 8% of those who had not been bullied.
Similarly, the largest group of 10th-graders with the lowest self-worth were those who had been bullied in the past and the present.
"Although bullying in the present was a stronger predictor of poor health than past bullying, past bullying predicted poorer present health after considering present bullying," the study said.
About 30% of participants said in at least one of the three rounds of interviews that they had experienced frequent bullying.
We can't conclude from this study design that bullying causes poor health outcomes. Researchers looked only at associations. Nonetheless, the findings corroborate many other studies linking bullying to worse health.
Also, the study was conducted in three U.S. metropolitan areas and only in public schools; the situations in different geographic areas, and different kinds of school, may vary.
"What these results show are a strong argument for an immediate intervention, early intervention, before the effects of bullying can get too serious for mental and physical health," Bogart said.
She encourages parents to have an open line of communication with their children and to always ask how their day went. There may be physical signs of bullying, such as bruises or scrapes, but a more subtle red flag might be a reluctance to go to school. Sadness, depression and isolation are also possible indications of bullying.
School-based interventions have shown to be successful, especially when everyone involved in the school participates.
"Parents and teachers can teach kids the importance of respect and acceptance of other people and to speak out when they see bullying," Bogart said.