Being bullied at school leaves a lasting mark on young victims' bodies and minds years after the bullying ends, a new study shows. 

Researchers found that bullying had its greatest influence on the health of kids who were repeatedly targeted, and it was linked with poorer mental and physical health, increased symptoms of depression and lower self-esteem.

The study also revealed that victims of recent bullying fared worse than students who had been picked on by their peers further in the past.

"The effects of bullying can snowball over time," said study researcher Laura Bogart, a social psychologist at Boston Children's Hospital.

Children who experience continued bullying, such as in more than one grade, had more severe effects, she said.

"The results are a strong argument for an immediate, early intervention of bullying," Bogart said. [10 Scientific Tips for Raising Happy Kids]

The findings were published online on Feb. 17 and will appear in the March issue of the journal Pediatrics.

Early intervention

In the study, the researchers surveyed nearly 4,300 students attending public schools in Los Angeles, Houston and Birmingham, Ala. They collected data from students when they were in fifth, seventh and 10th grades, and also from their parents.

To determine whether students were frequently or repeatedly bullied in elementary, middle and high school, they were asked how often another kid had kicked or pushed them in a mean way during the last year. Children also completed questionnaires describing their physical and mental well-being.

By 10th grade, about 30 percent of the students in the study had been bullied.

At all three grade levels, students who had never been bullied reported being the most psychologically healthy.

But even the kids who had been bullied in the distant past had lasting psychological wounds, although not to the same extent as those being bullied currently or who were repeatedly bullied. For those children, mental health scores declined over time, suggesting they didn't feel as good about themselves and experienced more negative emotions.

The study did not look at bullying's full range of physical effects, but the questionnaire asked children about basic physical activity, such as whether being bullied caused them to have trouble walking around the block, doing their chores or playing sports.

It did not consider physical complaints and injuries, such as stomachaches, pain, cuts and broken bones, Bogart told Live Science.

She also said the study only measured in-person bullying and didn't include cyberbullying.

"Cyberbullying would be a good future direction for this research," Bogart said.

Warning signs

The findings show that bullying's emotional scars may remain long after the teasing and taunting stop. So what can a parent do to help their child?

"The first step for parents is to strengthen communication with your child, so that bullying comes up in conversation, especially at younger ages," Bogart said. It can come up as part of a broader discussion about respecting other people and accepting differences among them, she suggested.

Bogart also recommended that parents notice and recognize the signs of bullying, which may not be obvious physical clues like a black eye, but could take the form of unexplained scratches or bruises.

Parents can also tune in to subtle changes in a child's behavior, such as not wanting to go to school or seeming more anxious, sad or depressed.

Bogart also suggested that parents should be even more vigilant to these warning signs if their child falls into one of the high-risk groups more likely to be bullied. These include kids who are obese or have disabilities, as well as young people who are lesbian, gay, bisexual or transgender. 

Although not all children are victims of bullying, many kids are affected by it and see it happening at school, and are aware that it's going on, Bogart said.

She advised parents to help teach kids that it's OK to speak up if they witness someone being bullied.

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This story was originally written for LiveScience and was republished with permission here. Copyright 2014 LiveScience, a TechMediaNetwork company.