Short writing assignments in which students discuss their most cherished value may be a powerful new tool to help struggling black youths reduce stress and boost their grades, U.S. researchers said on Thursday.

Twelve- and 13-year-old black students who did a series of 15-minute writing exercises saw their grades improve significantly over the course a year, and the benefit has persisted two years after the exercises stopped, they said.

"The effects were primarily among low-achieving African-American students," Geoffrey Cohen of the University of Colorado at Boulder, whose study appears in the journal Science, said in an audio interview on the Science website.

For these children, there was an increase of almost half a grade point (0.4 grade points) in their overall grade point average across two years (based on a 4.0 scale)," he said.

"This was as apparent at the end of two years as it was at the commencement of the intervention."

Cohen said these self-affirmation writing assignments have been proven to reduce stress in other settings. His goal was to see if reducing school stress might improve school achievement.

The team gave the assignments to seventh-grade students at a U.S. public school.

They were given a list of values, such as relationships with friends and family, creativity, interest in music or sports. They were asked to pick the value that was most important to them and write a paragraph about it.

"The exercise gives kids a chance to say, 'This is what I believe in.' It takes the sting out of potential failures," Cohen said in the audio interview.

Other children were asked to write about a value that was unimportant to them.

When they compared the results, they found the self-affirming exercise reduced differences in grades between black students and whites in the study by 40 percent, and that benefit has persisted for two years.

"We also found affirmation-treated African Americans were significantly less likely to be enrolled in remediation that is assigned by their school or held back a grade," Cohen said.

The writing assignments had only a marginal effect on high-achieving black students and no effect on white students of European descent. He said there were too few Asian-American or Hispanic students to study.

Cohen said the study suggests that even modest interventions, when done early, can interrupt a downward achievement spiral.

"Small changes to an individual's psychological state can have surprisingly large effect over time if they alter the angle of people's performance trajectory," he said, adding that the early benefit is compounded over time.