More must be done to help child soldiers deal with mental problems in order to break the cycle of violence in war-torn regions, a report has said.
The study said mental health issues needed urgent attention
The study by German researchers found that post-traumatic stress made former child soldiers less willing to reject revenge and consider reconciliation.
Their mental distress could "impose barriers to sustainable and long-term peace building", the study said.
An estimated 250,000 child soldiers are fighting in wars around the world.
For the study, published in the Journal of the American Medical Association, researchers interviewed 169 children aged between 11 and 18 who had been forced to fight in Uganda and the Democratic Republic of Congo.
They live in two rehabilitation centres which have been home to some 20,000 former child soldiers over the past 10 years.
Most of the child soldiers were subjected to violence to make them fight - they had seen beatings, shooting and rape, and more than half said they had killed.
The researchers found that about a third of the children were suffering from post-traumatic stress.
They were asked to agree or disagree with statements such as "I am ready to forgive the persons who harmed me", and "I am going to pay back the persons who harmed me for what they did".
Those with more severe symptoms of post-traumatic stress were significantly less willing to consider reconciliation and regarded acts of retaliation as a way to overcome their experiences, the study found.
The researchers said their findings underlined the urgency of dealing with the psychological effects of war on child soldiers.
"Post-traumatic stress might be an important factor influencing post-conflict situations and may contribute to cycles of violence found in war-torn regions," they said.