AUCKLAND, Oct 10 - In surveys of the same 13-to-18-year-olds over a period of five years, students with stunted emotional capacities were found to be especially vulnerable.

Their actions may include cutting, scratching or preventing wounds from healing, and were not always obvious to caregivers.

Nearly 20 per cent also had suicidal tendencies at some point.

Victoria University psychology professor and survey lead Marc Wilson said some students had not learned to manage their emotions healthily.

They thus resorted to self-harm to quell their sense of detachment.

"Managing emotions was the most common function of self-injury, followed by self-punishment and the desire to feel something," Professor Wilson said.

"As expected, young people who are less well-equipped to understand and manage their emotions are more likely to manage their emotions through self-injury."

Professor Wilson said the process was a vicious cycle in which emotionally damaged students self-harm and then subsequently feel more damaged.

Some young people may ultimately rely upon self-harm for relief.

Relationships with schools, teachers, fellow pupils and family were therefore crucial in preventing acts of self-harm, the report found.

The three 30-minute surveys - each of which encompassed more than 900 students - were carried out in classrooms under supervision.

The same students were surveyed as they progressed through high school.

* Readers seeking support and information about suicide prevention can contact Lifeline's 24-hour telephone counselling service on 0800 543 354.

Suicide Crisis Helpline (aimed at those in distress, or those who are concerned about the wellbeing of someone else) - 0508 828 865 (0508 TAUTOKO)

Youthline - 0800 376 633, free text 234 or email

Kidsline (aimed at children up to 14 years of age; 4pm to 6pm weekdays) 0800 54 37 54 (0800 kidsline)