The research found that girls were nearly twice as likely to think about suicide if they had only a few friends and felt isolated from their peers. Girls were also more likely to consider suicide if their friends were not friends with each other.
These relationship factors had no significant effect on whether boys considered suicide.
"Close friendships appear to be much more important for adolescent girls than they are for boys, and problems with these relationships have major impacts on girls' mental health," said James Moody, co-author of the study and assistant professor of sociology at Ohio State University.
"Boys tend to hang out in groups, and close relationships do not seem to be as important to them."
Another key finding was that there was no way to tell which teenagers who are thinking about suicide will actually attempt it.
Moody conducted the study with Peter Bearman of the Institute for Social and Economic Research and Policy at Columbia University. Their findings appear in the January 2004 issue of the American Journal of Public Health.
The researchers used data on 13,465 adolescents in grades 7 through 12 from across the United States. The students participated in the National Longitudinal Survey of Adolescent Health. They completed a survey in school and then were interviewed in their homes one year later. They were asked, among many other things, about their friendships, whether they had thought about suicide in the past year and whether they had attempted suicide in the past year.
Moody said he wasn't surprised that friendships played a larger role for girls than for boys, but he was surprised at the size of the difference.
"I think everyone expected there to be differences between boys and girls, but we didn't expect to find that friendships would have essentially no major effect on whether boys considered suicide," he said.
For girls, those who felt isolated and friendless were at as great a risk for considering suicide as girls who knew someone who had committed suicide.
"Whenever a young person commits suicide, a school will send out counselors to help other students," Moody said. "But there is just as much risk of suicidal thoughts among adolescent girls who feel isolated. We need to provide help to these students, too."
Another important finding was that girls had increased risk of suicidal thoughts if people they listed as friends did not name each other as friends.
"When girls are stuck between friends who don't get along, it puts them under a huge amount of stress," Moody said. "The more that happens, the more likely a girl is to think about suicide."
As other studies have shown, both boys and girls were more likely to attempt suicide if they had a friend who attempted suicide. Boys were less likely to attempt suicide if they attended schools in which the friendship network was dense and interlocked - in other words, where many of the boys shared the same friends.
But why do dense friendship networks seem to prevent suicide attempts in boys, but friendship doesn't seem to affect suicidal thoughts?
Moody said the question needs more study, but it may be that boys don't share their thoughts about suicide with friends, but they may admit when they are planning to attempt suicide.
"In a school where there are a lot of interlocking relationships, there are many people who may hear a suicidal boy talking and help him get the support he needs. Boys may not tell people what they are thinking about, but they may talk about what they might do."
But overall, Moody said there wasn't a clear pattern indicating which adolescents who were thinking about suicide would eventually attempt it.
"Essentially, once adolescents start thinking about suicide the factors that trigger an actual attempt are largely random. That means that we have to identify and help any teenager who is thinking about suicide. It is better to get a few false positives than to miss some of them," he said.
The results of the study offer several suggestions for identifying and helping adolescents at risk for suicide, Moody said. In addition to the friendship findings, the study mirrored other studies that show teens are less at risk if they participate in more activities with their parents and if they attend church regularly.
"Parents should talk often to their children, do things with them and, if they're religious, take them to church," he said.
Teachers and parents also need to look for adolescents, especially girls, who don't seem to be socially connected to their peers, or who have friends who don't get along.
"Changing schools, joining clubs, participating in more extracurricular activities -- all could help adolescents who are at risk for thinking about suicide," he said.
The National Institutes of Health funded the National Longitudinal Study of Adolescent Health, which was the data set used in this research.