School-aged girls who socialise with negative peers are more likely to not only experience depression during their youth, but are at an increased risk of mental health problems as adults, a world-first study by international economists has found.

New research from Monash University and the University of Southampton shows depression can be contagious for teenage girls and have lasting effects into adulthood. It is the first study to causally link depression among peers in adolescence to depression occurring later in adulthood.

The study by Monash Business School’s Professor Yves Zenou and co-authors Dr Corrado Giulietti and Professor Michael Vlassopoulos from the University of Southampton, shows that just an increase in one standard deviation of the proportion of female schoolmates who are depressed, increases the probability of depression in adulthood by 2.6 percentage points for women.

Alarmingly, researchers found that an increase in peer depression by one standard deviation is also associated with a probability of college attendance that is 3.5 percentage points lower, a likelihood of working that is 2.8 percentage points lower, and a reduction in income of $1,870 annually.

The study uses data spanning 14 years from adolescence into early adulthood, of 12,400 individuals (6,663 females and 5,737 males) across 128 schools, from the US National Longitudinal Survey of Adolescent to Adult Health.

It explores the impact of depression among peers when individuals are between the ages of 12 to 18 years on their own mental health when they are between the ages of 24 to 32 years.

The research paper titled: ‘Peers, Gender and Long-Term Depression’ is available at SSRN (Social Science Research Network).

“Our estimates suggest that a young girl who has a large share of depressed peers in high school will have a probability of being depressed in adulthood that is eight percentage points higher than one whose peer group has a low share of depressed peers,” Professor Zenou, from Monash Business School’s Department of Economics, said.

“It corresponds to an increase of over one-third of the average likelihood of female depression. The effect is even greater for women who were exposed to a higher proportion of depressed peers at school or those who already had weak mental health as a teen.”

Researchers found marked gender differences in sharing negative feelings with their friends. Seven in 10 girls reported discussing a problem with their female friends, while boys reported discussing their problems with just 39 per cent of friends.

Additionally, girls who come from a lower socioeconomic background and whose mothers have lower occupational statuses, are more influenced by the depression of their peers in adolescence.

“Teenage girls are more vulnerable to same-gender depressed peers than boys and this influence persists over time, even when they are not interacting with each other,” Professor Zenou says.

“It’s important for parents to understand just how this can lead them to a life of mental health issues. Parents, especially mothers, need to spend time with their daughters talking about mental health issues and, if necessary, change up the peer group by changing schools or residential location.”

Professor Zenou says the implications are clear – children must move schools.

“We have to avoid girls being exposed to negative peers and what we see is that the earlier that teen girls are moved away from these negative peer groups the better,” Professor Zenou said.

“Changing from a school with a high incidence of depression to one with a low incidence of depression has a big impact on a girl’s long-term depression and capacity to go to college and work in the future.”

According to the World Health Organization, 10 to 20 per cent of adolescents globally experience mental health conditions, with depression the leading cause of illness among teenagers.

In 2019, the Australian Government’s Productivity Commission estimated the annual economic cost of mental illness had reached $180bn and found that three-quarters of Australians developed mental illness symptoms before turning 25.