Investigators have linked difficult early life experiences with reduced quality and quantity of the white matter communication highways throughout the adolescent brain. This reduced connectivity is also associated with lower performance on cognitive tasks.

Research has shown that a biological brain mechanism called the 'kynurenine pathway' is imbalanced in adolescents with depression, and this imbalance is more pronounced in teenage girls than boys.

Research has tracked depressive symptoms in 7100 young people from Tokyo and London and shown girls have more depressive symptoms than boys in both cities. The study found that this gap is around twice as large in London and the year-on-year rise in depressive symptoms is around four times steeper for teenage girls in London than for teenage girls in Tokyo.