Depression is a widespread mental illness, affecting one in ten people. Twice as many women as men suffer major depression. Hormonal changes brought on by puberty, menstruation, menopause, and pregnancy may contribute to the higher risk, as these periods in a woman’s reproductive cycle have been associated with depression. Hormone replacement therapy has been prescribed to treat changing sex hormone levels, but a study found the health risks exceeded the benefits. Carolin Klein is investigating the impact of an alternative approach, cognitive-behavioural therapy (CBT), on hormone levels. CBT has been as effective as medication in treating depression, with no side effects. If depression and sex hormone levels are related, cognitive-behavioural therapy could also normalize hormone levels. Carolin is measuring hormones in depressed men and women before, during, and after CBT. The results could clarify if changing hormone levels cause depression, and lead to greater use of cognitive-behavioural therapy to treat abnormal hormone levels, without the side effects associated with some medications.