In this study, the research team conducted eight experiments (four on animal brains, four on brains of the deceased human kind) of varying sample sizes — from 20 to 90 brains in each — and found that the brains of deceased humans who’d been depressed had increased levels of hippocampal FGF9 and that live animals with increased FGF9 levels demonstrated depressive, anxious behavior. “This is not just a correlation,” study leader Huda Akil of the University of Michigan says.