Mental illness is the 21st century’s leading cause of disability, affecting an estimated billion people across the world. Depression is the number one contributor: more than 250 million people have this condition globally. The number of people prescribed antidepressant medications, the first-line treatment for depression, increases each year, and the market for them is valued at approximately $15bn (£11bn). Yet depression prevalence rates have not decreased since accurate record-keeping began. One reason for this paradox is the failure of science to adequately explain how and why depression occurs.