Stress doesn't just live inside your brain. The invisible chemical signature of stress and worry can be found all over your body – in your blood, in your saliva, and even in the hair upon your head. But just how far do these chemical markers of stress extend? Reading a saliva sample is one thing, but if we zoom out, is it possible to detect biological stress at the population level? In a new study, scientists showed that you can do this, and it's all thanks to human sewage.