Hormones are a class of signaling molecules that exist in all multi-cell organisms and, in humans, include commonly-known examples like melatonin, testosterone, and cortisol. Hormones are silent drivers of our behavior and personality, and their molecular fingerprints are on everything from attraction to appetite. Decades of biological and psychological research have found that certain hormones are associated with specific traits and behaviors, but in reality, the human endocrine system is so complex that the same chemical may behave radically differently from person to person.