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 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.
Hormones affect people in different ways throughout their lives, and hormone levels can spike dramatically at physical or emotional transitions. New parents are slammed with a rush of hormones, for instance, as are rookie players on sports teams and tweens entering middle school.
Hormones serve a wide range of functions in the body and can trigger medical challenges when they fall out of balance. For this reason, some, including steroids and insulin, may be administered for therapeutic purposes.