Of the primary emotions fear is the one that bears most directly on survival. Children show fear. Adults try not to maybe because it's shameful or in some circumstances dangerous. The fear response is automatic though and your body runs through its reflexes whether you want it to or not.
What they smell isn't the emotion of fear. What dogs can smell is the changes in a person's skin that suggest fear to the dog anxiety the way your skin sweats the amount of uric acid that suddenly pours out of your pores.
