There is perhaps no domain where misreading nonverbal signals causes more personal damage than the distinction between romantic interest and social politeness. The professional, warm, attentive, and kind person can be completely indistinguishable from the romantically interested person to an observer who does not know what to look for. Both smile. Both maintain eye contact. Both ask follow-up questions and remember things you said. Both show warmth in their tone. The behavioral overlap is so extensive that entire relationship histories are built on confusing the two, with predictably painful outcomes. The confusion is not a failure of intelligence. It is...