Helen Fisher's book "Why Him? Why Her?" explores how personality types determine who we fall in love with. Each person primarily expresses one of four personality types - Explorer, Builder, Director, or Negotiator - which are governed by different brain chemistry systems driving attraction. Fisher conducted a unique study of 40,000 people to identify each type and show which types are compatible or problematic matches. Her research into romance and attachment leads to new insights into dating, love, and marriage. The book is based on a questionnaire completed by 5 million people worldwide and will change understanding of why we love certain partners.