Leman and many other psychologists have discovered that people who are born in a certain birth place in the family share a lot of the same traits. This is normally divided into three different groups: first, middle, and last (only children are only mentioned briefly).
These traits are said to be especially noticeable when it comes to long-term relationships. In this book, Leman attempts to explain how the typical traits found in each person's birth order influences their perspectives and actions and how people of each birth place can better get along.
different exercises to see if the one your with is your perfect match. (i.e. the oldest sibling should be with the youngest, middle sibling either younger or older, and the youngest should be with the oldest in a family.