If you want to know more about the guy you're dating, his family tree is a good place to start -- or more specifically, his birth order or the order he was born into a family. "While other factors, such as age, race, and gender, all play a role in shaping personality, research indicates that the number one factor influencing personality is birth order," explains William Cane, author of The Birth Order Book of Love, who has analyzed the birth order of 6,000 celebrities, historical figures and couples.

Birth order personality
Firstborn
Because firstborns are usually given more responsibility from young to look after their siblings, they typically grow up to be dominant, super-responsible, organized and competitive.

Middle child
The middle child is a people-pleaser. He or she is diplomatic, and it's difficult to get them to take a stand. The middleborn also tends to be a little manipulative.

Lastborn
Lastborns are friendly, creative individuals who love challenging the status quo. They're more adventurous than their siblings and are the risk-taker in the family, although they also tend to be needy at times.

Only child
An only child is used to being in the spotlight within the family, so he or she tends to crave attention. Being an only child also means that he or she is unlikely to know very much about the opposite sex.

Twins
Twins have a strong bond and therefore always think of the other before making a decision. Their thoughtful nature makes them considerate romantic partners.