There's a new book out called "Birth Order:  What Your Position In the Family Really Tells You About Your Character".  And it lays out the most compatible people for you to date, based on their birth order.  Check this out

First-born with last-born works. Oldest siblings are more likely to want to take care of people, and youngest siblings want to be cared for.

Only child with youngest child works. People who are the youngest in their family inspire creativity and freedom in people who didn't have any siblings.

Two last-borns work. They might be too immature together, but they can also bring out the best in each other.

A middle child is compatible with anyone. Middle children tend to be able to adapt to anyone.  They can calm down an overbearing first-born, get along well with another middle child, support someone who was the youngest, or let an only child take charge.

Two first-borns DON'T work. There's too big a risk for conflict, since first-borns are usually competitive and want to be in charge.

First-born and only child DOESN'T work. Both are used to being in charge and getting their way.

Two only children DON'T work. It's just too much of both people thinking of themselves.

Via (MyDaily)