Birth Order Theory
The duplication theorem states that opposites attract. Put more simply, this means that if you're a firstborn, your best match will be a lastborn. The duplication theorem was formulated by psychologist Walter Toman, and it has been tested in clinical studies. (Born in Austria, Dr. Toman had dual Austrian and U.S. citizenship. He taught in Germany and was also an associate professor of psychology at Brandeis University in Massachusetts.) Researchers have found, for example, that people with many younger siblings are often quite compatible with people who have numerous older siblings. One study found that a lack of rank conflict leads to the most harmonious relationships. (Leonard Weller et al., Birth Order and Marital Bliss in Israel, J. Marriage and the Family, November 1974, 794-797.)
Rank conflict refers to the problems that individuals of similar rank experience in a relationship. Two firstborns, for example, have the same rank and would be expected to fight over who's the leader. Similarly two lastborns could be expected to experience conflict over who's to be considered the baby of the relationship. The notion of rank conflict was first formulated by Walter Toman in his book Family Constellation. Frank Sulloway found that rank conflict doesn't occur when two persons of the same rank are together, instead it occurs when two persons of opposite rank are together, for example when a firstborn and a lastborn are in a relationship. Sulloway's research suggests that especially during times of social upheaval those with opposite birth rank (a firstborn and a lastborn) can be expected to run into relationship problems. His research is contained in his 1996 book Born to Rebel.
William Cane has tested these birth order theories on over 6,000 celebrities, couples, and individuals. The duplication theorem has validity, he found, although it needs to be modified by considering the potential for social revolution to cause interpersonal conflict between firstborns and lastborns. His work on compatibility theory is contained in The Birth Order Book of Love: How the #1 Personality Predictor Can Help You Find The One. (Photo: Johnny Depp, Vanessa Paradis.)