All men may be created equal; but a look at their pay stubs will tell you that their incomes are not. Blame it on social class, education -- even luck, but according to Dalton Conley, New York University professor of sociology and public policy, inequality begins at home.

In his book "The Pecking Order: Which Siblings Succeed and Why," Conley says that 75 percent of the income inequality between individuals in the United States occurs between siblings in the same families. He points to the diverse fortunes of Bill and Roger Clinton, and Jimmy and Billy Carter as examples.

Research shows that first borns (and onlys) lead the pack in terms of educational attainment, occupational prestige, income and net worth. Conversely middle children in large families tend to fare the worst. (Marcia! Marcia! Marcia!)

"A child's position in the family impacts his personality, his behavior, his learning and ultimately his earning power," states Michael Grose, author of "Why First Born Rule the World and Last-borns Want to Change It." "Most people have an intuitive knowledge that birth order somehow has an impact on development, but they underestimate how far-reaching and just how significant that impact really is."

Conley concedes that birth order is significant in shaping individual success, but only for children of large families -- four or more siblings -- and in families where finances and parental time are constrained. (In wealthy families, like the Bushes and Kennedys, it has less effect.)

Here's a look at what impact your birth-order may have on you:

First Borns:
More conscientious, ambitious and aggressive than their younger siblings, first borns are over-represented at Harvard and Yale as well as disciplines requiring higher education such as medicine, engineering or law. Every astronaut to go into space has been either the oldest child in his or her family or the eldest boy. And throughout history -- even when large families were the norm -- more than half of all Nobel Prize winners and U.S. presidents have been first born. Famous eldest children include: Hillary Clinton, Bill Clinton, Richard Branson, J.K. Rowling and Winston Churchill. And macho movie stars are First Born, too, including Clint Eastwood, John Wayne, Sylvester Stallone, Bruce Willis and all the actors who have played James Bond.

Middles:
Middle children are more easy going and peer-oriented. Since they can get lost in the shuffle of their own families, they learn to build bridges to other sources of support and therefore tend to have excellent people skills. Middle children often take on the role of mediator and peacemaker. Famous middle children include: Bill Gates, J.F.K., Madonna and Princess Diana.

Youngest:
The youngest child tends to be the most creative and can be very charming -- even manipulative. Because they often identify with the underdog, they tend to champion egalitarian causes. (Youngest siblings were the earliest backers of the Protestant Reformation and the Enlightenment.) Successful in journalism, advertising, sales and the arts, famous youngest children include Cameron Diaz, Jim Carrey, Drew Carey, Rosie O'Donnell, Eddie Murphy and Billy Crystal.

Only Children:
Only children have similar characteristics to first borns and are frequently burdened with high parental expectations. Research shows they are more confident, articulate and likely to use their imagination than other children. They also expect a lot from others, hate criticism, can be inflexible and are likely to be perfectionists. Well-known only children include Rudy Guiliani, Franklin Delano Roosevelt, Alan Greenspan, Tiger Woods, tennis' teen queen Maria Sharapova and Leonardo Da Vinci.

Twins:
Because they hold equal status and are treated so similarly, twins turn out similarly in most cases. Consider advice columnists "Dear Abby" and "Ann Landers" (Abigail and Esther Friedman), and Harold and Bernard Shapiro, who became presidents of Princeton University and Canada's McGill University respectively.

Dr. Frank Sulloway, a behavioral scientist and visiting professor at the Institute of Personality and Social Research at University of California, Berkeley and author of the book, "Born To Rebel: Birth Order, Family Dynamics and Creative Lives," says first borns are more similar in personality to first borns in other families than they are to their own younger siblings and that youngest children are often more similar to the youngest child in another family than his or her own elder siblings. He says this is because the family is not as much a "shared environment" as a set of niches that provide siblings with different outlooks.

Conley agrees, but stresses that these are just general trends -- and that the whole birth-order theory can be turned on its head depending on the child's personality, the age gap between siblings and the family circumstances each child experiences during his or her formative years.

Kate Lorenz is the article and advice editor for CareerBuilder.com. She researches and writes about job search strategy, career management, hiring trends and workplace issues.

Copyright 2006 CareerBuilder.com.