If you're the oldest child in your family, you may complain, "Why do I have to do that chore just because I'm the oldest?" Your younger brother may moan, "Why does she get to stay up an extra hour just because she's the oldest? Being in the middle of this family stinks."

Both of you may ask about your youngest sister, "Why should she get away with everything just because she's the youngest?"

Where you fall in the birth order of your family does have an effect on what your life is like. If you're the oldest, your parents may expect you to be responsible for younger brothers and sisters. If you're in the middle, you may feel that you get less attention than your siblings. If you're the youngest, you may feel that no one takes you seriously.

Some psychologists believe parents treat oldest children differently from the rest of the family. They push their older kids harder and expect more from them. The result: Oldest children expect more from themselves, too. Do you think this theory, or idea, is true in your family?

The size of your family also influences your life. If you're one of nine children, you may have to share your room with someone else and wear "hand-me-down" clothes. While there are lots of brothers and sisters to play with, you may feel you don't get enough privacy.

On the other hand, if you're an only child, you may feel lonely and left out, even though you have your own room and get new clothes every season.

Social scientists, psychologists and psychiatrists -- experts who study how people behave and feel -- have done hundreds of studies about birth order. The studies are designed to find out how birth order affects the way you think, feel and act -- your personality.

As you might expect with that many people working on a single problem, the experts have come up lots of different answers. One question the experts hope to answer in their studies is: Are eldest children more likely to be leaders than younger siblings? Some studies suggest that they are. For example, a large percentage of the members of the U.S. Congress are oldest children. And a 1972 study showed that more than half of the United States' presidents were oldest sons. President Ronald Reagan is an exception; he's the younger of two boys. First Lady Nancy Reagan is an only child.

One psychologist did a study of middle and youngest children a few years ago. His data -- the results of his study -- suggested that middle and youngest kids are more likely to take up rough sports like football and judo than their oldest siblings are. The study doesn't explain why this happens -- it just demonstrates that it does.

The trouble with birth-order research is that studies often contradict each other, and real life often turns out to be different from the results of the studies. For example, one researcher found that youngest children were the most likely members of a family to enter creative fields like writing, drama or music. Bruce Springsteen is an exception to that rule; he's the oldest of three. Rocker Cyndi Lauper is also an exception. She's the middle child in a family of three. Another study showed that writers from big families were likely to be the first-born members of the group.

No one knows quite what to make of all these contradictions. But conducting research about personality is a difficult business. It's pretty hard to measure what kind of person someone is. Your doctor can weigh and measure you, and write down the color of your hair and eyes. But the kind of person you are is a lot harder to define.

If you're an eldest child, you may take life seriously, obey the rules and work hard to be responsible. If you're a middle child, you may be more rebellious than your older brother or sister, and you may play peacemaker between your older sibling and your younger one.

If you're the youngest, you may be easygoing and relaxed, and you probably get away with things your older brothers and sisters couldn't. But you may also have a hard time growing out of being the "baby" of the family.

If you're an only child, you may feel sure of yourself around adults, but find it hard to play with people your own age. Maybe none of these traits sound like you. Don't worry. Each human being is an individual.

Where you fall in your family's birth order may have some effect. But a lot of other things influence you too. The type of school you go to, how much money your family has, your health, the kind of neighborhood, state or country you live in, whether you go to church or synagogue or do not practice a religion -- all these ingredients and many more go into the unique recipe that makes you who you are.Tips for Parents

Birth-order research has consumed social scientists for at least a century. Swiss psychologists Cecile Ernst and Jules Angst, who believe much of the research is flawed because it leaves out too many variables, provide an overview of hundreds of studies in "Birth Order: Its Influences on Personality." The scholarly book, published in 1983, is still available from Springer-Verlag Publishers for $32. Catherine O'Neill is a free-lance children's writer based in Baltimore.