The only child has trouble sharing, the oldest is bossy, the baby always gets what he wants, and the middle child is—well, stuck in the middle. Are these merely stereotypes, or is there some truth to birth order differences?

Birth order only explains a small part of who we are, but personality changes definitely exist between siblings, says expert Frank Sulloway, PhD, author of Born to Rebel (Pantheon). "It's the roles siblings adopt that lead to differences in behavior," he says. And parents tend to reinforce these roles, whether they realize it or not. Here’s how to explain the personality differences between your only, oldest, youngest, or middle child.

Oldest Child Characteristics

Since firstborns follow their parents' lead, they like taking charge and have oodles of confidence, says Kevin Leman, PhD, author of The Birth Order Book: Why You Are the Way You Are (Revell). They don't have older siblings to tease them when they learn to tie their shoes or ride a bike. Adults take them seriously, and that boosts their confidence. When parents gush over the oldest sibling’s "firsts," it motivates them to achieve. Proof of this: Leman recounts a corporate seminar he conducted for CEOs in which 19 of the 20 attendees were firstborns.

It's easy for ambitious firstborns to become perfectionists; after all, they see adults coloring inside the lines and pouring milk without spilling. Your firstborn wants everything just so, Leman says, and he wants to get things right the first time around. To this end, he may resist pouring his own milk or coloring on his own because he doesn't want to make mistakes. These perfectionist oldest child traits also mean firstborns may have trouble admitting when they're wrong.

It's not difficult to see how firstborns can become so tightly wound: new to their roles as Mom and Dad, first-time parents can be overprotective and tentative while at the same time strict and demanding, says Leman. This can lead to “oldest child syndrome” and the conscientious desire to overachieve.

Parenting the Oldest Sibling

Parents tend to view firstborns as role models for younger siblings, and that can be a lot of pressure. "Watch for the effects of stress," cautions pediatrician T. Berry Brazelton, MD, coauthor of Touchpoints 3 to 6: Your Child's Emotional and Behavioral Development (Perseus Publishing). Be careful using "should"—as in, "you should've known better."

(Perseus Publishing). Be careful using "should"—as in, "you should've known better." While you're doling out extra responsibilities to your oldest, grant some privileges, too, like a later bedtime.

"It's easy to put too much responsibility on the firstborn," says Dr. Brazelton. Your oldest might volunteer to bring the baby a toy when he's fussy or hand you a diaper, but don't expect her to help all the time.

Middle Child Characteristics

Middle-born children are often completely different than older siblings. "Once a role is filled by the firstborn, the second-born will seek out a role that's completely the opposite," Leman says. Because of this, they’re hardest to label, since middle child personalities emerge in response to how they perceive the next-oldest sibling in the family. If the older sibling is a parent-pleaser, the middle child might rebel to get attention. “Middle child traits are the hardest to categorize, but whatever traits he develops play off the first born,” says Leman.

In the eyes of the middle child, oldest siblings reap all the privileges and the babies get away with everything, so middles learn to negotiate to get what they want. "Middle-borns are the most willing to wheel and deal," Sulloway says. They are agreeable, diplomatic, and compromising, and they handle disappointment well. They have realistic expectations, are the least likely to be spoiled, and they tend to be the most independent. Because they often feel left out, they tend to gravitate toward friends outside the family.

Parenting Your Middle Child

Thank her when she steps in to mediate a sibling squabble.

Respect his need for peers. Create opportunities for him to meet new friends at the park or on playdates.

Firstborns have their parents all to themselves initially, as do last-borns once their other siblings grow up and leave home. But the middle kids always have to share parental attention. Set aside extra time for your middle child to make her feel special, recommends Dr. Brazelton: "Do it for every child, individually, but especially for that middle child."

Youngest Child Characteristics

Parents tend to let things slide once the last child comes along—they aren't nervous, first-time parents anymore. As a result, lastborns usually do get away with more than their siblings do, says Leman. They shoulder less responsibility, so youngest child traits tend to be carefree, easygoing, fun-loving, affectionate, and sociable, and they like to make people laugh.

But being the youngest isn't all roses. Because lastborns view their older siblings as bigger, faster, and smarter, they may attempt to differentiate themselves by being more rebellious, says Sulloway. Leman, himself the family baby, agrees with this youngest child syndrome: "Lastborns have an 'I'll show them' attitude." And if older siblings baby the baby, lastborns might be spoiled and manipulative.

Parenting the Youngest Sibling

Lastborns often feel they aren't taken seriously. Let her make some family decisions—like where to go out for dinner or which video to watch together.

Acknowledge his "firsts." When he learns to tie his shoes or pees in the potty, call the relatives like you did with the firstborn. And be sure to make a big deal of his artistic accomplishments, displaying his drawings on the fridge, as you did for his older siblings.

Give the youngest child some responsibilities, even something simple like putting napkins on the table. Lastborns can end up with few family duties because they’ve learned to duck out of work or other family members have dubbed them too "little" to be able to handle things, says Leman.

Only Child Characteristics

Because only children spend so much time alone, they're self-entertainers and often tend to be the most creative of all birth orders. In fact, Leman calls only children "Super Firstborns." Like oldest siblings, they are confident, well spoken, pay enormous attention to detail, and tend to do well in school. Plus, spending so much time around grown-ups often makes onlies act like "little adults."

Only children have never had to compete for their parents' attention or share toys with their siblings, so they do run the risk of developing a self-centered streak. They're also used to feeling important and may have a hard time when things don't necessarily go their way, Leman says. Because their role models are competent adults, onlies are even more susceptible to perfectionism than firstborns.

Parenting Your Only Child

Since they aren't used to sharing with other kids at home, only children especially can benefit from playgroups.

Onlies lean toward perfectionism, so model acceptance of your own mistakes. Remind him that you couldn't cut out a perfect circle at his age either.

Don't seize every opportunity to teach her a better way to do something—if she makes the bed with a few wrinkles, don't remake it. You don't want to send the message that she is not measuring up.

What About Twins?

"Twins don't usually follow typical birth order roles," says Nancy Segal, PhD, twins expert and author of Indivisible by Two: Lives of Extraordinary Twins (Harvard University Press). "Most parents tend to be very fair and not emphasize order of birth, at least in Western nations." However, when twins are born vaginally, the firstborn is usually bigger, and the second-born has a greater risk for health problems. In these cases, parents may unconsciously treat the first twin more like a firstborn.

Factors That Affect Birth Order Personalities

"Many things contribute to human behavior," says birth order expert Frank Sulloway, PhD. "Birth order only explains a small chunk." Here are some other factors that alter traditional birth order roles.

Gender. Being born first doesn't necessarily guarantee firstborn status. In some cultures, a boy may be treated like a firstborn even when he has four older sisters, because he's the firstborn male.

Age differences. Birth order effects are strongest when siblings are two to four years apart. With large age gaps, siblings might act more like only children or firstborns. Siblings separated by fewer than two years are almost like twins. "When sibs are close in age, there's a physical equality," says Sulloway. It's hard to grab that truck from your younger brother when he's not all that little.