Simply by virtue of being a couple's first child, a firstborn will naturally be a sort of experiment for the new parents, a mixture of instinct and trial-and-error. Perhaps this will cause the parents to become by-the-book caregivers who are extremely attentive, stringent with rules, and overly neurotic about the minutiae. This in turn may cause the child to become a perfectionist, always striving to please his parents.
In contrast, if the couple decides to have a second child, they might raise their second-born with less of an iron first due to their experiences raising their firstborn. They might also be less attentive to the second-born since there's another child competing for attention, and they probably will be less inclined to impulsively dial 911 every time the child breaks a sweat. This may cause the second-born to be less of a perfectionist but more of a people-pleaser due to the lack of attention he gets in comparison to his older sibling.
In short, it's not necessarily the fact that a child came out of his mother's womb first that he grew up to be a leader who talks a blue streak. Rather, it's the fact that his parents treated him as their firstborn child that shaped his attitude and behavior.
Firstborn
As the leader of the pack, firstborns often tend to be:
Reliable
Conscientious
Structured
Cautious
Controlling
Achievers
Firstborns bask in their parents' presence, which may explain why they sometimes act like mini-adults. Firstborns are diligent and want to be the best at everything they do. They excel at winning the hearts of their elders.
Firstborns as Children
Lori Kiel McGowan, a public health consultant and mother of two boys in Cambridge, Massachusetts, can attest to that. She describes her 6-year-old firstborn son Kiel as a cautious boy who prefers the company of adults or younger children compared to that of his peers. "For his sixth birthday, we invited six friends of his choosing," McGowan recalls. "For maybe the first half hour to 45 minutes, he crawled into a corner behind a booth and cried. These were close friends from school and after-school and the neighborhood. But after [his cautiousness waned], he came out and had a great time." When presented with a new situation, Kiel's cautious nature manifested itself in temporary introversion. However, once he became acclimated to his new environment, his caution subsided.
Moreover, as for his behavior around his younger brother Fionn, Kiel plays the role of the "controlling big brother" to a T: "He alternately tries to hug him and push him down, or gets a toy or juice for him, and then takes something away to make him scream," McGowan says. "It's definitely a sibling love/hate thing."

The Grown-Up Firstborn
As firstborns grow older, their traits are not necessarily lost. Firstborn child Tracy Rackauskas, a 35-year-old from Denver, fully identifies herself as an achiever. "I want to be the best dressed, in a quirky-fashionable way; I want to have the best fantasy football team; I want to be the best editor; I want to be the most insightful and understanding partner; I want to be the sexiest and the smartest and the most interesting," Rackauskas says. "And it's not because I'm really competitive or want to be better than anybody else, but just because I want to be the best." Her ambition carried over to her studies in law school, where she studied all the time and prepared for exams by making her own outlines according to her specific organization system. Perhaps in part due to her diligence, Rackauskas graduated summa cum laude and now works as a legal editor, making sure everything that passes through her hands meets her standards.

Middle Child
"The middle child often feels left out and a sense of, 'Well, I'm not the oldest. I'm not the youngest. Who am I?'" says therapist Meri Wallace. This sort of hierarchical floundering leads middle children to make their mark among their peers, since parental attention is usually devoted to the beloved firstborn or baby of the family.
In general, middle children tend to possess the following characteristics:
People-pleasers
Somewhat rebellious
Thrives on friendships
Has large social circle
Peacemaker
Middle Kids as Children
Holly Schrock, a 31-year-old at-home mother of five in Newtown, Pennsylvania, describes her second-born child Maggie as an attention-getter with a mildly rebellious streak. "You'll say to her 'Go pick up the laundry right now' or 'Go put on your bathing suit so we can go to the pool,' and if she's in the middle of something, she'll look at you bald-faced and say, 'No!'" Schrock says.
However, "middle children are the toughest to pin down because they do play off their older sibling," says Dr. Leman. For instance, the sex of the child is a big variable. If the firstborn child is a boy and the middle child is a girl, she may possess firstborn characteristics because though she is technically second-born, she is also the firstborn female. As the second-born child with an older brother, 4-year-old Maggie also qualifies as the firstborn female in the family, which may explain why she also possesses the nurturing leader qualities of a firstborn child. "Maggie likes to be a little mommy, especially to the younger ones." Schrock says. "She picks up her 2-year-old brother and tries to tend to his needs in a positive way. She's protective of them."

The Grown-Up Middle Child
Schrock herself is also a middle child. Sandwiched between an older and younger sister, Schrock exhibited the same rebellious streak that her daughter Maggie does. "I wasn't a bad kid, but I was definitely pushing the envelope a little," Schrock says. In fact, at one point during her teen years, Schrock became embroiled in an argument with her parents that resulted in her running away for three days. Though Schrock admits she has since calmed down a bit, she still won't take anyone's guff. "I don't like being told what to do, period."
Last Born
Youngest children tend to be the most free-spirited due to their parents' increasingly laissez-faire attitude towards parenting the second (or third, or fourth, or fifth...) time around. The baby of the family tends to be:
Fun-loving
Uncomplicated
Manipulative
Outgoing
Attention-seeker
Self-centered
Lastborns as Children
Megan, an at-home mom in San Diego, says her 7-year-old daughter Kacey loves the spotlight and will wrestle it away from others, if need be. "Kacey loves to go out into our backyard and put on shows," Megan says. "One time she was out there roller skating with her older sister, Jessica, but when Jessica started skating in these pirouettes that Kacey couldn't do, Kacey deliberately fell down to get our attention."

The Grown-Up Lastborn
Lastborn child Janice Lee, now 25 years old and working as an architect in New York City, definitely identifies with the simplistic, uncomplicated nature of a last-born child. "Most girls would die if their boyfriends didn't get them something for their birthday," she says. "But my boyfriend and I don't exchange gifts on our birthdays or anniversaries. We're very low-key. We'll go out to dinner, but nothing extravagant." Lee also has a pie-in-the-sky, "everything will work out" worldview: "I don't need to have that much security in my life. I like being spontaneous. I moved to Germany from Toronto for a job last year, and I didn't even speak the language," she says.
Only Children
Being the only child is a unique position in a family. Without any siblings to compete with, the only child monopolizes his parents' attention and resources, not just for a short period of time like a firstborn, but forever. In effect, this makes an only child something like a "super-firstborn": only children have the privilege (and the burden) of having all their parents' support and expectations on their shoulders. Thus, only children tend to be:
Mature for their age
Perfectionists
Conscientious
Diligent
Leaders

Only Children as Kids
Just one meeting with 5-year-old Lilia, and you'll see. "She has a sophisticated sense of humor and is often one of the few girls her age to get a sarcastic remark or double-meaning," says Lilia's mother Razan Brooker, a software business owner in Boston. "Her teachers are very surprised at her level of understanding of adult humor." Even as a younger child, Lilia exhibited noticeable maturity and diligence. Like most children, Lilia sucked her thumb. But rather than throwing a temper tantrum when asked to break the habit, "she agreed to it and threw away her 'blankie,' claiming that is what makes her want to suck her thumb," Brooker says. "She then proceeded to make a chart for herself consisting of 30 squares for the number of days she was told it will take her to break the habit." A month later, Lilia was sleeping with her hands by her side.

The Grown-Up Only Child
Even when only children reach adulthood, they may not necessarily shed their need to be model human beings, able to run a five-minute mile and cook a seven-course meal without consulting a cookbook. "I hosted a Christmas party at my apartment and had to make sure the champagne was chilled, the music was on, the cats were locked in the kitchen," says Margaret Lloyd, a 27-year-old New York advertising associate. "Even after guests arrived, I kept fussing with things, even though it probably took away from some of my enjoyment that evening."
Exceptions to Traditional Birth Order Structure
Blended Families: In the case of divorce, remarriage, and the melding of stepchildren, Dr. Leman says, "blended families don't blend; they collide." Firstborn children who used to be the leader of the pack may find themselves unceremoniously thrown off the top of the hill by an older stepsibling, and the youngest of the family may suddenly have to deal with all the attention that's segued towards the new baby .
But despite a child's new position in a blended family hierarchy, he will not tailor his existing personality to his new position unless he is still in infancy. Many psychologists agree that personality develops tremendously during the first few years of life during the bonding stages. By about age 5, much of a child's personality has been established (although that doesn't mean it's fixed). In this way, a 10-year-old firstborn will likely have a more difficult time giving up his position as the eldest than a 4-year-old might.
Families Within Families: In cases such as with twins , you have a family within a family -- a unit that operates independently of birth order. "A twin will never act like a middle-born; he will always act like a firstborn or a baby," Leman says. Since twins are perceived as a single unit -- likely even referred to as "the twins" -- they separate themselves from the traditional family and revel in their special position.
Gap Children: According to Leman, if you have a gap of at least five years in between births, another family begins in the birth order structure. A 2-year-old boy with a newborn brother and an 8-year-old older sister isn't going adopt middle-child traits, but rather those of a firstborn.

Adoption: The same scenario occurs with adopted kids. The age at which the child is adopted is a key factor in which traits the child is most likely to exhibit. The younger the child is at adoption, the more time he will spend under the adoptive parents' care and adopt his position in the existing family tree. For instance, if a firstborn 1-year-old is adopted by a family with a 4-year-old child, the adopted child will likely fall into the role of the baby, despite the fact that he is biologically a firstborn child. However, if a firstborn child is 7 years old when he is adopted into a family with a 10-year-old child, the adopted child will still act like a firstborn even though he has an older brother. "You don't give up being a firstborn," says Leman. "You take the birth order with you."

source: www.parents.com/baby/development/social/birth-order-and-personality/