No matter when you were born, or who you've become, it is important to accept each person for who they are as an individual. It is important that we not assume we know someone because they fit into certain categories such as "first-born" or "only-child." That being said, there are some common threads that often appear in an individual's personality or propensities or sibling relationships, relating to their birth order.
The eldest child usually has received the most focused attention from her (or his) parents during those vital formative years. If the family is healthy and loving, this often means that child is the most advanced and the most capable of taking on additional responsibilities or expectations. The eldest often feels responsible. The eldest is often driven to succeed.
The eldest also often has the highest expectations and the toughest rules placed on him or her. They are the ground-breakers. The one the parents are learning on. Parents usually relax a bit on the later children because they have a better understanding of what's important to them as a parent, because there are more places for their limited attention to go, and because they have (and expect) the eldest child to help out. As adults, we see these characteristics retained, and the oldest child may continue to have rigid expectations of themselves and of what "right-behavior" looks like. Eldest children are often care-takers, especially in healthy loving households. I'll get to common realities for children and grown-up children of unhealthy homes in a minute.
The next-oldest child often shares many of the responsibilities, without the benefit of any special privileges that the oldest sibling receives. The second child is not seen as the oldest, the smartest, the... They often feel left out or over-burdened. The "ugly step-child" who never quite has enough, and doesn't know how to ask for more. This child is often driven to prove him or herself, usually against the accomplishments of the eldest, or the perceived expectations (not the same as the actual expectations!) of the parents. The second-oldest may go out of his/her way to be different from the eldest, or may give up on meeting any expectations, and act laid-back and fun-seeking (while somehow remaining disengaged from life and goals). Sibling children who are close together in age often take one of two routes-- either they are best friends and share everything, or they don't get along at all and want to emphasize their differences.
This relationship between first and second-born primarily depends on the eldest child. Is he/she able to enjoy having a live-in best friend without expressing superiority or a need for special recognition as the oldest? Or has she/he embraced the role of "third-parent" to the point that she/he resents the efforts of the second oldest to copy or equal him/her? The oldest may even feel that his/her role in the family is threatened by the second-oldest child's efforts to take on some of the "older child" responsibilities and power in relation to the younger children. At the same time, that second-oldest may feel that they are never good enough, or that they must take on the same role with the family that the eldest had, when he/she finally leaves home and the second gets to be the "oldest." They can be very disappointed when the skills and responsibilities and special rights they have so carefully practiced are no longer needed or appreciated now that everyone is older, and the eldest has moved out.
In a three-child family, the youngest is the peace-keeper and the comedian. It's his/her role to act the clown, to get attention by being funny or acting out, and to smooth over rifts among other family members either by pulling attention to herself or acting as a peace-maker directly. The youngest child in a family often gets more attention during the critical formative years because there is no new baby to "replace" him or her. Also, for each additional child, the parents' attitude and rules usually relax even further. This can cause a divide between youngest and older siblings who feel that by comparison, the youngest has it easy, and is "spoiled." There may be jealousy about the amount of un-interrupted attention the youngest child receives. This can translate into rougher treatment, and games like "keep-away" where the youngest never gets to have a turn with the ball. Older children may feel that he/she gets everything else, all the attention and love the parentsÃ‚Â give out, so the youngest won't get anything from them.
Conversely, older siblings may also love and coddle the youngest child, feeling more maternal/paternal toward him or her. Older children will share their treats and toys, will protect and enjoy special time with the youngest. Since older siblings often attend the same school or take care of the youngest by bringing him/her along to their own activities, the youngest can grow up "fast." They've been to rock concerts and had teens apply makeup to their faces. They know about the fun of "making out" and they have the freedoms of a much older child. This treatment often does result in a spoiled youngest. One who has no sense of cause and effect, and no understanding of direct consequences, respectful boundaries, or age-appropriate behaviors. The youngest may be protected from abusive parents by the older children, or abandoned to deal with those parents and problems on their own as older children left home as early as possible.
The youngest child in a family-- particularly a "surprise baby" who is born several years after the others-- can be every bit as spoiled and entitled as an only-child is said to be. They live in a well-padded cloud, with little understanding of the difference between their childhood and that of the older children. Parents usually have more money later in life, as well as fewer expectations, and the youngest often benefits from this as well. The youngest may receive financial assistance and expensive gifts (a car, money for college, etc) that was not available to older children, and this often results in sibling jealousy.
Of course, the other side of the sibling coin is that the youngest can become a people-pleaser. They can be very good at making others happy, and understanding what others expect of them. Someone, in an unhealthy home, who fears conflict or is very skilled at averting it, having watched his/her older siblings fight, act out, get punished over and over again. The youngest may have taken on the responsibility for preventing fights among their older siblings. In an unhealthy home, the youngest may not have received adequate attention as a young child. The parents may have been too focused on the challenges that the older children presented. This creates a weakness for anyone who shows them love or gives them positive attention. Youngest girls are more likely to seek out relationships with older boys and men, looking for the love they didn't get at home. Youngest boys are more likely to take risks and act out, looking for ways to get the attention they lack, and even negative attention is better than none.
In unhealthy homes, children often band together to survive the abuses and unpredictability (or predictable anger) of their parents. They look out for each other, keep each other's secrets, and help each other stay safe both in and outside of the home. The biggest burden often falls to the oldest child or children. Often, too, the oldest receives the harshest punishments and the greatest child-rearing responsibilities as more babies come along. Because of these challenges, the oldest child is most at-risk for depression and mental disorders, risk-taking behaviors such as drinking, drugs, self-mutilation, early sexual activity, etc. This child needs a way out, and a way to cope with the pain of their daily life. This eldest child is likely to get married (or have unplanned pregnancies) early, in their search for a more loving relationship.
Also, when the younger children are finally safe, the oldest child of an abusive family may not want anything more to do with the other children. This is particularly true of oldest sisters, though not always. The younger siblings she has raised are reminders of her own unhappy past, and all the burdens she took on when they were young. She may choose simply to disappear from their lives, or finally allow herself to succumb to mental breakdown or alcoholism. The oldest brother of an unhealthy home may continue to feel responsible for the safety and well-being of his younger siblings long after they have all matured. He may provide emotional shelter or financial resources to his siblings well into adulthood. Again, while these descriptions present common behaviors and traits, each person is unique, and copes with challenges (and sibling relationships) in their own way.
Children who grow up knowing they are not responsible for their parents' bad behaviors are the most likely to retain healthy and supportive sibling relationships as adults. It is common for each child in a family (healthy or not) to go through some period of withdrawal from the family dynamics. We each have a desire to find ourselves, and to define who "me" is, without the preset expectations of the family that has known us to be a certain way our whole lives. This withdrawal can happen in the midst of teen angst, it can happen when a sibling has finally gotten away to college or a first year of marriage. It can happen when the sibling finally has children of his or her own, or even after their own children are out of the house, and they have a chance to wonder who they are when no one else is around. Temporary withdrawal is even more likely when one is questioning his or her sexuality, religious beliefs, moral values, or other lifestyle choices. We must choose what we believe and how we will interact with others for ourselves. And once we know who and what we are, it is much easier to re-enter the family and establish positive and loving mature sibling relationships.
As parents, the best we can do is appreciate each child as unique from their siblings. We can explain to our children that we treat each of them differently because they are not the same person. They have unique needs, unique abilities, and thus our love and expectations for each of them-- while being the same mother-love and father-love for a child in our family-- will often be expressed differently by us, their loving parents. Some children have a strong sense of fair-play, and thus they only feel loved by being able to measure that the same time and money are spent on each child. Some children understand that Billy loves hugs, so he gets lots of hugs because I love him; and Suzie loves play time, so she gets extra-long tea parties because I love her. It's different, but it is the same love. One appropriate to each child's unique needs. It's even easier to see how similar the love is when both Billy and Suzie know that each month their parents put $10 in each college fund, regardless of anything else.
Here are some links to additional online resources about birth order, sibling relationships, and personality: