To understand this family we must look to psychology, which simply put, is the study of the mind and behavior. We'll be looking a fairly well-known concept known as birth order.
Birth Order:
The study of birth order relates personality traits to the order in which children are born into a family. Though nearly as popular as astrology for the prediction of love matches and career paths, there is firm logic behind the science of birth order. For instance, first-borns are used to being in control from their time spent as an only child before the birth of a sibling. Popular society quickly recognizes the tendencies of a middle child or the baby of the family. But the Weasley family is not like the census average of 2.5 children. Does birth order recognize a certain behavior for the fifth child of seven? Large families often create trouble for birth order studies. Behaviors are not assigned for every birth number in a family but rather follow a sequence. The term "third-born" refers to that personality even the child may be eighth in a family. A fourth child would the one who was actually born fourth.
Theory A: Birth order recognizes the first child through the fourth. The fifth child is assigned tendencies of being an only child and then the cycle starts over. The sixth child would have first born tendencies.
Theory B: Some experts argue though that large families start cannot follow that cycle. Rather a first born child may act more like an only child, the second born like a last-born, and the third-born would start the first born cycle.
Theory C and D: Twins can prove to be another problem in birth order studies. Birth order is about relations between siblings so when multiples are born together they are not coming into the family alone. The gaps between children cause the differences in behavior so what happens when one child follows the other within minutes? Twins take their own places in birth order (as seen in theory A and B.) Theory C and D are based off the idea that despite their assigned number which determines the follow placement of their younger siblings, one twin will have first born tendencies while the other takes on the tendencies of the last-born. These roles may flip during adolescent years.
Theory E and F: Correspond with basis birth orders in A and B. Given the twins similar personalities, these two theories treat them as a single unit, a single addition to the family.
| A | B | C | D | E | F | |
| Bill | 1st | only | 1st | only | 1st | only |
| Charlie | 2nd | last | 2nd | last | 2nd | last |
| Percy | 3rd | 1st | 3rd | 1st | 3rd | 1st |
| Fred or George | 4th | 2nd | 4th-first,youngest | 2nd-first,youngest | 4th | 2nd |
| George or Fred | only | 3rd | only-first,youngest | 3rd-first,youngest | 4th | 2nd |
| Ron | 1st | 4th | 1st | 4th | only | 3rd |
| Ginny | 2nd | only | 2nd | only | 1st | 4th |
Bill Weasley - First, only
"...with a horrible great fang on it, really, Bill, what do they say at the bank?"
Only children are characterized by feelings of loneliness. These kids tend to seek out truth. When the only child becomes the oldest with the addition of siblings, this child can develop a perfectionist personality. Only children will follow rules where they can be seen but will bend them behind the scenes, but as they become first-borns they more strictly adhere to rules. They are leaders who stay within social norms and avoid stepping over boundaries. Bill is a flexible character. We see him battling furniture on the lawn with his younger sibling and he follows an out of the norm style that drives his mother crazy. But he also shows leadership abilities which he displayed at a young age earning a Head Boy position at Hogwarts.
All first-borns start off as only children so we'll see characteristics of both groups in Bill Weasley. After the birth of a second child, the first-born perceives a loss of love from his parents and will seek out attention. Bill has a steady career (Curse-breaking which would likely involve details which are a focus for first-borns) and while his hair earns his mother's criticism, negative attention is still attention. First-borns can feel resentment with each addition child and known for being less involved in a family. It is not surprising that Bill works so far from his childhood home.
Charlie - Second, last
"He could have played for England if he hadn't gone off chasing dragons."
Second-born children in a family are greeted with a more relaxed parenting style than the first child. Where the first child is new, parents have past experiences to help when dealing with another child. The old joke goes that there are often twice as many photos for the first child than there are for the second.
The first-born in a family will develop anxiety over his or her parents' shared attention. The first born will often pick on his or her younger sibling and "so-and-so is getting into trouble" is a call most parents recognize. Until a third child is born, the second born is the youngest child. The "baby" of a family is usually out-going but being second to the first born means that there are often feelings of inadequacy. Despite being outward happy, the child may only use this to mask their insecurities. Second-born children are very aware of things and will often know more about each member of the family than anyone else.
While the first-born has a revered place in the family, the second-born struggles to find his or her identity. Like the first-born, this child may develop perfectionist behavior. To step out of their sibling's shadow, or perhaps in an effort to overpass him, the second-born develops very opposite interests from the older sibling. Look at Bill and Charlie. In school Bill was a Head Boy, a scholarly and orderly position. Charlie, on the other hand, excelled in Quidditch, a sport at Hogwarts. But when it came time to select a career, he chose to study dragons than compete in professional Quidditch. This may represent his ability to overcome his childhood insecurities. He chooses a career that would make him happy despite its lack of glory. He may have ended any old rivalries with Bill which would explain their closeness as adults.
Percy - First, third
"You could do with taking a leaf out of Percy's book!"
Just like the first-born turns his or her torment onto the second-born, the second-born will in turn show the same jealous feelings for the third child in a family. The second-born has lost his privileges as the baby of the family and will try and transfer his feelings of inadequacy onto his younger sibling. But a third-born will not develop the same insecurities as his brother, instead feeling vulnerable. This child will often freeze his emotions and close himself up. If the child succeeds he becomes fearless, if not he will become fearful. These two personalities will be a continuous battle all his life.
Percy shows signs of his placement. Especially in The Order of the Phoenix, we see his ability to shut down his feelings for his family to achieve a place out of the shadows. This desire to exceed his parents expectations (though at the same time separating from them) makes it clear to see why some experts consider third-borns to be first-borns at heart. Like his first-born brother Bill, Percy held positions of leadership during his time at Hogwarts. Like a first-born he seeks to "rule" his family.
Fred and George - Any or all placements
"Now, you two - this year, you behave yourselves. If I get one more owl telling me you've � blown up a toilet or -"
Twins are difficult to figure out and given the time when they were born, Fred and George could be any of the personalities between first- and fourth-borns.
We've looked at all of these places except fourth-borns, so let's start there. The line of torment continues. The vulnerable third-born will often tell the fourth-born that they are "not strong enough, big enough, or smart enough" to play with the older siblings. Fourth-borns feel unwanted and develop immature personalities. But here we see that the twins have each other. After being left out, these two will join in an alliance. Twins are notable for their special relationships and twins who are fourth-borns will most likely to allow them to be separated for their siblings. Fred and George are famous for their pranks and seem to be completely unique from anyone in their family. Their childish pranks are example of fourth-born immaturity.
That's the twins as fourth-borns (Theory E), in Theory F the twins would be second-borns. Percy would be the first-born child with his leadership abilities and perfectionist behavior. Like how Charlie and Bill were different, the same pattern is seen in Percy and the twins. Where Percy excelled academically, the twins are born rebels and star athletes like the families true second born.
One debate says that twins become their own birth cycle because they grow up together so closely. In this case they will display first-born and last-born tendencies. These two placements in the family share a common desire for attention. Like how Charlie's style attracts negative attention from his mother, the twins achieve similar critical attention from Mrs. Weasley. The problem with theory's A and B is that they would provide the twin's with slightly different personalities. From the books we do not have enough evidence to back it up. I believe either the C or D theories, or the E and F theories would work for them since Rowling has created two very similar twins.
Ron - First, third, fourth, only
"I'm the sixth in our family to go to Hogwarts. You could say I've got a lot to live up to. Bill and Charlie have already left -- Bill was head boy and Charlie was captain of Quidditch. Now Percy's a prefect. Fred and George mess around a lot, but they still get really good marks and everyone thinks they're really funny. Everyone expects me to do as well as the others, but if I do, it's no big deal, because they did it first. "
While Ron isn't the youngest member of his family, he is the youngest boy. More importantly, Ron is the Weasley we know the most about. A prominent part of his character is dealing with the fact that he has five older brother each successful (will not debate Percy's decision here) and unique in their own way. There have been athletes and Prefects and Hogwarts most infamous troublemakers.
In The Order of the Phoenix, not only does Ron make Prefect but he joins the Quidditch team as Keeper. However, as Hermione often points out, Ron lacks prefect discipline. Here he displays Theories B and D's fourth-born immaturity. In Quidditch, he struggles for most of the season. Again, he seems to suffer from the fourth-born insecurities that he will never be good enough. Birth order personalities may take years to develop. Just as Ron has proven his Gryffindor bravery, he may yet to have fully realized his leadership abilities. It is obvious he has the drive to succeed like a first-born character and making prefect may mean that others have recognized a dormant skill to take charge. He falls into Harry's shadow often but were he to be separated from Harry, it would likely he would make a great leader. For him, leadership may lie in his passion rather than his skills, which will develop with time.
Theory E suggests Ron would have an only child personality. With the cycle that runs all the way and starts over a child in the fifth-born position is more likely to be ignored. Fourth-borns are isolated early in life and the isolation becomes self-driven by the time the fifth child comes along. This new child is thus ignored since the fourth-born(s) have no desire to continue the cycle of teasing. In an already large family the fifth-born can offer no threat that might upset older siblings. By the time Ron is born both Bill and Charlie would be at Hogwarts. Percy who suppresses his emotions would not be likely to reach out to Ron and the twins who have been isolated by their brothers would focus on each other. A child can be an only child personality even when surrounded by brothers.
An "only child" tends to be very loyal in the bonds he or she does create. They also have the extraordinary ability to achieve what other call impossible. Here we see the Ron we know now at Hogwarts. Here the repeating birth cycle theory seems sounds and perfectly keyed for this member of the Weasley family.
Ginny - First, second, fourth, only
"You don't know how weird it is for her to be this shy. She never shuts up normally..." After six boys, we finally come to a girl; Ginny Weasley, the smallest of them all. A solitary girl in a family of boys will behave differently from a girl who has sisters. This girl will either create herself to be ultra-feminine or ultra-masculine. With her dating adventures we're clearly seeing a character embracing her gender in this family of men. Actually, it's her dating habits that tell us the most about Ginny. Youngest children are very outgoing, as mentioned before, and in relationships they tend to be the dominating partner controlling whoever they are with. We see her drag her "guy of the month" to the Dumbledore's Army meetings. Ginny could prove to be a more prominent character in the books. Youngest children are often very charming, though as Ron will argue, can quite easily shift to being the family's biggest pain. Only girls, especially when they're the youngest in the family, can prove trouble for overprotective, older brothers. But because she is a girl she is obviously left out. She seems to cope remarkably well suggesting she is not suffering by being a fourth-born personality as Theory F would suggest. I think we're already seeing a new side to Ginny. The Ginny we knew in her first year was obviously suffering from loneliness (found in only child and fourth-born personalities) and turned to Riddle for comfort. She seems to have overcome this and may develop into a very successful and involved character. Certainly here we would be seeing a first-born behavior, able to overcome childhood difficulties, like a first-born effectively coming to peace terms with another addition to their life. As we saw with the Riddle incident and with her dating habits, Ginny could prove to be a positive first-born personality. There is just not enough known at this point to predict how she change as she nears adulthood.
Many of the Weasley children fall into different placements. Birth order is a hint at understanding people but it is individuals and self-image that truly form a person. But also, there are other \ conflicts with this particular family. Mainly that the Weasley children all at one point attend Hogwarts, a magical boarding school. A child may not suffer from much older sibling teasing if this person is gone away at school for months at a time. But still, the knowing that a younger sibling is at home with a parent's attention can be infuriating for a first-born. A second-born may suffer from hearing how his brother is accomplishing so much in school.
So, considering all this information, what's the conclusion? Personally, I would say I find Theories C and D to be the most accurate. While we as readers can change this based on our interpretations, there is one true birth order and it will not change. We will simply have to keep reading and watching how Rowling lets these characters grow. There can remain little doubt that her characters are truthful to human nature and as real as any family.