What effects do you believe birth order has on how people develop, their character
traits, and values?
DEVELOPING FICTITIOUS CHARACTERS
All authors, when they first begin writing, search for ways to develop characters and their motivations out of nothing but an idea. None of the innumerable approaches are right or wrong. Whatever works for the writer.
My novels tend to be driven by external conflicts, so I usually have the story idea before I start developing the characters. That provides clues about the personality traits my characters will need to support and move the storyline. What sort of people, under what circumstances, could reasonably get themselves into that situation? And what kind of character would figure a way out? Other writers start with the characters, some meld both situation and characters.
There are innumerable books on how to develop characters, basic personality types, and what traits go with each. Those authors are more qualified to give advice to other writers.
A writer can always use real people as models for characters or create a written biography of the person. The biography approach has never worked for me. I like to discover who the characters are is and what they like and dislike. Part of the joy of writing is when they do or say something I never expected or planned.
A couple of other fun methods I've tried include: Astrological signs [which works best for matching the personalities of the hero and heroine and their compatibility issues]) and birth order [which can provide strong motivations].
To tell you the truth, I've never put much stock in either astrology or birth order … you can always make a case for or against the traits and/or motivations of any particular character … and real person. But they can provide a beginning point and are fun to play around with.
BIRTH ORDER
Although birth order has been studied extensively over the years, it's a difficult area to study, which makes the topic controversial … both the studies themselves and the findings. Studies have always been complicated by the genders of siblings as well as the birth order and the time span between children.
Today, making studies even more confusing, we have blended families of step-siblings and half-siblings which obscure the "real" birth order. We have single parent families. Ethnically blended families. Also, large spacing between siblings and other factors skew birth order effects.
Many families are different than the typical ▼

When I write, I use birth order to help me identify the characters' motivations.

These traits showed up in all or most of the sources I reviewed as characteristics of first-born children:
● Conscientious, reliable, serious, mature, cautious
● Natural leaders
● Diligent, high achievers, goal-oriented, want to be the best at anything they do
● Ambitious to succeed
● Structured, controlling
● Perfectionist
● Strong need for approval from people in charge
● Don't easily admit they are wrong
● Don't like surprises
● Protective

In my opinion, the character traits of middle-born children are questionable. After all, in many families with more than one child, the middle-born may be a last-born child for a number of years, and the poof … suddenly he or she finds himself [herself] now in the middle. I suppose that's why spacing between children is so important in studies.

● People pleasing, good friend, peace-maker, go-with-the-flow attitude
● Social, thrives on friendship
● Rebellious, doesn't like being told what to do, doesn't like to be boxed in
● Adaptable
● Independent and resourceful
● Insecure and secretive
● Obsessed about fairness
There seems to be more agreement among the experts on the middle-born personality traits, although to me rebellious seems to be in conflict with other traits such as adaptable.
Imagine - Four Older Brothers The Kennedy Family The Only Child
Only-children are similar to first-born, only more so. They are typically mature for their age. This may have something to do with spending time with adults. Many only-children are high achievers, some are rebels who march to the beat of their drum. Remember, first-borns are also only children for a period of time.
● Mature, confident
● Comfortable with adults as children which makes them comfortable with people when they are adults
● Perfectionist
● Conscientious
● Diligent
● Verbally precocious
● High achievers
● Don't take criticism well
● Tend to do well in school as children
WHAT IS YOUR EXPERIENCE?
According to Chinie Diaz [http://www.rappler.com/] "Whatever the case – whether you're true to you birth order personality type or not – it's always good to remember that personality type theories only serve as a tool for greater self-awareness. And any tendencies we many have are simply that – TENDENCIES.
If they're good ones, then by all means nurture them and allow them to blossom. If they're bad, well … that's the beauty of being free and intelligent human beings. We always have the choice and ability to overcome that which makes us less than great."
THAT ABOUT SAYS IT ALL!
Resources
http://www.parents.com/baby/development/social/birth-order-and-personality/
http://www.lexiyoga.com/birth-order-can-predict-romantic-compatibility
http://www.parents.com/baby/development/sibling-issues/raising-only-child/
http://www.rappler.com/move-ph/ispeak/35606-birth-order-personality
http://www.cracked.com/blog/5-things-people-dont-understand-about-only-children/