One of the interesting things that I have seen is the way kids adopt "roles" within a family.

My uncle was once commenting on how his daughter, the youngest of the three, was so completely different than her older brothers.  She was rebellious, while the two older boys were straight arrows.

I saw very clear roles develop for the three of them

The oldest boy was an athlete

The middle one was a good student

The youngest became the rebel

it seems like the second is easier than the first, mostly because the parents finally have a clue.

If 13/41 presidents were first-borns, then that means that (and this assumes that the remainder are not only children, but I have not idea about that)  28/41 were not first-borns.  I wonder if that doesn't fit the stereotype that second-and-beyond-borns are more dynamic and outgoing....

I have two children.  My eldest (male,8) is quiet, bright, inquisitive, studious and eager to please.  
My youngest (female,4) is rebellious, daring, sneaky, artistic, not eagar to please and charming.

I'm very into birth order theory.  As an only, with all of the anti-only-child messages out there in society, I had to read up on it to educate myself.  People really think onlies are freaks and I shudder when I hear people saying that they are going to have another kid to give the other kid a sibling.  It's nice to have a sibling (that you get along with), but not a reason to have another one.

Basically, onlies are first-borns as far as the theory goes with a few extra quirks, since they never have to compete for attention.

I don't believe any of the intelligence stuff though, but I do think that your birth order plays into your personality.

Most of the only child stuff I have read said that you are better off to marry someone with a different birth order.  In other words, only and only isn't good.  I married a 5th out of 7, and we are perfect together : )

And I was the "only" for close to nine years.  A lot of parental expectation that probably made me bossy and arrogant, if nothing else.  I'm still bossy.  I still think I'm always right.  I probably had seven children so I'd have lots of people to boss around.  

My husband was the youngest by several years.  I have opinions about that, too, ofcourse.  The youngest always seem to wait around for everyone to do for them.  I see it with others who are the youngest...and I certainly see it in my own youngest two children.  By the time we get to the youngest, we're old and tired.  So much easier to just do for them than to always teach them to do it for themselves.  We're tired of making points and setting examples...we just want quiet!

One thing I've heard is that older siblings tend to marry younger siblings, which is true in my relationhip.  I'm the older sibling who is used to being in charge, and my husband is the younger sibling who is used to having someone else take the lead, so we're very compatible in that way.

My birth family is very typical in terms of birth order.  I'd say we follow the typical IQ thing too, with each kid having a lower IQ than the one before.
#1 daughter (me) - ambitious, organized, list-maker, smart, bossy, inflexible
#2 daughter - artistic, social, trouble getting along with parents during teen years
#3 son - charmer, super-social, horrendously disorganized and impulsive, lots of academic trouble in school and never held responsible for anything (allowed to the baby indefinitely -- he's now 22)

So I'm really curious to see how things play out in my own family.  So far, my kids are pretty typical.  My oldest is 3 and is introverted, thoughtful, calm, but needs everything to go his way.  My youngest is 1 and is very social, charming, flexible, and more physically active.  It's impossible to say about IQ at this point, but my older kid seems really bright in some areas, but has only average verbal skills, whereas the younger one is very verbal at a young age.  We'll see how it goes as they grow up.

I'm a youngest of two who married a youngest of two. I don't think either of us are typically "youngest" in the same way that a "youngest of many" is. My brother is only 11.5 months older than me, so I am not THAT much younger. My DH was 5 years younger than his older brother, but the brother was profoundly mentally retarded so DH is a lot more like an only than a youngest.

I do think it's interesting, though, to see how and if the stereotypes play out in a family.

I doubt birth order affects IQ that much.  There are so many variables with IQ.

I'm an oldest married to an oldest.  Our relationship is pretty good.  We've been married 13.5 years.  I think what makes it work is our strengths and weaknesses compliment each other.  

I find the comments about having an older sister for a male interesting.  My father was an only, had a terrible mother and in turn some not so great theories about women.  DH has a younger brother who is 6 years younger than him, and a really messed up mother.  DH has some trust issues as well, which imagine are rooted in his upbringing.  I wonder how I came to pick this person who has some commonality with my own father.  I wonder if it's birth order, or something else entirely.

out of 4 in my family, every last one of us  married a spouse with same birth order...strange isn't it?

As a second child married to a second child, here's my (somewhat tongue-in-cheek and grossly generalized) take:

Oldest children love to seize on studies such as these that show they are better than their sibs. It feeds their need to constantly be superior. They probably felt deeply jealous and displaced by subsequent children and need the constant affirmation.

Second children, like myself and DH, are not to be confused with youngest children or the "baby" of the family. Second children also have esteem issues but we're so used to playing second fiddle it becomes our lot in life. Between jealous older siblings constantly putting us down, and parents who wondered why we weren't achieving the same things as our older sibs, we feel like we're constantly trying to catch up and prove ourselves-- and always failing, because just as soon as we start to think we're just as OK as everyone else in the family, our threatened older sibling has to pull out a study like this one.

Older siblings think the world revolves around them. Second children are great at putting everyone else's needs first, not that that doesn't put us in therapy at some point.

In my family, my sister is the oldest, my mom is the baby, and I am the middle child. It took me a while to figure it out but once I did it made everything clearer. My husband is also the middle child between his sister and his brother. So between the two of us, we are excellent peacemakers and also incapable of making a decision. We also feel AOK keeping a healthy distance from anyone in our families.

Among my closest friends, I have an older, a second, a youngest, and a middle. We do often fall into our birth-order roles, though! The oldest is usually the planner and decision maker, who gets mad if she doesn't get what she wants or if other people disagree with her. The other second child and I are sometimes resentful yet afraid to speak up if we disagree with her. The middle child steadfastly takes no position whatsoever. The youngest is oblivious to the drama and just does whatever the hell she wants, even if it inconveniences everyone else. But we're all pretty awesome people and I love them all.