I am the last born in my family and I must say the order is reversed here. Not to say I am an uber-citizen of the Major Ganglion Club. But the IQ numbers in my brood were destined to rise from the start.

I live far, far, far away from home. *LOL*

oldest children had an average IQ of 103, second children came in at 101, and third-borns were at 100.

The effect of birth order, if it really exists, seems to be utterly trivial.

I don't know about I.Q.'s, but there are tendencies for certain roles to be played out, adapted to,or whatever, by those born in certain spots in the family.Check Bradshaw's model -- but he's kind of dogmatic about it. I've seen the real examples often enough to think his thing is pretty right on.

The IQ spread is ... three points? And that's supposed to decide your destiny? Yeah, whatever.

I don't think IQ and personality traits are necessarily correlated. Both dumb and smart people can be rebellious. Both dumb and smart people can be conscientious etc. I think the minuscule spread in IQs in fact shows that IQ has nothing at all to do with the apparent "typical" differences there are between siblings. Something else must be at play to explain the stereotypes suggested and the order of birth (and therefore the way siblings are treated differently or experience things differently because of that) seems a quite plausible factor IMO in determining how.you "turn out".

I think it is from the back ground of how a child is raised, from when did the parents start to work with a child.
A child's mind is like a sponge from the ages of 0-6 and are the most vital to that child.
My only son was born in 1989 and from the time he was still inside of the womb, I was singing him ABC's and reading him stories, and teaching him mathematical equations.
Did this work, Yes it did.
He made A's throughout school, and scored the highest scores on the S.A.T.'s throughout the years.
Was this in his DNA to to be intelligent, and if We would have had another child would they have been just as smart?
I don't know if it was in his DNA however I do feel that the child would have been just as smart.

I never heard I. Q. being compared in all my reading about birth order. And IMO, there isn't a "debate" about the effect of birth order. There are generalizations we can make, taking all the other traits of the family of origin into account as well, including sexes of the kids and how far apart they are born. See Kevin Leman's work. I didn't realize it 'til after I started reading his books but he had his day on Oprah in the '90's.