Birth order is a particularly interesting combination of biological and environmental factors.
It seems that the mother's body is harsher to later borns, which accounts for the differences in birth rate and other consequences that manifest themselves later in life.
At the same time, there's the environmental factors you mentioned, and those the article deals with (vaccination rates, etc.).
Then you get into development, where later borns tend to have a lower IQ than their siblings through early childhood, but that this reverses as they grow into older childhood and adolescent.
I asked him about it offline and got to researching the biology of birth order.
I'm pretty sure I'd read this before, but did you know that, statistically speaking, fraternal birth order affects male sexual orientation?
Here's a great, clear, concise synopsis: O brother, where art thou? The fraternal birth-order effect on male sexual orientation .
For each additional brother that precedes him, a boy's chance of growing up to be gay increases by a third.
And it's not social influence, because the effect is seen with biological brothers out of the house (eg, adopted) and is not seen with step-brothers in the house.
Other interesting facts:
A brain nucleus that is larger in men than women is also smaller in gay men than straight men...
For women, a bewildering array of body parts (ears, fingers, eyes, arms) all indicate that lesbians were, on average, exposed to more fetal testosterone (T) than straight women (3–6). The idea that the brains of lesbians might have been masculinized by exposure to fetal T fits easily with animal models, where researchers can make a mammalian brain as masculine or feminine as they like just by controlling how much T reaches the brain, especially early in life (7)...
Twin studies found that sexual orientation is heritable in both sexes (9), and a portion of the X chromosome was implicated in some cases of brothers who were both gay (10)...
One estimate is that approximately one in seven homosexual men in North America are gay because of older brothers (12). That means that about a million Americans are either gay men today or boys who are going to grow up to be gay because their mother had sons before them...
Here's the one I was really looking for:
Each subsequent baby a mother carries tends to weigh more, but boys whose mother carried brothers before them did not weigh as much as boys with older sisters (13).
If you've met my 'little' brothers, you know they're big, strapping bruisers who managed to grow over 6 feet tall. And I had heard and rightly remembered that younger children tend to be bigger.
But in this fact I have my revenge: They would have been even bigger if they'd had an older sister instead of me! ;-)
A few more facts:
The FBO effect on birth weight was greater among boys who turned out to be homosexual than those who became straight, suggesting that those boys who are made gay by older brothers are also more susceptible to the FBO effect on birth weight. Scouring the literature, these authors found an old report that placenta weight, which also tends to get heavier with each subsequent child carried, weighs even more for boys if they have elder brothers rather than elder sisters (14).
Older brothers increase the probability of a boy becoming gay only if that boy is right-handed. Among left-handed men, there's no difference in the incidence of homosexuality no matter how many brothers they have. What's curious about this finding is that, by itself, left-handedness makes males and females slightly more likely to be gay (16).
So, what's the cause? Maybe
the "maternal immunization hypothesis" (Fig. 1). A mother carrying a first son has very little exposure to the proteins he is making because of the placental barrier. But upon delivery and the inevitable mixing of fetal and maternal blood, her immune system will now see proteins it has never seen before, including proteins encoded on her son's Y chromosome. If she mounts an immune response to these proteins, then any subsequent sons will be exposed, via active transport across the placenta, to maternal antibodies directed against the male-specific proteins. These maternal antibodies might then perturb development of the younger son, decreasing birth weight and affecting his brain to increase the probability that he will grow up to be gay (13).
Now, before anyone attacks, let me say that some parts of this article imply that biology causes homosexuality. I don't believe that and never have. Biology affects behavior, but it is not behavior. There is human choice. Two other behaviors commonly associated with male heterosexuality are violence and promiscuity. I may very well have those biological proclivities. But we all agree, societally, that violence and promiscuity are not acceptable.
I'm not arguing societal consensus. What I am arguing is the effect of values and choice in behavior. I stipulate some biological propensities to homosexuality. The point is: what do people do with it? What choices do they make?
Further, homosexual tendencies need not be any more overpowering than promiscuity or violence.
Besides, it's not a zero-sum game. It needn't be the case that someone with biological homosexual tendencies also has zero biological attraction to the opposite sex.
Oh, and, by the way: none of my three younger brothers are gay. Not that there's anything wrong with that ;-)