Its easy to make up just-so stories to explain differences across siblings as being caused by birth-order. This article casts doubt on the significance of birth order.
But we can ask the question of whether birth order should matter and in what ways. Should natural selection imply systematic differences between older and younger siblings? Here is one argument that it should. Siblings “share genes” and as a consequence siblings have an evolutionary incentive to help each other. Birth order creates an asymmetry in the ways that different siblings can help each other. In particular, oldest siblings learn things first. They are the first to experiment with different survival strategies. The results of these experiments benefit all of the younger siblings. (Am I a good hunter? If so, my siblings are likely to be good hunters too.) Younger siblings have less to offer their older siblings on this dimension.
As a result we should expect older siblings to be more experimental than their younger siblings and more experimental than only children.
Here is evidence that older siblings have more years of education than younger siblings and more years of education than only children.
Cheap Talk 
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September 11, 2009 at 8:54 am
Michael Turner
And yet, Frank Sulloway, in his Born to Rebel, came to something like the opposite conclusion: first-borns tend to align more with the existing social order compared to later-borns. (They also react more violently against it than later-borns, if they rebel at all — not exactly a very creative reaction.)
Flexibility and creativity when it comes to survival strategies are important mainly when conditions are unstable and when nothing is known about the instability. But for most of human evolution, indeed most of human history, things have been pretty monotone, and when things changed, they usually changed in ways that the elders usually had some acquaintance with, if only from lore passed down to them.
Not that you need to appeal to sociobiology, particularly — as Sulloway says in that article, birth-order per se doesn’t *cause* anything. His analysis was more game-theoretic, almost economic, and somewhat more nuanced than anything relying entirely on birth order as the dependent variable. For example, birth-*spacing* turns out to matter a fair amount as well, as an amplifier for the basic mechanism of competitive role-differentiation. Sulloway’s model is also quite pleasingly elegant in certain ways. For example, an only child is both first-born and last-born. It turns out, the only child shares a lot of important and useful traits with both categories.