Most of the time, when people talk about birth order they are, they think it's self evident, at least if you're a first or lastborn. Middleborns can create a bit more confusion...are they the exact middle child (2 of 3 or 3 of 5) or anyone who isn't a first or last child? In The Secret Power of Middle Children, we spend a chapter talking about the different factors that can influence how typical someone is for a middle child, but the same factors apply, in many ways, to other birth orders as well. Because when it comes down to it, birth order is a proxy for parental investment (whether that investment comes in the form of attention, affection, financial support, etc) and while many societies have tended to favor their firstborns, there are a number of factors that have an impact on how parents allocate their resources among their children.
Sex can make a difference. In some families and in some cultures, what matters is not being the firstborn but being the first boy. So regardless of whether the first boy in the family is actually born first, or second, or third, he may be the recipient of a larger share of parental attention and support...as well as parental expectations (which can be hard and stressful work to live up to). Such a boy's personality/attitudes/behavior may be more firstborn typical than that of the birth position he is actually born into. Of course, there are also some cultural and social examples where being female results in greater parental investment, such as low caste Indian families (Mildred Dickemann, 1979) or Hungarian Gypsies (Bereczkei & Dunbar, 2002, 1997).