Thank you Debbie, I agree completely.

The impression I got from the (admittedly second-hand) observations in the research study is that birth order has no genetic or biological impact on a person's personality. To me, this is intuitive.

But to say that situational factors are insignificant - to say that having all younger siblings, or all older, or some of both, has no impact on the way a child grows and develops - is ridiculous. And to dismiss the way parents treat their children differently (based not only on their ordinal position but also their age relative to the other kids) is disingenuous and missing the point. There IS an effect of birth order on a person's development. That it's situational and not biological doesn't reduce its significance.

That said, it is of course necessary to recognize that our personalities are not shaped SOLELY by birth order. There are so many other variables, many of which we're still uncovering. And of course it's unfair to dismissively categorize people based on their birth order; it's always tempting to fit people into a box, and our confirmation bias allows us to ignore those messy, unique bits of a person's personality that don't fit into those preconceived boxes.

No, we are not solely defined by our birth order. But we are, in part, shaped by it.