I'm my biological parent's only child, but my mom remarried when I was young and I have 2 older step brothers. My parents weren't wealthy-my mom is a teacher and my step dad is a maintanance planner a utility plant-but they were smart with their money. My parents paid for my college 100% as long as it was an in state public university. One of my step brothers went to a private college but he did so with the help of a basketball scholarship--I don't thnk it was a full ride though. My oldest step brother is a career Army guy, but they were prepared to pay for his school as well if he had wanted college. All this without my dad paying much in the way of child support and my stepdad paying child support to his ex. It can be done, but it takes careful planning. My mom's parents had four kids (and my grandma never worked outside the home), and all four kids went to college and they paid for all of their undergraduate degrees. Two went onto masters degrees and one is a doctor. There are 8 kids in DH's family and it is a mixed bag as far as birth order and education. There are 8 kids in my stepdads family and all but my stepdad finished college and are teachers, engineers, nurses, etc. When I think of my cousins, in one family the older child is more educated, in another the younger child is more educated, and in the third the middle child is more educated.
These studies look at broader trends, but don't really account for factors like parental commitment to education, the individual personalities if the kids, etc.
While I don't thnk there is anything wrong with looking at studies, I tend to take studies (especially those whose subjects are living things) with a grain of salt.