One of the problems with studying birth order effects - and the reason why our results may not match some older beliefs/studies - is that children are not raised together anymore. With so many kids in daycare, young children spend most of their waking hours in the care of others surrounded by children other than their siblings. Instead of developing with their siblings, they are surrounded by a group (larger than most family sizes) of similar aged peers. Even if the siblings are in the same daycare class, the effects are watered down due to the presence of many other same-aged children and being cared for by someone other than a parent.

I think what we will find now is a whole new effect. Children raised in daycare won't have the typical birth order effects. Eldest and only children won't have the same one-on-one treatment throughout the day and will have to learn to compete for attention, youngest children will have same-aged or younger peers to compare themselves to (and therefore won't have the elder siblings as their main mentors), and a host of other differences can occur when preschool kids are put into groups or classrooms all day long. Whether you are a first, middle, last, or only child, you will spend most of your day in a similar environment and therefore will develop in a very similar manner.

I'd like to see more information about the effects of being raised in a large group of non-related children by someone other than a parent, rather than trying to make sense of data that may no longer be applicable.