Being the second oldest in a family with four kids birth order is definitely something relevant in my life. The article I found has very similar findings to the textbook but I also found many of the same flaws in their reasoning. The major one is replicability. Although there can be many families whose children's characteristics fit that of the studies the percentage it applies to the population is too small and inconsistent to be taken with credibility.
The other critical thinking aspect I saw representative was correlation vs. causation. In my opinion, I see how the parents treat each child separately as a major factor to their upbringing rather than birth order. For example, the first child is considered to be a natural leader, reliable, perfectionists, and aggressive. Growing up the older kids tend to be role models for the younger ones so the younger siblings already look up to the oldest one and put him or her in a leadership role. My older brother fits these qualities in all except being a perfectionist. The middle child is supposed to be secretive, independent, and inventive. Since the oldest child tends to receive most of the attention the middle child copes with this by keeping his feelings withheld and being secretive. I can see many of the middle child characteristics within me. Finally, the last born is outgoing, irresponsible, and spoiled. Parents usually are not as strict with the last child since they have had a few kids already they are not as worried as parents. This results in the kid becoming irresponsible with so much freedom but at the same time spoiled. This is perfectly representative of my youngest sister. I think birth order differences are more a result of them being "nurtured" as they grow up rather than something that predestined.