Popular books use birth order for self-discovery, relationship tips, business advice, and parenting guidance, however, when scientist scrutinized the data, they found no convincing findings that linked birth order to personality or behavior. I believe our common perception that birth order matters is due to the confirmation bias. I think it is so popular because when people first hear about it they immediately look to their own family and other families they know and try to match the birth order characteristics with the oldest, middle, and youngest child in that family. More times than not they are able to find at least one characteristic that matches and so they believe the theory.
Correlation versus causation is something to be aware of when researching the birth order effect. There are other factors that come into play when determining the reason for one's personality traits. One article I read discussed how birth order is linked to family size. There are many reasons family size could affect our personalities, as family size is associated with important social factors. These include ethnicity, education, and wealth.
Another principle of scientific thinking to keep in mind is replicability. Many studies fail to show significant effects of birth order and cannot be replicated.
http://www.scientificamerican.com/article.cfm?id=ruled-by-birth-order
I think this is a very interesting topic, considering I am in a family with two younger brothers. I am one who has fallen for the whole 'confirmation bias' trick thinking that all of the specific personality characteristics match to my brothers and I. It makes more sense to me now, after reading so much in the psychology book about principles of critical thinking, that correlation versus causation would pose a problem when assigning birth order to personality traits.
When I learned that there was no scientific evidence that supports the birth order effect, I was very surprised. I learned about this concept multiple times during high school and junior high and I also thought that it applied to my family so well. But now that I think about it, I have realized that I been a victim of the confirmation bias.
It is interesting that, in China, people believe the first child is often the most beautiful one and second child is often the smartest one. I did believe it when I was young. However, there is definitely no evidence showing this causation.
This post is really amusing to me because this has definitely happened in my family. Now I realize it's probably entirely due to confirmation bias, but for years my family has though that birth order was clear due to our different personalities.