Throughout the years there has been much debate on whether or not birth order in families affects their IQ scores.
One of the first studies done was in 1874 by Francis Galton which came to the conclusion there was a high correlation between birth order and IQ scores. The reasoning for his finding was...
1. Firstborn sons would be more likely to have the financial resources to continue their education.
2. Firstborns had the advantage of being "treated more as companions by parents." This means that they also undertake more responsibility than their younger siblings.
3. Firstborn children would get more attention and better nourishment in families with limited financial resources.
Therefore according to Galton there was a correlation between birth order and IQ scores. (His reasoning is similar to those offered by modern scientists.)
Another study done was in 1973 by Lillian Belmont and Francis Marolla. Now this study included the aspect of family size with birth order and intelligence. Here were their results...
1. "Children from large families tend to make poorer showings on intelligence tests and on educational measures, even when social class is controlled."
2. "Within each family size (i) firstborns always scored better on the Raven than did later borns; and (ii) with few inconsistencies, there was a gradient of declining scores with rising birth order, so that firstborns scored better than secondborns, who in turn scored better than thirdborns, and so forth."
3. "In general, as family size increased, there was a decrease in Raven performance within any particular birth order position." For example, a thirdborn born child from a 3-child family would be expected to score higher than a thirdborn child from a 4-child family. A thirdborn child from a 5-child family would be expected to score even lower, and so on.
This study brought about a very important aspect in whether or not birth order affects intelligence.
Due to these two studies I can understand why scientists can come to the conclusion that there is a casual relationship between birth order and intelligence. It has been disproved that birth order and intelligence have a high correlation by the idea that families with lower IQs have more children than families with higher IQs.
I have definitely witnessed these findings throughout my time in high school. It seemed like the families with the largest amount of kids had lower IQs rather than the families with only one or two kids. Through my personal experience I couldn't be sure of the family's IQ or the children's but from witnessing them in classes and throughout their high school career it became pretty obvious.
Now, putting my own family into this study I can definitely see the relationship between birth order, family size, and IQ.
I was an only child for twelve years and so I got a lot of attention from my parents and nurturing of intelligence. Once my sister came along that attention decreased to level out to the both of us and for some time more for her. I can see this affecting children who get siblings at a younger age. If I were to get a sibling at the age of six versus twelve the attention and nurturing I would have gotten from my parents would have decreased sooner and could have affected our overall IQ.
Correlational vs Causation was a very important principle to look at throughout this study, as well as replicability.
Overall, I think that birth order and intelligence have a casual relationship and the aspect of family size is very important. These findings were evident in my own family as well as families throughout my school.