How birth order effects personality

According to recent information, there is a small but reliable negative correlation between IQ and birth order. This is that the more older siblings one has, the lower the person's IQ is. This result came after many speculations of the finding; before, there was little evidence that birth rate and personality coincided. Before, research was flawed primarily because the size of the family played a large role. According to Joshua Hartshorne, "A child from a two-kid family has a 50 percent chance of being a firstborn, whereas a child from a five-kid family has only a 20 percent chance of being a firstborn." Therefore, if you are part of a smaller family, it is much more likely that you will be a first born than if you were part of a larger family.
The first born tending to be more intelligent could be linked to many factors. The major reason is that larger families have to distribute their resources more thinly (time, money, etc). Also, well educated and wealthy parents tend to have fewer children, while less educated and economically unstable families tend to have more children.
Even though new evidence supports these claims, there is still not a strong correlation between intelligence and birth order, so the evidence, to me, seems unreliable.
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