Are you smarter than your younger brother or sister? Well, if so there are multiple theories about why this is so. The correlation between birth order and IQ has long been discussed in the field of psychology. In the 1970's American psychologist Robert Zajonc hypothesized that later born children usually have lower IQ's than their older siblings. While the data does back this claim up, it may not be the most logical explanation of it. A much simpler explanation is that lower IQ parents tend to have more children than higher IQ parents. Due to this, and the fact that IQ is genetic, the evidence leads me to believe that IQ and birth order have no correlation. This thought is supported by studies performed within families that show that there is little to no correlation between birth order and IQ.
When examining the relationship between birth order and IQ I was not surprised in the least that there is little to no correlation between them. The reason that it did not surprise me was that the evidence supporting it seems to violate Occam's Razor. This means that there is a more parsimonious, or simple, explanation to the question of whether birth order is correlated with IQ. That answer being that lower IQ parents have more children than higher IQ parents thus skewing the data to say that birth order and IQ are correlated.