Birth Order and IQ: Some researchers have found that the oldest child tends to be the smartest, but what does the General Social Survey say? It asked 838 people about how many siblings were born before them, and also gave them a ten-question vocabulary test which is highly correlated with IQ.
Mean vocabulary score
Only child 7.11
The first kid is not the smartest; the second is although the difference is not statistically significant. The first two siblings are, however, significantly smarter than subsequent children.
On average, an only child is almost a half of a standard deviation smarter than a first child. Is this due to smart parents being more likely to have one kid? Let's look at mom and dad's educational level:
Mean father's years of schooling
Only child 10.16
Mean mother's years of schooling
Only child 11.53
Moms seem a bit smarter but dads look dumber. Sample size for the fathers with one child is only 25, but that's not terrible for a mean. Does all the attention that an only child gets (or even the first couple of kids) pay off?