Thursday, January 22, 2009

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
First 6.27
Second 6.47
Third 5.91
Fourth 6.10
Fifth 5.94
Sixth 5.51

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
First 11.28
Second 11.22
Third 10.93
Fourth 10.87
Fifth 10.41
Sixth 8.33


Mean mother's years of schooling

Only child 11.53
First 11.32
Second 10.98
Third 10.30
Fourth 9.83
Fifth 9.19
Sixth 8.73

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?

6 comments:

The Undiscovered Jew said...

"Does all the attention that an only child gets (or even the first couple of kids) pay off?"

The difference could be caused by hormonal changes in the womb after the first birth.

25% of IQ is due to environment and I would bet a good chunk of that 25% is explainable by hormones.

Ron Guhname said...

"The difference could be caused by hormonal changes in the womb after the first birth."

But what about the difference between an only child and a first one?

Blode032222 said...

I haven't worked out exactly how family size plays into this. It would be interesting to compare only strict apples to apples, i.e., figure out how all the fourth-of-seven children compared to the third-of-seven children. Maybe I've got it wrong, but it looks like right now, most of the First and Second children could well be from small families (2 children total) while of course all the fourth children are from 4+.

If that's so, then this may mess with our estimates of how much environment is affecting this. There must be genetic factors influencing, at least a little, how many kids someone chooses to have. Naturally there would also be tons of environmental factors influencing the same thing!

Interesting data though.

The Undiscovered Jew said...

"But what about the difference between an only child and a first one?"

It could be due to a number of factors such as higher IQ parents having fewer children. A good study would have to take into account IQ of parents as well as the children's IQ and birth order.

However, we do know that hormones and birth order can have an effect on the brain.

We know for instance that the more older brothers a boy has the more likely he is to be gay and homosexuality is estimated to be 60-70 hormonal in origin. The brains of homosexuals appear to be "feminized" because of hormonal effects in the womb.

Lover of Wisdom said...

Could someone use GSS to see if there is a greater degree of only children being left handed than those children with siblings?

This could confirm the hormonal/testosterone theory.

Michael Ventura said...

How old were these children when these Vocab Scores were taken?

If it's just Childhood IQ it doesn't mean anything.

We already know that there’s a practically meaningless boost in Childhood IQ for Children who grow up in more culturally enriching homes.

We also know that this boost does not persist into Adulthood, where a higher IQ would actually be of any value.