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# The Birth Order Illusion

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Once one of my wife's law professors polled her class on birth order. "How many of you are first-borns?" Two-thirds of the students raised their hands. Clear evidence that first-borns are achievers, right?

Hardly. An alternative hypothesis is that law students come from affluent families with few kids. Imagine that birth order has nothing to do with law school attendance. If half the students are only children, and half come from two-child homes, then three-quarters will be first-borns.

I decided to race these hypotheses using the General Social Survey. If you regress real income on birth order, you get the same pattern as my wife's law school class. The first-born averages \$1900 more than the second-born, who averages \$1900 more than the third-born, and so on.

However, if you regress real income on birth order AND family size, you get a totally different picture. Birth order makes essentially no difference (in fact, the sign reverses), but average income falls by about \$2400/child in your family. First-born only child? You'll make more than average. First-child child in a big family? You'll do no better than the fifth-born child - maybe a little worse!

Does this show that big families hurt incomes? Possibly, but the simpler story is more plausible: Poor people have more kids, and kids of poor people tend to be poor themselves.

NCA writes:

I thought today's post would have commented on Angelina Jolie playing Dagny Taggart

Randy writes:

I saw a similar demonstration in a communications class I took while in the military. The instructor predicted that most of the first borns would be in the front rows, second borns in the middle, and third borns in the back. And he was right. This was a room full of enlisted military people and therefore mostly lower to middle middle class and mostly from rural backgrounds. On the other hand, part of the contention was also that those in the front rows were also the best students, but I saw no evidence of that. For example, I'm freakin brilliant and I always sit in the back :)

Robert Scarth writes:

I like that post; it reminds us how counter intuitive even simple stats can be.

According to my simple spreadsheet calculations, if family size is Poisson distributed then two thirds of students being first born means that the average family size is about 1.5. If this is the case I'd expect 23.5% to be second born, 7.4% to be third born, and 2% to be forth born.

dearieme writes:

I have a vague recollection of reading that many of the birth order claims break down if you treat all only children as last born. :-))

I suspect there is also an effect having to do with spreading limited resources across multiple children. Looking a the experience in terms of investment in children of the two child families I knew growing up that where otherwise similar socio-economically to the four child family I grew up in I noted that there were significantly better educational opportunities in the two child family. The children in the two child family went to top private schools in the area, we went to mediocre to poor public schools. The children I knew in the two child family had much more exposure to early computing resource and other enrichment. Overall, the children in the two or one child families where simply MUCH MUCH better capitalized.

Ragerz writes:

And since kids from poor families have no choice about who their parents are, current distributions of wealth and income are not based on merit.

Randy writes:

Ragerz,

True, but irrelevant. It is of value to society to distribute the most resources to the most productive, regardless of how they became the most productive. It is also of value to society that the unproductive can fall into poverty and that the productive can achieve wealth. To remove the motivators for productivity is to create an unproductive society, and consequently a higher level of objective poverty.

Fabio Rojas writes:

Bryan - this is old hat: Check out Jeremey Freese's article:

http://www.jeremyfreese.com/docs/FreesePowellSteelman%20-%20rebel%20without%20a%20cause%20or%20effect.pdf

Michael Stack writes:

Repeat the experiement and ask how many of them were born last (I think somebody above made a similar observation counting only-children as last-born).

writes:

What explains why the poor have more children?

Are children more valuable to poor families, or is the cost of children cheaper for poor families?

Can the same model be used to explain why poor nations have higher birth rates than wealthy ones?

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