Do small families lead to higher IQ?

Okay, so this is just me thinking (and mathing) out loud. Suppose we have two different groups (A and B) of 100 people each (arbitrary number chosen for ease of dividing.) In Group A, people are lumped into 5 large “clans” of 20 people each. In Group B, people are lumped in 20 small clans of 5 people each.

Each society has an average IQ of 100–ten people with 80IQs, ten people with 120IQs, and eighty people with 100IQs. I assume that there is slight but not absolute assortative mating, so that most high-IQ and low-IQ people end up marrying someone average.

IQ pairings:

100/100    100/80    100/120    80/80    120/120 (IQ)

30                 9                9                 1               1            (couples)

Okay, so there should be thirty couples where both partners have 100IQs, nine 100/80IQ couples, nine 100/120IQ couples, one 80/80IQ couple, and one 120/120IQ couple.

If each couple has 2 kids, distributed thusly:

100/100=> 10% 80, 10% 120, and 80% 100

120/120=> 100% 120

80/80 => 100% 80

120/100=> 100% 110

80/100 => 100% 90

Then we’ll end up with eight 80IQ kids, eighteen 90IQ, forty-eight 100IQ, eighteen 110 IQ, and 8 120IQ.

So, under pretty much perfect and totally arbitrary conditions that probably only vaguely approximate how genetics actually works (also, we are ignoring the influence of random chance on the grounds that it is random and therefore evens out over the long-term,) our population approaches a normal bell-curved IQ distribution.

Third gen:

80/80  80/90  80/100  90/90  90/100  90/110  100/100  100/110  100/120  110/110  110/120  120/120

1             2            5             4            9             2              6                9               5              4             2             1

2 80         4 85      10 90      8 90     18 95      4 100       1,4,1       18 105     10 110        8 110       4 115        2 120

3 80, 4 85, 18 90, 18 95, 8 100, 18 105, 18 110, 4 115, and 3 120. For simplicity’s sake:

7 80IQ, 18 90IQ, 44 100IQ, 18 110IQ, and 7 120IQ.

Not bad for a very, very rough model that is trying to keep the math very simple so I can write it blog post window instead of paper, though clearly 6 children have gotten lost somewhere. (rounding error???)

Anyway, now let’s assume that we don’t have a 2-child policy in place, but that being smart (or dumb) does something to your reproductive chances.

In the simplest model, people with 80IQs have zero children, 90s have one child, 100s have 2 children, 110s have 3 children, and 120s have 4 children.

oh god but the couples are crossed so do I take the average or the top IQ? I guess I’ll take average.

Gen 2:

100/100    100/80    100/120    80/80    120/120 (IQ)

30                 9                9                 1               1            (couples)

60 kids        9 kids       27 kids       0              4 kids

6, 48, 6

So our new distribution is six 80IQ, nine 90IQ, forty-eight 100IQ, twenty-seven 110IQ, and ten 120IQ.

(checks math oh good it adds up to 100.)

We’re not going to run gen three, as obviously the trend will continue.

Let’s go back to our original clans. Society A has 5 clans of 20 people each; Society B has 20 clans of 5 people each.

With 10 high-IQ and 10 low-IQ people per society, each clan in A is likely to have 2 smart and 2 dumb people. Each clan in B, by contrast, is likely to have only 1 smart or 1 dumb person. For our model, each clan will be the reproductive unit rather than each couple, and we’ll take the average IQ of each clan.

Society A: 5 clans with average of 100 IQ => social stasis.

Society B: 20 clans, 10 with average of 96, 10 with average of 106. Not a big difference, but if the 106s have even just a few more children over the generations than the 96s, they will gradually increase as a % of the population.

Of course, over the generations, a few of our 5-person clans will get two smart people (average IQ 108), a dumb and a smart (average 100), and two dumb (92.) The 108 clans will do very well for themselves, and the 92 clans will do very badly.

Speculative conclusions:

If society functions so that smart people have more offspring than dumb people (definitely not a given in the real world,) then: In society A, everyone benefits from the smart people, whose brains uplift their entire extended families (large clans.) This helps everyone, especially the least capable, who otherwise could not have provided for themselves. However, the average IQ in society A doesn’t move much, because you are likely to have equal numbers of dumb and smart people in each family, balancing each other out. In Society B, the smart people are still helping their families, but since their families are smaller, random chance dictates that they are less likely to have a dumb person in their families. The families with the misfortune to have a dumb member suffer and have fewer children as a result; the families with the good fortune to have a smart member benefit and have more children as a result. Society B has more suffering, but also evolves to have a higher average IQ. Society A has less suffering, but its IQ does not change. Obviously this a thought experiment and should not be taken as proof of anything about real world genetics. But my suspicion is that this is basically the mechanism behind the evolution of high-IQ in areas with long histories of nuclear, atomized families, and the mechanism suppressing IQ in areas with strongly tribal norms. (See HBD Chick for everything family structure related.)




4 thoughts on “Do small families lead to higher IQ?

  1. In real life, families who have fewer children tend to start them later. Does that have any effect on IQ? Also, is there any correlation between birth order and IQ? Because in a small-family society there would be a greater portion of first and second born children.


    • Those are interesting questions, but I think the data on them is highly confounded. In situations where families are actually all having as many kids as humanly possible and raising all of them to adulthood, I suspect you will get selection for earlier age of menarchy, which in turn is likely to cause lower IQ.

      But historically, I think most societies were more limited by things like food and disease than sheer physical ability to bear children, so that people who could afford medical care or food could raise more children than those who could not, even if they did marry later (and of course often men who married later married women younger than themselves, and if their wives died in childbirth, would just marry another and carry on having more children.)

      For example, in the book I just read about a Pygmy family in the Congo, the grandparents (uneducated hunter-gatherers) had 6 surviving children, and the main character, (one of their kids), had 11 surviving children–despite having married later. Among important differences, he used his relative wealth and education to attract a bride from outside his own tribe (reducing the chance of sickle cell anemia) and he was able to afford medical care for his kids.


  2. True or not, it’s not significant enough to fully mitigate the benefit of maxing smart-people-babies and minning dumb-people-babies. Intredasting idea tho tbqhimho


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