The Bell Curve Part One: there goes the ball game

The Bell Curve's chapters are grouped into four parts, the first of which is entitled "The Emergence of a Cognitive Elite" and its three introductory pages before the first chapter do not bode well for the rest of the book.

The Bell Curve is now twenty-five years old and even in just the past quarter century the book's claim about a correlation between intelligence and wealth is weakened by evidence of the increasing rise in the number of people who are wealthy thanks to inheritance.

But where the book really falters is in its argument about "primogeniture" which is the term used for the custom of the first born (almost always first born son) inheriting most of the family wealth (my highlight):

This differentiation by cognitive ability did not coalesce into cognitive classes in premodern societies for various reasons. Clerical celibacy was one. Another was that the people who rose to the top on their brains were co-opted by aristocratic systems that depleted their descendants' talent, mainly through the mechanism known as primogeniture. Because parents could not pick the brightest of their progeny to inherit the title and land, aristocracies fell victim to regression to the mean: children of parents with above-average IQs tend to have lower IQs than their parents, and their children's IQs are lower still. Over the course of a few generations, the average intelligence in an aristocratic family fell toward the population average, hastened by marriages that matched bride and groom by lineage, not ability.

In 2016 The Journal of Human Resources published a study "The Early Origins of Birth Order Differences in Children’s Outcomes and Parental Behavior" demonstrating that in fact first-born children score higher on intelligence tests. And it was not the first study to make that claim.

How is that for an ironic twist on The Bell Curve's primogeniture claim? As this University of Edinburgh summary of the 2016 study states:

First borns score higher than their siblings in IQ tests as early as age one, the study has found...

...Economists at the University of Edinburgh, Analysis Group and the University of Sydney examined data from the U.S. Children of the National Longitudinal Survey of Youth, a dataset collected by the US Bureau of Labor Statistics.

Nearly 5,000 children were observed from pre-birth to age 14. Every child was assessed every two years. The tests included reading recognition, such as matching letters, naming names and reading single words aloud and picture vocabulary assessments. Information was also collected on environmental factors such as family background and economic conditions.

It is extremely improbable that the first long-term surviving offspring of two people would be smartest across a population due to genetic variation. Not even hereditarians argue for that.

In other words, genetic causality is completely ruled out as a factor in the results of intelligence tests vs birth order. Which leaves only one causative factor: nurture.

As the study states (typos in the original were left in):

Researchers found that parents changed their behaviour as subsequent children were born. They offered less mental stimulation to younger siblings also took part in fewer activities such as such as reading with the child, crafts and playing musical instruments.
Mothers also took higher risks during the pregnancy of latter-born children, such as increased smoking.

And there goes the ball game for hereditarianism. It's clear that environment alone is responsible for significant differences in intelligence - at least as measured by intelligence tests.

But I will continue to cover The Bell Curve, chapter by chapter.