Another common misunderstanding is to think that the heritability of a trait refers to individuals. Mathematically, heritability refers to a whole population. Suppose that genes explain 70 percent of a population’s variance in height. You can use this information to conclude that “genes probably have a lot to do with how tall Joe is,” but it does not mean that “genes explain 70 percent of how tall Joe is.”
Heritability is not a fixed number for a given trait. It can vary by age, for example. We will encounter an example of this when we get to the heritability of IQ: Counterintuitively, it increases as people get older.
Heritability also varies by population. For example, suppose you want to know the heritability of performance on the SAT and you compare two sets of students. One sample is from an ordinary New York City public high school and the other is from Stuyvesant, a famous high school for the intellectually gifted. For practical purposes, Stuyvesant scores will be concentrated in a narrow range — probably 1500 to 1600. The scores for the sample from an ordinary high school will vary from 400 to 1600. The denominator for the heritability ratio calculated from students at Stuyvesant will be smaller than the denominator from the sample from the ordinary high school. Other things equal, the heritability of SAT scores in the Stuyvesant sample will be higher than the heritability for the sample from the ordinary high school.
Heritability can also vary over populations, or over the same population over time, for an important reason that is too seldom recognized: As society does a better job of enabling all of its citizens to realize their talents, the heritability of those talents will rise.
For instance:
For the first half of the twentieth century, Norway was a country in which the amount of schooling you got depended strongly on where you lived (many remote places did not have secondary schools) and your family’s social class. In 1960, the average years of education for Norwegian adults was 5.9. After World War II, access to elementary and secondary school became nearly universal. By 2000, the average Norwegian adult had 11.9 years of education. Norwegian allele frequencies for the SNPs that are associated with years of education cannot have changed appreciably from 1960 to 2000. The absolute genetic contribution was effectively constant. But the heritability of educational attainment for Norwegian male twins born before 1940 was 40 percent. For their counterparts born after 1940, it was approximately 70 percent.
“…Suppose that genes explain 70 percent of a population’s variance in height. You can use this information to conclude that “genes probably have a lot to do with how tall Joe is,” but it does not mean that “genes explain 70 percent of how tall Joe is.”…”
Eeeh. I believe there’s a little sophistry here because after all if you are deciding how tall Joe should be and he IS a part of a gene pool that is tall then he’s likely to be tall because he’s in the tall gene pool. He doesn’t stand alone. He’s part of the tall gene pool no matter if you “magically” separate him or not.
I see this as a trick used to pretend that there are NOT traits in different gene pools. They use this sort of magic mathematical foolishness to pretend that, things are not, what they are. That it’s dishonest I’ve proven by simply noting that you can’t remove that which is then declare it not a part of the set it is in in the first place honestly.
Not entirely sure I read it right but this:
“Heritability can also vary over populations, or over the same population over time, for an important reason that is too seldom recognized: As society does a better job of enabling all of its citizens to realize their talents, the heritability of those talents will rise.”
coupled with the passage about Norway sound like an argument for the traditional meritocratic scouring of the lower orders for talent, marrying them into the elite, and maintaining an otherwise broadly assortative mating pattern among the same.
Actually, so does the Stuyvesant section.
But, questions for which I would need an Isegoria-style precis/guide:
I could see how using SAT scores from one group of ordinary public HS students and comparing them to the SAT scores of their own parents, or even their own ethnic group if coherent enough could yield something interesting, if not conclusive, about inheritance.
But are there enough “ordinary”, not to say rough, public high schools in America where the student body over generations reflects enough parents and children or even the same ethnic mixes, to do that on this scale?
I mean, he didn’t just take samples of SATs from Chester A Arthur High of Brooklyn in 1950 and then the same school in 2000 and draw conclusions about heredity? There might not be any heredity.