The following paragraph is taken from Scientific American:



Before you ask your financial advisor for a saliva sample, though, bear in mind that only about 20 percent of the difference in risk-taking is due to genes, says study co-author Camelia Kuhnen, an assistant professor of finance at Northwestern’s Kellogg School of Management. “I wouldn’t want to oversell this as a screening device to find good traders,” Kuhnen tells ScientificAmerican.com. “Even if I have a gene that predisposes me to taking a lot of financial risk, I could go through a stock market crash that will make me less risk-taking.”



The 20-percent-claim is taken from a 2009 study published in the Quarterly Journal of Economics in which the classical twin design was used to estimate “heritability.” Unfortunately, heritability has nothing to do with the genetic contribution of genes to a trait of interest, but that doesn’t prevent psychologists, neuroeconomists, and other ignoramuses from using “heritability” as a measure of genetic influence.

There are many models for deriving heritability values. In one of the simplest methods, heritability (h) is estimated as



For example, the coefficient of relatedness between monozygotic twins is 1. Let us now compare the correlation coefficient in IQ scores in a sample of twin pairs. If the correlation is 0.7, then h = 0.84. A news article may then report that close to 85 percent of human intelligence is genetic. This statement will be wrong on at least two counts. First, the relation of a test score such as the IQ to what we intuitively understand as intelligence is unknown. Second, although the term “heritability” sounds like something that has to do with genes, it doesn’t. As pointed out by Moore and Shenk (2017), contrary to popular belief, the heritability of a trait does not tell us how “genetically inheritable” the trait is. Further, it does not inform us about what causes a trait, the relative influence of genes in the development of a trait, or the relative influence of the environment in the development of a trait.

“Heritability” is especially pernicious when used to compare the mean values of a trait between two populations.



It all started in 1969 with an article by Arthur Jensen entitled “How much can we boost IQ and scholastic achievement? In it, Jensen presented five arguments:

(1) Races are real genetic entities.



(2) Black people perform, on the average, more poorly than whites on standard IQ tests and their scholastic achievements, on average, are below those of whites.

(3) Special programs of compensatory education have not had much success in reducing this difference.

(4) The reason for the failure of compensatory educational programs is that IQ and scholarly achievements are highly heritable, with most of the variation among individuals arising from genetic rather than environmental sources.

(5) Remedial educational intervention programs are useless, and the best thing that can be done for black children is to capitalize on those skills for which they are biologically adapted.

Richard Lewontin recalls that Jensen’s article first came to his attention when, “at no little expense, it was sent to every member of the National Academy of Sciences by the eminent white Anglo-Saxon inventor, William Shockley*, as part of his continuing campaign to have the Academy study the effects of inter-racial mating.”

In a paper published in 1970 in the Bulletin of the Atomic Scientists, Lewontin exposed the methodological fallacies in Jensen arguments, particularly in the use of heritability to compare different populations.

Lewontin, first dissects the concept of heritability.

We cannot speak of a trait being molded by heredity, as opposed to environment. Every character of an organism is the result of a unique interaction between the inherited genetic information and the sequence of environments through which the organism has passed during its development. For some traits the variations in environment have little effect, so that once the genotype is known, the eventual form of the organism is pretty well specified. For other traits, specification of the genetic makeup may be a very poor predictor of the eventual phenotype because even the smallest environmental effects may affect the trait greatly.

The heritability of a [phenotypic] measurement is defined as the ratio of the variance due to the differences between the genotypes to the total variance in the population. If this heritability were 1.0, it would mean that all the variation in the population resulted from differences between genotypes but that there was no environmentally caused variation around each genotype mean. On the other hand, a heritability of 0.0 would mean that there was no genetic variation because all individuals were effectively identical in their genes, and that all the variation in the population arose from environmental differences

in the development of the different individuals.

Defined in this way, heritability is not a concept that can be applied to a trait in general, but only to a trait in a particular population, in a particular set of environments.

In other words, heritability tells you nothing about the genetic basis of eye color, height, IQ, shoe size, or scholarly achievements. It can only tell you something about a a particular population, at a particular time, under a particular set of environments. As a measure, heritably is useless in comparative studies. Nor is a heritability value determined for one population applicable to another population.

The fundamental error of Jensen’s argument is to confuse heritability of a character within a population with heritability of the difference between two populations. Indeed, between two populations, the concept of heritability of their difference is meaningless. This is because a variance based upon two measurements has only one degree of freedom and so cannot be partitioned into genetic and environmental components.

To illustrate Jensen’s main error, Lewontin came up with an example involving plants. Many of us are all familiar with a version of the following illustration.

A genetically variable population of seeds is divided into two. The two halves are planted and grown under different controlled conditions. Half of the seeds (left panel) are grown in a completely uniform environment supplanted with a uniformly rich nutrient solution. The other half (right panel) are grown in a completely uniform environment supplanted with a uniformly deficient nutrient solution. The plants on the left attain different heights, but because all the plants have grown under a painstakingly uniform environment, the variation is entirely genetic. The plants on the right also attain different heights, and again because all the plants have grown under a painstakingly uniform environment, the variation is again entirely genetic. Because of their enriched nutrient solution, the plants on the left attain a greater height on average than those on the right, but because the seeds were taken from the same population, the difference in mean height between the plants on the left and those on the right are entirely attributable to environmental factors.

This is all I knew from reading secondary sources describing Lewontin’s article. Reading his article, as I did recently, I discovered that his descriptions of the thought experiments are much more elaborate and contain many more controls.

Let us take two completely inbred lines of corn. Because they are completely inbred by self-fertilization, there is no genetic variation in either line, but the two lines will be genetically different from each other. Let us now plant seeds of these two inbred lines in flower pots with ordinary potting soil, one seed of each line to a pot. After they have germinated and grown for a few weeks we will measure the height of each plant. We will discover variation in height from plant to plant. Because each line is completely inbred, the variation in height within lines must be entirely environmental, a result of variation in potting conditions from pot to pot. Then the heritability of plant height in both lines is 0.0. But there will be an average difference in plant height between lines that arises entirely from the fact that the two lines are genetically different. Thus the difference between lines is entirely genetical even though the heritability of height is 0! Now let us do the opposite experiment. We will take two handsful from a sack containing seed of an open-pollinated variety of corn. Such a variety has lots of genetic variation in it. Instead of using potting soil, however, we will grow the seed in vermiculite watered with a carefully made up nutrient, Knop’s solution, used by plant physiologists for controlled growth experiments. One batch of seed will be grown on complete Knop’s solution, but the other will have the concentration of nitrates cut in half and, in addition, we will leave out the minute trace of zinc salt that is part of the necessary trace elements (30 parts per billion). After several weeks we will measure the plants. Now we will find variation within seed lots which is entirely genetical since no environmental variation within lots was allowed. Thus heritability will be 1.0. However, there will be a radical difference between seed lots which is ascribable entirely to the difference in nutrient levels. Thus, we have a case where heritability within populations is 1.0, yet the difference between populations is entirely environmental! But let us carry our experiment to the end. Suppose we do not know about the difference in the nutrient solutions because it was really the carelessness of our assistant that was involved. We call in a friend who is a very careful chemist and ask him to look into the matter for us. He analyzes the nutrient solutions and discovers the obvious-only half as much nitrates in the case of the stunted plants. So we add the missing nitrates and do the experiment again. This time our second batch of plants will grow a little larger but not much, and we will conclude that the difference between the lots is genetic since equalizing the large difference in nitrate level had so little effect. But, of course, we would be wrong for it is the missing trace of zinc that is the real culprit. Finally, it should be pointed out that it took many years before the importance of minute trace elements in plant physiology was worked out because ordinary laboratory glassware will leach out enough of many trace elements to let plants grow normally. Should educational psychologists study plant physiology?

Given the many misuses and abuses of the word “heritability,” I wish more than ever that we find another name for this variable. How about “gesplatz”?

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*To those unfamiliar with the name William Shockley, let me just say that he was a physicist and inventor, who shared the Nobel Prize for research on semiconductors and transistors. As a result of commercializing his work and founding California’s “Silicon Valley,” Shockley became a very rich man. In the last 25 years of his life, Shockley became interested in the relationships among race, intelligence, and genetics. Like many eugenicists before and after, he argued that a higher rate of reproduction among the less intelligent was having a dysgenic effect, and that a drop in average intelligence would ultimately lead to a decline in civilization. His solution was “voluntary sterilization” for those with IQ scores below 100. He outlined his eugenic views in a 1971 paper in Review of Educational Research entitled “Negro IQ deficit: Failure of a ‘malicious coincidence’ model warrants new research proposals,” and later in a 1992 book entitled Shockley on Eugenics and Race: The Application of Science to the Solution of Human Problems. In 1981 he filed a libel suit against the Atlanta Constitution after its science writer compared Shockley’s advocacy of sterilization to Nazi experiments. After three years of litigation, Shockley won, but received only one dollar in actual damages and no punitive damages. Shockley’s biographer wrote that the Atlanta Constitution article was indeed defamatory, but that Shockley’s reputation at the time was not worth more than one dollar.