Publication: Daedalus
Publication Date: 22-MAR-07
Delivery: Immediate Online Access
Author: Ehrlich, Paul ; Feldman, Marcus W.
Article Excerpt
Our large brains are surely at the center of our humanity. But it is equally certain that few organs are the subject of more misinformation in scientific and public discourse--especially in the widespread notion that most behaviors controlled by our marvelous brain are somehow programmed into it genetically. A typical treatment in the popular press is this overexcited claim by columnist Nicholas Wade in the New York Times: "When ... [the human genome] ... is fully translated, it will prove the ultimate thriller--the indisputable guide to the graces and horrors of human nature, the creations and cruelties of the human mind, the unbearable light and darkness of being." (1)
Wade may get a pass for being a journalist, but some scientists are equally confused. Molecular biologist Dean Hamer wrote: "People are different because they have different genes that created different brains that formed different personalities," and "[u]nderstanding the genetic roots of personality will help you 'find yourself and relate better to others." As distinguished a neurobiologist as Michael Gazzaniga is guilty of the misleading claim that "all behavioral traits are heritable"; (2) and molecular evolutionists Roderick Page and Edward Holmes have asserted that "genes control 62% of our cognitive ability." (3) In fact, an entire neo-field labeled evolutionary psychology has sprung up based on the misconception that genes are somehow determining our everyday behavior and our personalities. It is a field that believes there are genetic evolutionary answers to such questions as why a man driving an expensive car is more attractive than one driving a cheap car. (4)
So even well-educated and thoughtful observers have been persuaded by the language of heritability. With expressions such as 'genes are responsible for 50 percent of,' or 'genes contribute 50 percent of,' a behavior, this language gives the impression that genetic and environmental contributions to human behaviors are actually separable. They are not.
Heritability was originally introduced in the 1930s in the context of agriculture. It is an index of amenability to selective breeding under environmental conditions that the breeder could control. This index, now often termed 'narrow-sense heritability,' is the fraction of all variation in a trait that can be ascribed only to genes that act independently of one another and whose joint effect is the sum of their individual effects. One easy-to-understand way of measuring heritability is through a one-generation selection experiment. Individuals with extreme values of a trait are bred to one another--for example, the heaviest individuals from a hog population. The offspring are then raised in the same environment, and their average weight calculated. If the average weight of the offspring doesn't increase over that of the entire population (not just of the heavy parents) in the previous generation, the heritability is zero. On the other hand, if the average weight of the offspring equals that of their heavy parents, the heritability is 100 percent.
In the 1960s, the term 'heritability' was adopted by some students of human behaviors who wanted to know what fraction of the variation in these behaviors was primarily attributable to genetic differences and what percentage to environmental differences. Because controlling the environments of human subjects is not possible, however, this fraction--now called 'broad-sense heritability'--includes variation from interactions between genes and environments. That fraction of variation is nevertheless interpreted as determined by genes, thus inflating the heritability.
In other words, this new heritability statistic assumes no relationship between genetic transmission and environment, e.g., that the IQ scores of parents cannot affect those parts of the environment that might interact with genes to influence a child's IQ. The amount of stimulation parents provide their young children, the nature of dinner-table conversations, and the number of books in the home are thus taken to be independent of any genetic influences on children's IQ. When this independence assumption is violated, there is gene-environment correlation--exactly the correlation that...
NOTE: All illustrations and photos have been removed from this article.
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