Twins: And What They Tell Us About Who We Are by Lawrence Wright

intelligence by quite a margin. Indeed, the Pittsburgh team calculated that the

 

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broad heritability of intelligence was about 0.48giving a slight majority to the influence of the environment, including the prenatal environment. Such a figure would be far too low to support Jensen's argument, reiterated and expanded by Herrnstein and Murray in The Bell Curve , that the intermarriage of highly educated, intelligent people will lead inevitably to separate castes based on IQ scores. It is also considerably lower than the 0.66 figure that Bouchard cites as a consensus for all twin studies. "Overall, the study results have two implications," says Devlin. "A new model may be required regarding the influence of genes and environment on cognitive function, and interventions aimed at improving the prenatal environment could lead to a significant increase in the population's IQ."
Much of Devlin's information is arrived at by comparing fraternal twins with ordinary siblings. Despite the fact that on average fraternal twins are no more genetically alike than ordinary brothers and sisters, each having about half their genes in common, fraternal twins show much more similarity in intelligence. That increased similarity, says Devlin, is the maternal effectthe experience of sharing the same womb serves to make twins more alike than ordinary siblings. However, the experience of sharing the womb also tends to make twins different from each other. They compete for space and nutrition and even blood, which can create striking dissimilaritiesso much so that many twin researchers believe that their data on heritability actually underestimates the heritability for the population as a whole.
Bouchard disputes Devlin's analysis on several grounds. For one, he points to studies demonstrating that fraternal twins actually grow to be less alike in their intelligence as they agemore like ordinary brothers and sistersso that whatever effect Devlin is demonstrating disappears

 

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as the twins become adults. "If they were just drawing conclusions about the sample they worked with, which is heavily children, I'd say, 'Well, who knows?' But I know there's information about older adults, I know that these common family environmental influences disappear and that the postulate of maternal effects runs counter to all the work done by people who've studied prenatal effects. And so I just don't believe that this is the best model. They may be right in the long run. I just don't think so." The diminishing effect of prenatal experience is a quandary that the environmentalists acknowledge but have been unable to solve.
A more sinister charge is that the separated twins are making up stories in order to get into the press. "I don't blame the twins," says Kamin. "There's enormous implied pressure on them to exaggerate the degree of their separation. Nobody would be interested in them, they would not appear in the newspapers, they would not appear on TV shows and so on, if they said, 'Yeah, we saw quite a bit of each other and we went to the same schools.' If they convince other people and themselves that they saw very little of one another, then they're going to be valuable scientific resources and people will beat a path to their door. I think it is beyond cavil that these twins tend to gild the lily." Kamin points out that there is an economic interest as well: several of the separated twins, including the Jims and Jack and Oskar, have signed movie or book deals. Kamin claimed to have spoken to Jim Springer's mother, who he says admitted to him that the twins had met repeatedly in Florida when they were young but kept it a secret from Springer's father. "Now that appears, of course, in all the apocryphal literature about this as some mysterious things in the genes led the two of them to go each year

 

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to Saint Petersburg. I know she spoke to me about the fact that she had to keep these meetings secret from her husband." Kamin said he had