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Theodorik
14-09-14, 18:41
Twin Studies and IQ http://ecx.images-amazon.com/images/I/41qPjw4CP4L._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA300_SH20_OU01_.jpghttp://www.amazon.com/Entwined-Lives-Twins-About-Behavior/dp/0452280575

Nancy Segal is a Jewish liberal who is a professor of developmental psychology and director of the Twin Studies Center at California State University, Fullerton. She is also an identical twin. On page 322 she writes "Like Dr. Jerome Kagan of Harvard University, a former environmentalist who was 'dragged by the [genetic] data,' I believe many environmentally oriented psychologists and psychiatrists are finding twin research difficult to ignore."


Her book is crammed with facts and scientific studies on the subject of twins. Studies of twins raised apart show that they are very similar in IQ and personality, proving that IQ is mostly genetic.


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Children who are adopted may show a slight increase in IQ in the first few years, but by adulthood, there is no correlation between them and their step-siblings. On the other hand, identical twins raised in separate homes are nearly identical in I.Q.

Page 53: "The same degree of IQ resemblance between identical twins reared apart has been shown across five different studies, conducted between 1937 and 1992 by investigtors in the United States, Great Britain, Denmark and Sweden. This level of consistency is rare in human developmental research, matched only by the finding that identical twins are nearly as alike in IQ as the same person tested twice." This book has hundreds of footnotes referring to books and articles in scientific journals such as Developmental and Behavioral Pediatrics, American Journal of Human Genetics, Behavior Genetics, Annual Review of Neuroscience, Physiology and Behavior, Nature, and Child Development. Articles from over a hundred journals are cited.


Page 50: "Typical IQ correlations are .86 for identical twins reared together, .75 for identical twins reared apart, .60 for fraternal twins, .42 for parents and children and .15 for cousins."

Page 213: IQ is genetic to the same extent as height. "Studies show striking height resemblance in identical twins, relative to fraternal twins, suggesting that genetic factors explain 90% of individual differences." The correlation was the same for identical twins raised apart as for those raised together.






The book mentions some remarkable identical twins, including two football stars, two female professors of cardiology, two identical twins who became presidents of universities, and the Bessie and Jessie twins, who were raised apart.

Page 150: "Bessie and Jessie were studied more thoroughly by geneticist Hermann J. Muller in 1925. He found only a three-point difference in their intelligence test scores (153 and 156) despite the fact that Jessie completed college courses and Bessie had only four years of formal schooling--in fact, Bessie obtained the higher score. Both twins were avid readers, probably compensating for Bessie's sparse education--current resarchers would say that she created an environment compatible with her inherited potentials."


The book also has a chapter on pseudo-twins, unrelated children of the same age who are raised together. There is some similarity in IQ at a young age, but it evaporates later. Page 155: "...studies of adopted children were producing increasing evidence that shared environmental influences associated with modest IQ similarity in childhood essentially evaporated by adolescence, at which time adoptive siblings were no more alike than children raised in different families."
Page 163: Another interesting statistic is that the average IQ difference between unrelated individuals is 17 points, between adopted siblings raised together 15 points, between fraternal twins 10 points, and between identical twins 6 points. When the same person takes IQ tests more than once, scores typically differ by 2-4 points.


UST-SA's are children of the same age adopted and living together.


Page 163: "The most striking result for this study is that IQ scores of same-age un-related siblings are much less similar than scores of identical twins., fraternal twins and full siblings. The UST-SA IQ correlation (measure of association between siblings in each pair was .17, in contrast with correlations of .86 for identical twins, .60 for fraternal twins twins and .50 for full siblings. Remember that shared environment accounts for all the similarity in UST-SA pairs and in this study it explained only 17% of the individual differences. This tells us that shared environment makes a small contribution to the resemblance of people living together, and that genetic factors and nonshared environmental influences account for the remaining 83% of differences among people."


Most people don't know that IQ tests were originally developed to predict academic performance by people who didn't believe that intelligence was genetic. IQ tests predict academic performance very well. This means that academic potential is mostly genetic. Our society is in denial about this reality. People who don't have the genetic potential to learn university courses will not succeed, no matter how much money is spent trying to prepare them.


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Angela
14-09-14, 19:10
I think that denying the genetic component of intelligence is rather...unintelligent. However, it can be overstated.

This statistic struck me:
Page 50: "Typical IQ correlations are .86 for identical twins reared together, .75 for identical twins reared apart, .60 for fraternal twins, .42 for parents and children and .15 for cousins."

This explains a few of my cousins. :)
(I have 24 first cousins on my paternal line, by the way, so I suppose some variation is to be expected. I guess it all comes down to whom you choose to marry.)

LeBrok
15-09-14, 00:26
I remember reading a study and watched nice documentary, pointing to the fact that identical twins, even raised separately, ending up having similar interests, jobs, characters, personalities. It was a real eye opener for me how much genetics shapes our lives.