The study of twins / Peter Mittler.

  • Mittler, Peter J.
Date:
[1971]
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The study of twins / Peter Mittler. Source: Wellcome Collection.

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    86 Cognitive Processes mental differences between families which is 54 per cent, and differences within families of 6 per cent. But it should also be noted that the heritability estimates for scholastic achievement vary over a much wider range than do H values for intelligence. In general, H for scholastic achievement increases as we go from the primary grades up to high school and it is somewhat lower for relatively simple forms of learning (e.g. spelling and arith¬ metic computation) than for more complex learning (e.g. reading comprehension and arithmetic problem solving). Yet large scale twin data from the National Merit Scholarship Corporation show that the between families environmental component accounts for about 60 per cent of the variance in students' rank in their high school graduating class. This must mean that there are strong family influences which cause children to conform to some academic standard set by the family and which reduce variance in scholastic performance among siblings reared in the same family ... this means that there is much more that we can do to improve school performance through environmental means than we can do to change intelligence per se (Jensen, 1969, pp. 58-9) [italics added]. Specific mental abilities The distinctions made above betv^^een results of intelligence and attainment tests is an important one which has not in general been at all widely exploited in twin studies. Thus, most researchers have used general 'intelligence tests', yield¬ ing global IQs, which have then been used to compute correlations. We can now turn to the small number of studies which have attempted to examine specific cognitive functions from a genetic point of view. This is in line with current interest in isolating specific cognitive skills, and reflects a revival of interest in underlying constituent pro¬ cesses. Probably the first twin study to attempt this kind of analysis was that of Blewett (1954) on London school¬ children. He administered the Primary Abilities Test and the Nufferno Tests of Intellectual Level and Speed to twenty-six MZ and twenty-six DZ pairs, equally divided by sex. Blewett predicted that the second order factor