"
Meantime other investigators, principally followers of Karl Pearson in
England, were working out correlation coefficients in other lines of
research for hundreds of different traits. As we show in more detail in
Chapter IV, it was found, no matter what physical or mental trait was
measured, that the coefficient of correlation between parent and child
was a little less than .5 and that the coefficient between brother and
brother, or sister and sister, or brother and sister, was a little more
than .5. On the average of many cases the mean "nature" value, the
coefficient of direct heredity, was placed at .51. This gave another
means of measuring nurture, for it was also possible to measure the
relation between any trait in the child and some factor in the
environment. A specific instance will make this clearer.
Groups of school children usually show an appalling percentage of
short-sightedness. Now suppose it is suggested that this is because they
are allowed to learn to read at too early an age. One can find out the
age at which any given child did learn to read, and work out the
coefficient of correlation between this age and the child's amount of
myopia. If the relation between them is very close--say .
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