differences in their ancestry. All attempts to change human nature must accept as their most important condition the limits set by original nature to each individual."
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 .7 or .8—it will be evident that the earlier a child learns to read, the more short-sighted he is as he grows older. This will not prove a relation of cause and effect, but it will at least create a great suspicion. If on the contrary the correlation is very slight, it will be evident that early reading has little to do with the prevalance of defective vision among school children. If investigators similarly work out all the other correlations that can be suggested, finding whether there is any regular relation between myopia and overcrowding, long hours of study, general economic conditions at home, general physical or moral conditions of parents, the time the child spends out of doors, etc., and if no important relation is found between these various factors and