(5) "They were never alike either in mind or body, and their dissimilarity increases daily. The external influences have been identical; they have never been separated."
(6) "The two sisters are very different in ability and disposition. The one is retiring, but firm and determined; she has no taste for music or drawing. The other is of an active, excitable temperament; she displays an unusual amount of quickness and talent, and is passionately fond of music and drawing. From infancy, they have been rarely separated even at school, and as children visiting their friends, they always went together."
And so on. Not a single case was found in which originally dissimilar characters became assimilated, although submitted to exactly the same influences. Reviewing the evidence in his usual cautious way, Galton declared, "There is no escape from the conclusion that nature prevails enormously over nurture, when the differences of nurture do not exceed what is commonly to be found among persons of the same rank in society and in the same country."
This kind of evidence was a good start for eugenics but as the science grew, it outgrew such evidence. It no longer wanted to be told, no matter how minute the details, that "nature prevails enormously over nurture." It wanted to know exactly how much. It refused to be satisfied with the statement that a certain quantity was large; it demanded that it be measured or weighed. So Galton, Karl Pearson and other mathematicians devised means of doing this, and then Professor Edward L. Thorndike of Columbia University took up Galton's problem again, with more refined methods.
The tool used by Professor Thorndike was the coefficient of correlation, which shows the amount of resemblance or association between any two things that are capable of measurement, and is expressed in the form of a decimal fraction somewhere between 0 and the unit 1. Zero shows that there is no constant resemblance at all between the two things concerned,—that they are wholly independent of each other, while 1 shows that they are completely dependent on each other, a condition that rarely exists, of course.[4] For instance, the correlation between the right and left femur in man's legs is .98.
Professor Thorndike found in the New York City schools fifty pairs of twins of about the same age and measured the closeness of their resemblance in eight physical characters, and also in six mental characters, the latter being measured by the proficiency with which the subjects performed various tests. Then children of the same age and sex, picked at random from the same schools, were measured in the same way. It was thus possible to tell how much more alike twins were than ordinary children in the same environment.[5]
THE EFFECT OF NURTURE IN CHANGING NATURE