Again (2) if similarity in training is the cause of
similarity in mental traits, ordinary fraternal pairs not over four or
five years apart in age should show a resemblance somewhat nearly as
great as twin pairs, for the home and school condition of a pair of the
former will not be much less similar than those of a pair of the latter.
Again, (3) if training is the cause, twins should show greater
resemblance in the case of traits much subject to training, such as
ability in addition or multiplication, than in traits less subject to
training, such as quickness in marking off the A's on a sheet of printed
capitals, or in writing the opposites of words."
The data were elaborately analyzed from many points of view. They showed
(1) that the twins 12-14 years old were not any more alike than the
twins 9-11 years old, although they ought to have been, if environment
has great power to mold the character during these so-called "plastic
years of childhood." They showed (2) that the resemblance between twins
was two or three times as great as between ordinary children of the same
age and sex, brought up under similar environment. There seems to be no
reason, except heredity, why twins should be more alike. The data showed
(3) that the twins were no more alike in traits subject to much training
than in traits subject to little or no training.
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