"
Here again Professor Thorndike's exact measurements explode the fallacy.
They are actually, measurably, less alike at the older age; their inborn
natures are developing along predestined lines, with little regard to
the identity of their surroundings. Heredity accounts easily for these
facts, but they cannot be squared with the idea that mental differences
are the products solely of early training.
5. _Differential rates of increase in qualities subject to much
training._ If the mind is formed by training, then brothers ought to be
more alike in qualities which have been subject to little or no
training. Professor Thorndike's measurements on this point show the
reverse to be true. The likeness of various traits is determined by
heredity, and brothers may be more unlike in traits which have been
subjected to a large and equal amount of training. Twins were found to
be less alike in their ability at addition and multiplication, in which
the schools had been training them for some years, than they were in
ability to mark off the A's on a printed sheet, or to write the
opposites to a list of words--feats which they had probably never before
tried to do.
This same proposition may be put on a broader basis.
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