460 
of their surroundings. Heredity ac- 
counts easily for these facts, but they 
cannot be squared with the idea that 
mental differences are the products 
solely of early training. 
THE EFFECT OF 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 
much training, than they are in qualities 
which have been subject to little or no 
training. Thorndike’s measurements on 
this point show the reverse to be true. 
The likeness of various traits is deter- 
mined by heredity, and they may be 
most 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 their 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.'° ‘In so far as the 
differences in achievement found 
amongst a group of men are due to the 
differences in the quantity and quality 
of training which they had had in the 
function in question, the provision of 
equal amounts of the same sort of 
training for all individuals in the group 
should act to reduce the differences.” 
“If the addition of equal amounts of 
practice does not reduce the differences 
found amongst men, those differences 
cannot well be explained to any large 
extent by supposing them to have been 
due to corresponding differences in 
amount of previous practice. If, that 
is, inequalities in achievement are not 
reduced by equalizing practice, they 
cannot well have been caused by in- 
equalities in previous practice. If diff- 
erences in opportunity cause the differ- 
ences men display, making opportunity 
more nearly equal for all by adding 
The Journal of Heredity 
equal amounts to it in each case should 
make the differences less. 
‘The facts found are rather startling. 
Equalizing practice seems to increase 
differences. The superior man seems 
to have got his present superiority by 
his own nature rather than by superior 
advantages of the past, since, during a 
period of equal advantages for all, he 
increases his lead.” This point has 
been tested by such simple devices as 
mental multiplication, addition, marking 
A’s on a printed sheet of capitals and 
the like; all the contestants made some 
gain in efficiency, but those who were 
superior at the start were propor- 
tionately farther ahead than ever at the 
end. This is what the geneticist would 
expect, but fits very ill with the 
popular psychology which denies that 
any child is mentally limited by nature. 
MEASURING RESEMBLANCE 
6. Direct measurement of the amount 
of resemblance of mental traits in 
brothers and sisters shows that it is 
on the average equal to that of physical 
traits. It is manifestly impossible to 
assume that early training, or parental 
behavior, or anything of the sort, can 
have influenced very markedly the 
child’s eye color, or the length of his 
forearm, or the ratio of the length of 
his head to its breadth. If we measure 
the amount of resemblance between 
two brothers in such traits, we may say 
very confidently that our measurement 
represents the influence of heredity; 
that the child inherits his eye color and 
other physical traits of that kind from 
his parents. The resemblance, meas- 
ured on a scale from 0 to 1, has been 
found to be about 0.5. 
Pearson measured the resemblance 
between brothers and sisters in mental 
traits—for example temper, conscien- 
tiousness, introspection, vivacity—and 
found it on the average to have just 
the same intensity—that is, about 0.5. 
Further measurements of this sort with 
other traits are needed; but if future 
investigations confirm Pearson’s finding 
that the resemblance between brothers 
'°The quotations in this and the following paragraph are from Thorndike’s Educational 
Psychology, pp. 304-305, Vol. III. 
