there are, despite the lack of evidence
for hereditary mental differences, practical differences in the
mental activity of different races that are of social importance.
These differences, which seem so fundamental, have been
explained primarily by the powerful control exercised over the
individual by the habits which he acquires even before the
age of five years. These, though unconscious, may be, as the
Freudian psychologists maintain, all the more important for
that reason. This would appear to be the only explanation of
significant racial differences. Cultural differences cannot,
biologists are generally agreed, be transmitted in the germs
that pass from generation to generation. One may say, in
effect, that an individual is differentiated in his mental traits
by early association with a certain race, and by his immediate
ancestry or family, rather than by the fact of belonging
physically to a certain race.

THE INFLUENCE OF IMMEDIATE ANCESTRY OR FAMILY. A factor
that is, on experimental evidence, rated to be of high importance
in the determination of the differences of the mental
make-up of human beings, is "immediate ancestry" or family.
Stated in the most simple and general terms this means
that children of the same parents tend to display marked
likenesses in mental traits, and to exhibit less variation among
themselves than is exhibited in the same number of individuals
chosen at random. A great number of experiments have
been conducted to determine how far resemblances in mental
traits are due to common parentage. The correlation between
membership in the same family and resemblances of
social traits has been found to be uniformly high.

The inference was made that children of the same family
would show great resemblances in mental traits, when accurate
experiments showed marked similarity in physical traits
under the same conditions. The coefficient of correlation
between brothers in the color of the eye, is, according to the
results obtained by Karl Pearson, .52.[1] The coefficient of
fraternal correlation in the case of the cephalic index (ratio
of width to length of head) is .40. The correlation of hair
color is found to be .55. The fact of high correlation between
resemblance of physical traits and membership in the
same family is of crucial importance, because these traits
are clearly due to ancestry, and not to environmental
differences. If physical traits show such a correlation, it is likely
t