shown that the morbidity from tuberculosis is largely due to
heredity--a point on which most medical men are still uninformed.
Measurement of the direct correlation between phthisis in parent and
child shows it to be about .5, i. e., what one expects if it is a matter
of heredity. This is the coefficient for most physical and mental
characters: it is the coefficient for such pathological traits as
deafness and insanity, which are obviously due in most cases to
inheritance rather than infection.

But, one objects, this high correlation between parent and child does
not prove inheritance,--it obviously proves infection. The family
relations are so intimate that it is folly to overlook this factor in
the spread of the disease.

Very well, Professor Pearson replied, if the relations between parent
and child are so intimate that they lead to infection, they are
certainly not less intimate between husband and wife, and there ought to
be just as much infection in this relationship as in the former. The
correlation was measured in thousands of cases and was found to lie
around .25, being lowest in the poorer classes and highest in the
well-to-do classes.

At first glance this seems partly to confirm the objection--it looks as
if there must be a considerable amount of tubercular infection between
husband and wife. But when it is found that the resemblance between
husband and wife in the matter of insanity is also .25, the objection
becomes less formidable. Certainly it will hardly be argued that one of
the partners infects the other with this disability.

As a fact, a correlation of .25 between husband and wife, for
tuberculosis, is only partly due to infection. What it does mean is that
like tends to mate with like--called assortative mating. This
coefficient of resemblance between husband and wife in regard to
phthisis is about the same as the correlation of resemblance between
husband and wife for eye color, stature, longevity, general health,
truthfulness, tone of voice, and many other characters. No one will
suppose that life partners "infect" each other in these respects.
Certainly no one will claim that a man deliberately selects a wife on
the basis of resemblance to himself in these points; but he most
certainly does so to some extent unconsciously, as will be described at
greater length in Chapter XI. Assortative mating is a well-established
fact, and there is every reason to believe that much of the resemblance
be