Many of the world's most brilliant musicians, mathematicians and even
military leaders have been men of one-sided mental development, whose
ability in other lines was so slight that they were little better than
imbeciles, and it is not at all surprising that their children are
sometimes truly idiotic.

[Footnote 63: Barr, op. cit., p. 301 _et seq._]

The best writers of the present day no longer recognize consanguinity
as a cause _per se_, of idiocy. The heredity of neuroses, however, is
so strongly established that few would dispute the proposition that
where the morbidity is inherited through both parents it appears more
frequently and in a more marked degree than where one parent is
entirely free from taint. This is what occurs when a consanguineous
marriage takes place between descendants of a neurotic family. The
percentage of idiotic children would then be somewhat higher from
consanguineous marriages than from the average marriage purely through
the action of the laws of heredity.

Dr. Barr finds 49 out of 4050 cases of idiocy or 1.21 per cent, in
which there was a family history of consanguinity. This is little
higher than the average frequency of first cousin marriage, and an
analysis of 41 of these cases does not show one case that can be
attributed to consanguinity alone. To quote: "Two were the result of
incestuous connection--one of brother and sister, the other of father
and daughter, and in the others there was an undoubted history, of
grave neuroses."[64] "Beach and Shuttleworth find in the consideration
of their 100 cases (out of 2,380 idiots), giving 4.2 per cent (of
consanguineous parentage) that the bad effects are due rather to the
intensification of bad heredity common to both parents."[65]