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49
The percentage of defectives in the group covered by the inquiry was 10-5. In the rural
areas investigated by Dr. E. O. Lewis it was 3-4, and in the urban areas 1 -9.
How far the environmental conditions have an influence on the mentality is doubtful. The
effect after infancy is apparently slight, for the children in residential schools are not markedly
better mentally than those of their original environment, and the influence on the earlier life
cannot be estimated.
It has sometimes been suggested that the mentality of the children is more affected by the
mother than the father. The findings for 600 children were:—
Supernormal.
Normal.
Retarded.
Mentally
defective.
Unstable.
Father defective
(111 children)
1.8 64.9 28.8 4.5 (8.0)
Mother defective
(489 children)
2.3 60.9 25.2 11.5 (3.7)
There was a larger proportion of mentally defective children where the mother was defectiveThis
fact might suggest that the very earliest environment, including the pre-natal life, is important,
but there are many factors that complicate the issue. The percentage of retarded children
aDDears to be ereater in the cases where the father is defective.

Noteworthy features are:—
(а) The good mentality of the children one of whose parents was mentally defective and the
other apparently normal.
(b) The markedly worse mentality when the other parent was dull or unstable.
(c) The large proportion of defectives born to unmarried defective mothers. It is likely
that many of the fathers in this group were unstable, but their mentality is, of course, not
ascertainable.
(d) The very small numbers in class IV, but in each of four unions of two mentally defective
parents there were some normal children.
If the Mendelian law were applicable to mental deficiency as a unit recessive character this
would be impossible. All the parents in class IV, except one, were simple primary aments, the
exception had had encephalitis lethargica, but was probably previously defective.
The relatively low percentage of defectives and the good general level of intelligence in the
offspring of the union of a defective with a normal parent support the view that it is the in-breeding
within sub-normal population that is so harmful.
There has been a suggestion that dullness is an intermediate type of inheritance. This fact
is not specifically shown in the findings of this inquiry, but in another group of 200 defectives the
ancestry was, as far as could be discovered, as follows:—
Mental condition of
ancestors of defective.
Fathers,
(per cent.)
Mothers,
(per cent.)
Grandparents,
(per cent.)
Mentally defective .0 0.5 .0
Dull 4.5 6.0 1
Nervous and unstable 7.5 14.0 1.5
Psychotic 5.0 1.0 1.6
Epileptic 0.5 1.6 .1
Asthmatic 1.5 3.0 1.8
Phthisical 7.5 4.0 1.6
Normal 73.5 70.0 93.3
The knowledge of the ancestry in this group is certainly incomplete ; probably there were
really much fewer normal and many more defective progenitors. As the Mental Deficiency Act
did not operate until 1913, most of the mentally defective would pass unnoticed. It is not unlikely
that many of those called dull by their relatives were really mentally defective. The interesting
feature of this table is the high percentage of the various kinds of mental instability and
tuberculosis.