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134
The number of siblings of a defective who are also affected is a question of some
importance from the stand point of mental deficiency as a family problem. In 1912
Dr. Shrubsall analysing the records of 2,000 children who had been in attendance at
special schools for the mentally defective found that in 1,741 cases the child at school
was the only defective member and that in 259 cases another member or members
of the sibship were also mentally defective giving an incidence of 13 per cent. Dr.
Shrubsall has this year analysed the records of 1,360 cases dealt with under the
Mental Deficiency Act in which fuller histories are available as a result of home
inquiries. In this series it was found that in 1,231 only the one member of the sibship
was defective, and in 129 one or more others were also mentally defective,
an incidence of approximately 9.5 per cent., the members affected being as
follows:—
No. of brothers and sisters
of defective who were
also defective.
1928 and 1929.
1912.
Idiot.
Imbecile.
Feebleminded.
Moral
defective.
Special school
cases.
None 44 410 722 5 1,741
One 2 18 66 — 137
Two — 4 27 — 87
Three — 2 7 — 21
Four — 1 1 — 6
Five — — 1 — 6
Six — — — — 1
Seven — — — — —
Eight — — — — 1
In the cases analysed this year it appeared that all but seven of the instances
in which more members than one of the sibship were mentally defective the type of
defect found was simple primary amentia, often of the sub-cultural variety. The
frequency with which other members of the sibship were affected varied inversely
with the grade of defect, the lower the mental status, the fewer the others were
affected; the percentages of those in whom there were defective brothers or sisters
being:—Idiots 4.3, imbeciles 5.7, feeble-minded 11.6. It was also noted that
in 12 instances brothers or sisters of defectives had won scholarships, of these
defectives 7 were instances of primary amentia, 2 of cerebral paralysis and 3 of
mongolism. In one instance 5 brothers and sisters of a defective individual had
won scholarships and in another case two brothers had so done, both of these defectives
were examples of simple primary amentia. The general effect of these analyses
is to show that second instances of mental deficiency in a sibship are to be expected
in less than 15 per cent. of the cases.