HUMAN HEREDITY AS REVEALED BY PEDIGREES 



461 



Statistics seem to indicate that this defect is on the increase; certainly 

 it is far too common to be ignored. Records show 178,000 mental de- 

 fectives in Great Britain and Ireland in 1921, and these include only 

 the very positive cases. It has been estimated by one expert that in 

 the United States one person in every 294 is feeble-minded; by another 

 expert, one in every 138. In Indiana it is claimed that over 2 per cent 

 of the population of ten unselected counties are mental defectives. 

 If the incidence is the same in the whole state, there are 56,000 defec- 

 tives in Indiana alone, and Indiana is in no way exceptional. Calcula- 

 tions indicate that in the United States as a whole there are not less 

 than half a million feeble-minded individuals, and several times that 

 many individuals phenotypically normal but carrying the gene or 

 genes for feeble-mindedness. A large proportion of these individuals 

 are charges of the various states and cost the public many millions of 

 dollars annually without contributing anything of value to the com- 

 munity. 



Insanity and heredity. — -A great deal of careful work has been done 

 on the heredity of insanity. For example, David Heron in Scotland 

 has studied 331 family histories exhibiting insanity. There are sev- 

 eral distinct and different conditions included under the blanket term 

 "insanity." It is obvious that they could not all be determined by 

 the same gene or genes. Heron's data on insanity are summed up in 

 the following table: 



HERON'S DATA ON THE HEREDITY 

 OF INSANITY 



These figures are not at all readily fitted into any simple Mendelian 

 ratio. Doubtless the various insanities are not due to single Mendelian 

 units, but are complex cases. That they are to some extent and in 

 some manner hereditary, seems beyond question. 



RosanofJ and Orr, two prominent American psychiatrists, seem 

 somewhat more sanguine about fitting the insanities into the Mendel- 

 ian system. Their data are given here in tabular form: 



