THE KALLIKAK FAMILY 621 



a majority of the cases of mental deficiency are traceable 

 to heredity, possibly as large a proportion as two-thirds 

 or three-fourths. The remainder are due to a variety of 

 causes, including injury at birth, diseases or accidents 

 during infancy, and syphilis or alcoholism of parents. 



Social studies have been made of many feeble-minded 

 or otherwise degenerate strains, good examples of which 

 are the Kallikaks, the Nam Family, the Hill Folk, the 

 Pineys, the Jukes and the Ishmaelites. Representatives 

 of such families are to be found in almost any neighbor- 

 hood, and everywhere they make up a large proportion 

 of the ne'er-do-wells, paupers, criminals, and sex- 

 offenders. 



Some investigators, notably Goddard and Davenport, 

 are inclined to believe that mental deficiency is due to a 

 single factor or gene which behaves as a Mendelian re- 

 cessive. This would mean that defective mated with 

 defective would give only defectives in the Fi. Defec- 

 tive mated with normal would give an Fi composed 

 entirely of normal but hybrid individuals. If the latter 

 should mate with others who were hybrid like themselves, 

 the result would be an F2 yielding the familiar 3 : 1 ratio. 

 It is by no means certain, however, that this interpreta- 

 tion of the data is correct. The graded character of 

 mental defect would suggest that probably more than 

 one factor is involved. 



The Kallikak Family. — This family is of special 

 interest by reason of the fact that it contained two 

 branches, a normal and a defective. Both branches 

 trace back to a mentally normal young soldier of the 

 Revolutionary War. During the war this soldier mated 

 temporarily with a feeble-minded woman, giving rise to 

 a feeble-minded strain which exists to this day. From 

 this union 480 individuals have been traced, 143 of whom 

 are known to have been feeble-minded and many others 

 criminal, immoral, or alcoholic. Only 46 are known to 

 have been normal. After the war this same soldier mar- 



