Twin Data on the Genetics of Ageing 



139 



deterioration. In the last seven years, their digit symbol 

 scores have declined at a similar rate, from 13 to 2 and from 

 11 to 1. Interestingly enough, even the mistakes they make in 

 substituting symbols for digits are precisely the same. 



These two sets of aged twins are among the 72 paired 

 survivors of a series of 120 same-sexed pairs, tested psycho- 

 metrically in 1947 and, where possible, retested with the 



60-. 

 50-- 



40 

 30 



■ MONOZYGOTIC 

 D DIZYGOTIC 



u I 



1947 



■ ill I 



VOCABULARY 



DIGIT 

 SYMBOL 



BLOCK 

 DESIGN 



SIMILARITIES 



h 



DIGIT 

 SPAN 



Fig. 10. Comparative mean intra-pair differences in test scores (1947 and 1955). 



same battery in 1955. It is indicated by the results of this 

 longitudinal study, summarized diagrammatically in Fig. 10, 

 that the mean intra-pair differences in the test scores of two- 

 egg twins consistently exceed those of one-egg twin partners. 

 The detailed test data will be presented at the International 

 Congress of Human Genetics in Copenhagen, as long-needed 

 evidence for the essential part played by genetic factors in 

 intellectual performance until the period of senescent decline 

 (Feingold-Jarvik, Kallmann, Falek and Klaber). 



