Twin Data on the Genetics of Ageing 137 



pathological conditions as coronary artery disease, essential 

 hypertension and cerebral arteriosclerosis (Friedman, Rosen- 

 man and Byers, 1955; Wilkinson, 1950). Some investigators 

 regard dominant genotypes as the cause not only of a specific 

 predisposition to cerebral arteriosclerosis, but also of its 

 particular localization and frequent combination with nephro- 

 sclerosis (Sobye, 1948). However, despite the present evidence 

 for a gene-specific metabolic error being the basic cause of 

 hypertensive disease, many of the aetiological aspects of this 

 serious affliction of the ageing are still unsolved. 



Equally doubtful is the theory that the total group of senile 

 dementias may be due to the effect of a dominant genotype of 

 low penetrance (Cresseri, 1948; Meggendorfer, 1939). Either 

 one or two dominant genes have been assumed to be involved, 

 one controlling longevity and one producing the pathological 

 changes associated with senile dementia. According to the 

 results of our study of ageing twin family units, senile psy- 

 choses seem more adequately explained by an age-specific 

 intensification of long-existing but minor deficiencies in 

 general emotional adjustment than by a single genetic factor 

 causing a specific type of psychopathology. This theory im- 

 plies that the genetic components in the aetiology of a senile 

 psychosis consist of polygenically determined variations in 

 age-susceptible personality traits, a generally reduced level 

 of adaptive plasticity, and those gene-specific biochemical 

 phenomena controlling growth and decline. 



Graded differences in general ageing and longevity poten- 

 tials have been demonstrated by family statistics (Dublin, 

 Lotka and Spiegelman, 1953; Gianferrari, 1954; Jalavisto, 

 1951; Pearl and Pearl, 1934) as well as by twin studies (Kall- 

 mann, Feingold and Bondy, 1951; Kallmann and Sander, 

 1948; von Verschuer, 1954) and are most certainly polygenic 

 in origin. Longitudinal twin data show that compared to 

 the limited degrees of similarity between two-egg twins 

 or ordinary sibs, all measurable similarities between ageing 

 one-egg twins are consistently more pronounced, frequently in 

 spite of very different environments. The observed similarities 



