138 Franz J. Kallmann 



extend to physical and mental signs of ageing, social ad- 

 justment, intellectual performance, and its rate of decline. 

 The comparative histories of two typical twin pairs may help 

 to illustrate this point. 



The twin brothers in Fig. 8 belong in the series of two-egg 

 pairs with very similar life conditions. Reared on a farm, 

 they attended a rural school together, and then became 

 prosperous farmers in the same district. When they were in 

 their early thirties, each married a local girl and had one 

 daughter. However, despite the similarity of their lives, they 

 developed rather disparate personalities and patterns of 

 ageing. 



The younger-looking twin on the left, described as prankish, 

 excitable and somewhat extravagant, complained of ill- 

 health for thirty years before retiring at age 70. The other 

 twin, who still runs his farm on his own, has always been 

 placid, frugal and self-sufficient. He takes pride in never 

 having seen a doctor, except for an appendectomy. 



As to general intelligence, this twin has maintained a two- 

 point advantage over his brother, with vocabulary scores of 

 23 and 22 at the ages of 71 and 78. But in this period of 

 seven years, he has deteriorated from a score of 63 to 30 on 

 the tapping test, and from 25 to 17 on the digit symbol test. 

 The corresponding decline in the twin with the slower ageing 

 patterns has been only from 78 to 67, and from 27 to 21, 

 respectively. 



The one-egg twins in Fig. 9, on the other hand, are still 

 remarkably alike, both physically and mentally, although 

 their fortunes have differed considerably. Both were employed 

 in factories, married at a young age, and fathered six children 

 each. At 78, one is maintaining a stable home life in a subur- 

 ban community where he is still active as a cemetery worker. 

 The other neither works any longer, nor does he have a home 

 of his own. For forty years he has been separated from his 

 wife, for whose sake he had changed his religion. 



Nevertheless, both twins are in good physical health and 

 blissfully unaware of their equally advanced state of senile 



