24 



Eeva Jalavisto 



of the decrease in mean length of Hfe is due to inconsistency 

 in the distribution of young and old mothers. Whether other 

 factors were operative, possibly in the reverse direction, 

 during the earlier centuries is difficult to assess. It seems 

 essential, therefore, to avoid calculation of the entire expecta- 

 tion of life and to search for some other method of study. 



Maternal age: 

 cTlI-^" < 25 years 



Expectation of life as 

 function of age (years) 



10 15 20 

 ■►AGEjx) 



30 



^0 



50 



60 



Fig. 3. Expectation of life (Ex) as function of age in the male 

 progeny of young (<25 years) and old (>40) mothers; first- 

 born excluded. 



However in one respect the expectation of life calculated for 

 each age and for different maternal age groups is instructive. 

 Fig. 3 shows the expectation of life as a function of age for the 

 progeny of young and old mothers (first-born excluded). It 

 may be noted that from 6 years of age onwards the difference 

 in expectation of life between the progeny of young and old 

 mothers begins to diminish and between the ages of approxi- 

 mately 15 to 40 years the difference is constant. This can be 

 interpreted as evidence that at least in the middle-aged the 



