Parental Age Effects on Man 29 



Discussion 



One-third of the total material is made up of deaths under 

 the age of 30 years (5,590 cases). These are, however, divided 

 into several parental age groups and groups with different 

 ages at death. The cases in a group are thereby reduced to 

 quite a small number, always less than a thousand and some- 

 times as low as 200. A further reduction, e.g. resulting from 

 grouping according to birth order, would not give relevant 

 information because of the smallness of the material. Since 

 parity and maternal age tend to correlate it is not possible to 

 study the effects of birth order on infant mortality in this 

 series. However, the main result of this investigation suggests 

 that the maternal age effect is restricted to the first two years 

 of life. It is therefore not necessary to use genealogical material 

 for the elucidation of questions on the effects of parity, etc., 

 since recent statistics are available and more appropriate for 

 this purpose. 



The difficulties when dealing with family histories are many, 

 and lie mostly in their heterogeneity. The only uniform 

 feature is the rather high social rank of the families. Family 

 records of labourers' or peasants' families, for example, are 

 rarely available. The advantage of the uniformity is, however, 

 lost by the necessarily long time period, usually covering 

 several centuries, needed to compile a population sample of a 

 sufficient size. During such a long period of time famines, 

 epidemics, general hygiene and the standard of living have 

 changed and influenced mortality rates and causes of death. 



The results concerning maternal age effects are rather con- 

 troversial. Whereas there is agreement concerning perinatal 

 mortality, which increases with age of the mother, infant 

 mortality is found sometimes to increase and sometimes to 

 decrease with maternal age. When family histories have been 

 used for investigation of effects on longevity a slight de- 

 leterious effect of advanced maternal age has usually been 

 found. Gibson and McKeown (1950) have pointed out that 



