Discussion 33 



age at first birth, and there must be considerable differences in its 

 distribution now, compared with populations where parenthood 

 was not so optional. Children born to very young mothers now are 

 often also socially underprivileged or illegitimate. But in spite of 

 this, they have a statistically better performance than first children 

 born to predominantly prosperous mothers between 35 and 40 

 (Baird, D., Hytten, F. E., and Thomson, A. M. (1958). J. Ohstet. 

 Gynaec. Brit. Ejnp., 65, 865). There is also the point in regard to 

 parental age effects that when there are very large disparities in age 

 between husband and wife, the proportion of cases where the husband 

 is not the father increases very considerably. 



Maynard Smith: There is another point on this genetic question of 

 whether, if more old mothers are having children which survive, this 

 could have a genetic consequence on the expected longevity of a 

 population. It would be very dangerous to assume without evidence 

 that there will be a positive correlation between the longevity of 

 parents and offspring. Beeton and Pearson (1901) found a correla- 

 tion, but it was very small compared with, for example, the cor- 

 relations for stature or for other metrical characters. There are 

 genetic reasons why one might expect, for a character closely 

 associated with fitness, to get small parent-offspring correlations. 

 Certainly Dr. Comfort and I in independent work have found low 

 parent-offspring correlations for longevity in Drosophila. Yet Prof. 

 Rockstein has implied that he gets quite a considerable correlation 

 in the housefly, for the male offspring only. We require better 

 evidence in the human population as to whether there is or is not a 

 high correlation between the longevity of parents and their children. 



Danielli: We do not really know whether there are significantly 

 more children of older parents surviving, because once there is a 

 probability through advancement in social techniques that a woman 

 of potentially childbearing capacity will survive longer, then her 

 opportunities for not having children, so to speak, also increase and 

 this effect may be working in the opposite direction. So unless we 

 have actual evidence on this point, we have not even got the raw 

 material to find out whether there is any genetic effect at all. 



Muhlbock: In our inbred mice some of these factors are not 

 apparent. We can control the genetic constitution equally well in all 

 the animals. We observed first that there are more deaths in utero 

 with a higher age and that the litter size is also much smaller. This 

 is not just the effect of the number of eggs, but it is also a uterus 

 effect. Maybe the hormonal stimulation in the endometrium of the 

 uterus is not good enough in old age. Then we tried to find out 

 whether the lifespan of these inbred mice is influenced by the age of 



AGEING — V — 2 



