DISCUSSION 
actual sex-ratio and its age distribution. To revert to Professor 
Crow’s argument, if there is a hereditary disposition for choice of 
one sex, the ratio will even out through natural selection of the 
counterbalancing genes (or traditions). 
Clark: ‘There is some statistical evidence about this. I am 
speaking as rather an abnormal case, as a father of eight sons 
followed by one daughter. The mathematical probability of 
that can be worked out. Gini in Italy counted the number of 
such families and also of the converse—eight daughters followed 
by one son—and found that the latter were far more frequent; 
so that in Italy there is preference for sons. Parkes raised the 
point about young mothers having more sons. The crude stati- 
stics indicate, as Bronowski mentioned, that first births also 
show a higher male proportion. Is that fully explicable by the 
age of the mother, or does sequence of birth make some 
difference? 
Parkes: There is a correlation of course, but just how great 
I cannot say. 
Medawar: ‘There is another variable, namely, the age of the 
father, because the children of younger mothers tend also to be 
the children of younger fathers. I believe that the age of the 
mother as such, and the parity of the mother as such, are not 
important variables, but that the age of the father is. 
Bronowski: I can add a footnote to what Colin Clark has 
just been saying about the statistics. There is some statistical 
evidence to answer Crick’s question, but it is indirect. If you 
take the distribution of male and female children in large 
families, then you find that, in families of 3 or 4 or 5 children, 
the distribution is random; runs of all boys or all girls occur only 
as often as they would if we were tossing heads and tails. But 
when you get to families of 6 or 7 or 8 children and upwards, 
then there are too many runs of all boys or all girls. This is a 
clear indication that parents with one-sex families go on having 
children because they want to have a child of the opposite sex— 
either sex. I have had four daughters and no son, and I assure 
Lederberg that if I knew of a chemical which would ensure that 
I should not have another daughter I would use it! 
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