World Population 
but there is still the very large prenatal mortality to consider. 
And if one accepts that it falls differentially on males, then its 
decrease will further increase the number of males at birth. 
Medawar: It depends what causes those deaths of males 
before birth, doesn’t it? If they are genetic deaths, due to 
chromosome aberrations of some kind, it is probable that they 
won’t be within reach of medical treatment. 
Parkes: Lagree. But in this very large prenatal mortality— 
and I am thinking right back now to the time of implantation, 
not merely to the time when overt abortions occur—what is the 
evidence that genetic non-viability is a big factor? 
Medawar: I am not saying there is evidence in favour of 
this idea; only that we have no evidence against it. 
Lederberg: I predict that in 10 or 20 years we will have found 
a simple chemical method for biasing the sex-ratio. But what 
would happen then? Suppose the sex-ratio were under 
voluntary control, and suppose there were a small excess of 
males, would we have to anticipate a sudden increase in the 
sex-ratio even over that? Would there be a “business cycle” 
with a substantial degree of inertia and overshoot from genera- 
tion to generation? These are important questions. 
Parkes: With the present social set-up, and I keep on 
emphasizing that qualification, I think the result of voluntary 
choice, with a certain amount of overshoot and so forth, would 
be a sex-ratio much the same as we have now. 
Lederberg: What is the evidence for this? Do we have any 
data on people’s attitudes or choices in any range of our society 
which would enable us to write the econometric model of the 
future development of such choices? This is the urgent business 
of this kind of symposium. 
Crick: Surely it would be quite easy to obtain; you have 
only to ask different classes of people, who have had different 
numbers of children, what sex they would like their next child 
to have if they had a choice. 
Lederberg: You must do that, but you must also find out 
how they behave as well as what they say they want. And you 
must anticipate what this behaviour will be as a function of the 
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