DISCUSSION 
There is another avenue to sex determination which Professor 
Parkes might have mentioned, and this is the effect of the 
steroids on sex determination or on other sex-determining 
factors. It is well known that if you take frog eggs and immerse 
them in an oestrogen solution you get 100 per cent females. 
Recently in our laboratory we have been immersing rabbit eggs 
in oestrogen solution, and so far we have go per cent females. 
Medawar: I should like to question Parkes’ belief that we 
are only at the beginning of a tendency for the sex-ratio to 
increase, and that we can look forward to a still greater pre- 
ponderance of males. I don’t think that is true. I think the 
present tendency for the sex-ratio to increase is due to a number 
of demographic tendencies which are self-limiting. Parkes 
mentioned one of them—the tendency towards a younger age 
at marriage; a second one, which he didn’t mention, is probably 
equally important, namely, the tendency for families to be 
completed earlier and earlier in married life, and that is also a 
self-limiting factor; the third tendency is differential mortality 
itself, which has hitherto favoured females throughout the whole 
of life, anyhow from birth onward: that is also self-limiting 
because mortality up to the ages at which most people marry is 
in fact getting somewhere near as low as it can ever be. So I 
don’t think that we need fear that the sex-ratio is going up to 
ludicrous proportions over the marrying years. 
Brain: Another factor, of course, is homosexuality. We don’t 
know the incidence of it, but apparently it is substantial 
enough, if there is a differential between the two sexes, to 
counteract completely the other comparatively small figure. 
Comfort: ‘The incidence of male homosexuality might also be 
influenced of course by the unavailability of females in the 
future. 
Crick: In the light of all that, Professor Parkes, could you 
estimate, for 10 or 20 years ahead, what the sex-ratio is going to 
be, making reasonable extrapolations in the way that Medawar 
has pointed out? 
Parkes: So far as infant and postnatal mortality is concerned, 
as Medawar said, we may be getting near the limits of decrease, 
116 
