The Sex-Ratio in Human Populations 
A. S. PARKES 
T= study of human biology is handicapped in many ways, 
but it has some advantages for quantitative work because 
human beings are produced in huge numbers compared 
with any experimental animal and are comparatively well 
documented. This advantage is well shown in the study of the 
sex-ratio. The proportions in which the sexes occur has attract- 
ed attention from early times and it is a matter of general 
interest because the simple facts and their more obvious social 
implications can be understood by anybody. 
The best known fact is that the ratio of the two sexes at 
birth is not quite equality, as would be expected on the chromo- 
some basis of sex determination, but fluctuates around 105 
males per 100 females. Over millions of births, this slight excess 
of males cannot be due to chance and we must consider its 
significance. In England and Wales at the present time there 
are about 106 males per 100 females at birth. The proportion 
has not always been as high as this; it has been increasing irre- 
gularly for some 70 years. Fig. 1 shows the sex-ratio of births 
by 10- or 5-year periods from 1841 up to the present time. The 
decrease in the number of males at birth during the first 30 
years of this period is not easy to understand; possibly registra- 
tion was incomplete. For the next 40 years the ratio hovered 
below 104 males per 100 females and then coincident with 
World War I rose rapidly to about 105. A subsequent decrease 
was followed by another sharp rise reaching a peak coincident 
with World War II. On the huge numbers concerned these 
changes must mean that something happens from time to time 
to the sex-ratio of live births, and that the general level for this 
century is higher than the level for last century. 
