Different sex ratios at birth in Europe and North America
BMJ 2002; 325 doi: https://doi.org/10.1136/bmj.325.7359.334/a (Published 10 August 2002) Cite this as: BMJ 2002;325:334All rapid responses
Rapid responses are electronic comments to the editor. They enable our users to debate issues raised in articles published on bmj.com. A rapid response is first posted online. If you need the URL (web address) of an individual response, simply click on the response headline and copy the URL from the browser window. A proportion of responses will, after editing, be published online and in the print journal as letters, which are indexed in PubMed. Rapid responses are not indexed in PubMed and they are not journal articles. The BMJ reserves the right to remove responses which are being wilfully misrepresented as published articles or when it is brought to our attention that a response spreads misinformation.
From March 2022, the word limit for rapid responses will be 600 words not including references and author details. We will no longer post responses that exceed this limit.
The word limit for letters selected from posted responses remains 300 words.
Fryer also seems to confuse statistical significance with practical
significance.
His unreferenced suggestion that the differences in sex ratios are
the result of environmental pollution strikes me as highly speculative.
However, even if the differences in sex ratio between Europe and North
America were entirely due to environmental pollution, the fact that the
proportion of boys is decreased only from 0.514 to 0.511 (a change of less
than one percent) gives me reassurance that any such effects are tiny and
of no cause for concern.
Competing interests: No competing interests
Sir,
Adam Jacobs asks if it matters that there are significant differences
in the male to female raios at birth. I say it must be a matter for
concern and a triumph for meticulous science.
The sex of autistic children and cot death victims also shows a clear
and prominent bias to males.
If we assume the over use of pesticides or other environmental causes
may be a factor, then we can also assume that in places of excessive
environmental pollution that this may also have other unforseen effects.
The figures which are significant would suggest that in the USA their
is indeed environmental pollution at a higher level than in Europe and the
results of this study in a sense can almost be predicted. USA is truly a
giant in the chemical and pharmaceutical industry.
A reduction in the levels of pollution should result in the self
correction of this small anomaly.
More worrying is the "thermometer" effect showing that even those
born apparently healthy may be having health problems.
More imporant than anything is the improvement and reduction in
uneccesary environmental damage to our planet.
If we still get significant differences in sex ratios then this will
no longer be a possible cause of the problem.
As a matter of interest the sex ratio has been altered before by
warfare - man against man.
It is ironic and predictible that war on insects may be indirectly
causing this new significant finding of decreased males.
To the credit of the USA they do have a book to warn of ill health
from chemical pollution. Does the UK need an equivalent version? The
poisons book in its greatly improved second edition is not quite good
enough to alert doctors to lesser health problems from chronic pesticide
and chemical exposure.
John Fryer Scientist
Competing interests: No competing interests
More on gender ratio by latitude in Europe and North America
Re:
Research pointers: Grech V, Savona-Ventura S, Vassallo-Agius P.
Unexplained differences in sex ratios at birth in Europe and North
America. BMJ 2002; 324: 1010-1011
Dr. Jacobs criticizes the above paper on two points. First of all, he
states that significance testing is questionable as we tested an entire
population, and not a sample. The reason for our analysis was the usual
reason – to find out how likely these results would have come about by
chance alone. Secondly, I agree that there is no practical/clinical
usefulness that we can see at this time. However, I disagree with his
contention that the practical outcome is always going to be 51 males out
of 100 live births. We clearly showed that there was a male birth deficit
of 1193407 over the study period – not a small number.
Voracek and Fisher comment that since the onset of the study period
in the 1950s is slightly different for Europe and North America, the post
WWII peak in male live births may have skewed our results. However, after
the war, the ratio rapidly declined to the baseline and is therefore
unlikely to have affected our overall results.(1,2) In any case, we will
be publishing our analysis of trends in the near future. They also comment
that the area of the North American continent is 4.8 times the 24 European
countries in our study. However, that data on which the paper is based
shows that the number of actual births is quite similar (N. Am. 127034732,
Europe 157947117 – ratio 0.80). They finally also agree that there is a
latitude change in M/F ratio, but do not comment on the reversed ratio
trend in the North American continent.
The interesting comment by Shields et al re. maternal CMV positivity
being associated with female births further goes to show that a truly
complex interplay of factors influences the gender ratio at birth.
References
1. Bromen K, Jockel KH. Change in male proportion among newborn infants.
Lancet 1997;349:804-805
2. van den Broek JM. Change in male proportion among newborn infants.
Lancet 1997;349:805
Competing interests: No competing interests