RESEARCH REPORT

Secular trends in sex ratios at birth in North America and
Europe over the second half of the 20th century
V Grech, P Vassallo-Agius, C Savona-Ventura
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

J Epidemiol Community Health 2003;57:612–615

Context: A previous study showed that significantly more boys were born in southern latitudes in
Europe than in northern latitudes and the converse pattern was observed in North America.
Objective: This study analyses secular trends in gender ratios for live births over the second half of the
20th century.
Design, setting, participants: Analysis was carried out from a World Health Organisation dataset
comprising live births over the above period. This included 127 034 732 North American and
157 947 117 European live births.
Main outcome measures: Analysis of trends in gender ratios for countries in both continents.
Results: The findings show a highly significant overall decline in male births in both Europe and North
America (p<0.0001), particularly in Mexico (p<0.0001). Interestingly, in Europe, male births declined
in North European countries (latitude>40°, p<0.0001) while rising in Mediterranean countries (latitude
≅35–40°, p<0.0001). These trends produced an overall European male live birth deficit 238 693 and
a North American deficit of 954 714 (total male live birth deficit 1 193 407).
Conclusions: No reasonable explanation/s for the observed trends have been identified and the
causes for these trends may well be multifactorial.

S
ex in reptiles and fish is determined after conception,

during embryogenesis, according to ambient environ-

mental temperature.1 In contrast, in mammals and birds,

sex is determined at conception. In mammals, male births

invariably occur slightly in excess.2 The male to female ratio of

live births is generally expressed as the ratio of male live births

divided by total live births (M/F), and the human M/F is

expected to be 0.515, with approximately 3% more males born

than females.3 The reason for this discrepancy is uncertain as

testicles produce equal numbers of X-bearing and Y-bearing

spermatozoa.4

Several theories have been put forward to explain why this

ratio should not be 0.5, with equal number of male and female

births. These theories include the timing of conception within

the ovulatory cycle,3 endocrine effects,5 and immunological

effects.6

We have shown that M/F varies in geographical space,

exhibiting a latitude gradient,7 and that this gradient is differ-

ent in Europe and North America, with more males born

towards the south of Europe, compared with the North

American continent where more males are born towards the

north of the continent.8

Other studies have also shown that M/F may vary with

time. Several authors have shown that M/F has declined over

the second half of the 20th century in various industrialised

countries. These include Denmark,9 the United Kingdom,10 the

Netherlands,11 Germany,12 Canada,13 the United States of

America,14 and Finland.15 In contrast, M/F has been noted to

have risen in Ireland over the same period.16 A non-significant

rise in M/F has also been noted in Australia17 and Japan.18

James has proposed that M/F fluctuates over a 30 year

cycle.19 In this study, we identify secular trends in M/F in dif-

ferent countries and continents from a World Health Organis-

ation dataset for the second half of the 20th century. We also

analysed these trends by geographical latitude in Europe and

briefly review the relevant literature regarding this topic.

METHODS
Data sources
Annual male and female live births were obtained directly

from WHO. Data were available for 1950–97 for the North

American continent (Canada, USA, and Mexico), and for

European countries for the period 1950–99, with the following

exceptions:
Data were not available for Mexico for the period 1950–8

and for 1996–7. Data were not available for the years 1998–9
for the following countries: France and Spain. Data were not
available for 1999 for the following countries: the Nether-
lands, Poland, United Kingdom, Germany, Norway, and
Greece. Data for Romania were not available for the period
1950–54. Stillbirths are not included in this study and neither
were data from small countries.

Statistics
The quadratic equations of Fleiss were used for exact calcula-

tion of 95% confidence limits for ratios.
20 Linear regression was

used for the calculation of association of annual M/F with

time. Graphs are drawn as five year moving averages. A p value

<0.05 was taken to represent a statistically significant result.

RESULTS
There has been a significant decline in M/F ratios during the

second part of the 20th century in both the European and

North American continents (fig 1). Live birth data and

regression calculations for countries studied are shown in

table 1.

Europe
This continent included a study (live birth) population of

157 947 117. A statistically significant decrease in M/F was

noted in Greece, Hungary, Poland, and Sweden. A decline in

M/F, albeit not statistically significant, was found in Austria,

Belgium, Bulgaria, Norway, Portugal, Romania, and Switzer-

land. In contrast, M/F rose significantly in France and Italy. M/F

rose and then fell in Spain, with an overall significant rise.

See end of article for
authors’ affiliations
. . . . . . . . . . . . . . . . . . . . . . .

Correspondence to:
Dr V Grech, Paediatric
Department, St Luke’s
Hospital, Guardamangia,
Malta;
victor.e.grech@govt.mt

Accepted for publication
2 January 2003
. . . . . . . . . . . . . . . . . . . . . . .

612

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Summation of live births for Mediterranean countries that
span latitude ≅35–40° (Bulgaria, Greece, Italy, Portugal, and
Spain) showed a significant rise in M/F. Live births for the
remaining European countries above latitude 40° (Austria,
Belgium, Denmark, Finland, France, Germany, Hungary,
Ireland, the Netherlands, Norway, Poland, Romania, Sweden,
Switzerland, and the United Kingdom) showed a significant
fall in M/F (fig 2).

Assuming an expected M/F of 0.515, these trends produced
a male birth deficit of 12 744 in the Mediterranean region,
212 780 in central Europe and 13 169 in the Nordic countries;
a total male birth deficit of births 238 693.

North America
This continent included a study (live birth) population of

127 034 732. The combined North American continent data

(Mexico, USA, and Canada) showed a significant decline in

M/F (fig 1). M/F declined significantly in the USA and Mexico

(fig 3).
Once again, assuming an expected M/F of 0.515, these

trends produced a male birth deficit of 21 993 in Canada,

410 932 in the United States, and 521 789 in Mexico; a total

male birth deficit of 954 714.

The combined live male birth deficit for the two continents

was 1 193 407.

Figure 1 M/F ratios for Europe (combined), North America
(combined), and both continents combined.

Table 1 Male, female, total births and linear regression of sex ratio with time for countries included in this study

Country Male live births Female live births Total live births r p M/F 95% CI

Denmark* 1680922 1588490 3269412 −0.29 0.49 0.5141 0.5136 to 0.5147
Finland* 1741682 1652473 3394155 −0.10 0.50 0.5131 0.5126 to 0.5137
Germany* 25678261 24239453 49917714 −0.75 <0.0001 0.5144 0.5143 to 0.5146
Ireland* 1498481 1416062 2914543 0.45 0.002 0.5141 0.5136 to 0.5147
Netherlands* 5160590 4899973 10060563 −0.60 <0.0001 0.5130 0.5126 to 0.5133
UK* 20441737 19330020 39771757 −0.61 <0.0001 0.5140 0.5138 to 0.5141
Austria 2600186 2466512 5066698 −0.13 0.38 0.5132 0.5128 to 0.5136
Belgium 3138073 2965932 6104005 −0.17 0.26 0.5141 0.5137 to 0.5145
Bulgaria 3176760 3000118 6176878 −0.22 0.13 0.5143 0.5139 to 0.5147
France 19579581 18634567 38214148 0.52 <0.0001 0.5124 0.5122 to 0.5125
Greece 3395944 3168572 6564516 −0.43 0.002 0.5173 0.5169 to 0.5177
Hungary 3815036 3587994 7403030 −0.65 <0.0001 0.5153 0.5150 to 0.5157
Italy 18671513 17652250 36323763 0.73 <0.0001 0.5140 0.5139 to 0.5142
Norway 1444600 1363935 2808535 −0.15 0.31 0.5144 0.5138 to 0.5149
Poland 15295234 14359427 29654661 0.80 <0.0001 0.5158 0.5156 to 0.5160
Portugal 4283717 4015694 8299411 −0.03 0.81 0.5161 0.5158 to 0.5165
Romania 8082232 7649545 15731777 −0.27 0.07 0.5138 0.5135 to 0.5140
Spain 13600342 12805240 26405582 0.69 <0.0001 0.5151 0.5149 to 0.5152
Sweden 2607559 2459796 5067355 −0.51 <0.0001 0.5146 0.5141 to 0.5150
Switzerland 2103563 1995775 4099338 −0.07 0.65 0.5131 0.5127 to 0.5136
Mediterranean (35-40°) 43128276 40641874 83770150 0.66 <0.0001 0.5147 0.5148 to 0.5149
Northern Europe (>40°) 114818841 108564205 223383046 −0.84 <0.0001 0.5139 0.5140 to 0.5141
All Europe 157947117 149206079 307153196 −0.68 <0.0001 0.5142 0.5142 to 0.5143
Canada (>50°)* 8012882 7588817 15601699 0.26 0.79 0.5136 0.5133 to 0.5138
USA (30-50°)* 76827922 73150435 149978357 −0.73 <0.0001 0.5123 0.5122 to 0.5123
Mexico (<30°) 42193928 40749212 82943140 −0.76 <0.0001 0.5087 0.5086 to 0.5088
North America 127034732 121488464 248523196 −0.73 <0.0001 0.5112 0.5111 to 0.5112

*Previously reported.

Figure 2 Summation of M/F ratios for Mediterranean and northern
European countries.

Figure 3 M/F ratios for Canada, the United States of America,
and Mexico.

Trends in sex ratios at birth 613

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DISCUSSION
In utero, the male fetus is more prone to morbidity and mor-

tality from external influences than the female fetus.

Moreover, the male fetus is at greater risk of all obstetric com-

plications than the female fetus and readers are referred to the

review by Kraemer.21 Despite these adverse factors, males are

invariably born in excess of females,2 implying an even higher

conception rate of males over females than evinced by the sex

ratio at birth.
In the first half of the 20th century, antenatal care improved

dramatically in industrialised countries, which led to a fall in
stillbirths, most of which would have otherwise been male.
This therefore led to an increase in the M/F ratio.

9 22

M/F then declined in several countries in the second half of
the 20th century, and one study has proposed that M/F could
be used as a sentinel health indicator.23 This is supported by
data from Italy that showed that in metropolitan areas, M/F
declined in comparison with the rest of the country where
M/F increased.24

However, there are several arguments against this hypoth-
esis. Firstly, in Finland15 and in Malta,7 the decline in M/F pre-
dated the countries’ industrialisation or the widespread use of
pesticides. Secondly, it has been noted that in the United
States, M/F was overall higher in the black population than in
the white population,14 and that M/F rose and then fell in the
white population (1964–1988), while rising throughout the
study period in the non-white population.25 The overall decline
is accounted for by the fact that over 80% of the USA popula-
tion falls in the “white” category. Thirdly, Ireland has reported
a rise in M/F despite increasing industrialisation,16 and in this
study, M/F was also seen to rise in France, Italy, and Spain.
These findings do not support the sentinel health indicator
hypothesis.

Our earlier study provoked several useful comments.8

Jacobs commented that the differences that we showed were
very small,26 which is perfectly true. However, this small
difference has resulted in an overall deficit of 1 193 407 male
births in the European and North American continents for
this same study period.8 Voracek and Fisher commented that
as the onset of the study period in the 1950s is slightly differ-
ent for Europe and North America, the peak in male live births
after the second world war may have skewed our results.
However, after the war, the ratio rapidly declined to the base-
line and is therefore unlikely to have affected our overall
results.

11 12 They also pointed out that that the area of the
North American continent is 4.8 times the 24 European coun-
tries in our study. However, the data on which the paper is
based show that the number of actual births is quite similar
(North America 127 034 732, Europe 157 947 117, ratio 0.80).
They also reanalysed the dataset and showed a curvilinear
relation of latitude and sex ratio that is consistent with an
effect related to photoperiod. However, the secular trends can-
not be explained, to our mind, by such an effect.

27 Shields et al
showed a significant relation between cytomegalovirus sero-
positivity in cord blood and female sex, but again, we cannot
see how CMV infection may possibly be implicated in these
secular trends.28

James also reiterated the influence of maternal oestrogen
levels on birth weight and the likelihood of dizygotic twins.29

While an interesting hypothesis, we cannot see how this fac-
tor alone could explain the widely varying secular trends in
different countries over the same timespans.

An earlier study had noted declining trends in several
countries in various continents, but a latitude effect had not
been perceived.30 This same study also showed no overall
change in M/F over 1926–1990, whereas our study only looked
at the second half of 20th century. It may be that Parazzini et
al’s early part of the dataset trends nullified the trends in the
second half of 20th century.31

External factors that influence M/F have also been
described. Germany, for example, experienced two M/F peaks

that were related to the two world wars,12 and these peaks
were also supported by data from the Netherlands.32

James has proposed that M/F fluctuates over a 30 cycle, and
that this is attributable to a homeostatic mechanism that cor-
relates sex at birth negatively with the adult sex ratio at the
time of conception.33 The current decline in M/F would there-
fore be a negative feedback response to the increase in M/F in
the first half of the 20th century. This hypothesis is partly
supported by data from the USA that demonstrated a degree
of cyclicity over a 55 year period.30 More interestingly, this
same study showed a strong degree of correlation between
parental age and birth weight. However, the overall decline in
M/F in our 50 year study seems constant in both Europe and
North America (fig 1) with no evidence of a 30 year cycle.

Diverging cultural attitudes may also potentially influence
M/F. For example, Latino and Eastern cultures prefer male
over female offspring.

34 35 In such cultures, families would be
more likely to settle for a single son, than for a single daugh-
ter, and on first having a daughter, may therefore opt to have
additional children in order to have a son. Male offspring bias
may also be evinced by female infanticide and/or sex selective
abortion. However, it is unlikely that such factors could have
played an important part in determining the observed M/F
ratios in the developed countries considered in this study.

36 37

Moreover, any such skew would have mitigated against our
finding of an overall male deficit.

Several other hypothesis have been put forward to explain
different trends in M/F ratios, with various factors assumed to
influence the female genital tract environment in ways to
favour the Y-bearing spermatozoa.25 For example, it has been
claimed that caloric availability per capita correlates positively
with M/F.37

In an earlier study we showed a latitude gradient for both
Europe and the North American continent.8 Our current
European results are interesting in that they show a rising M/F
ratio in Mediterranean countries and a falling M/F ratio in
more northern European countries (fig 2). Since the early
1980s, in Mediterranean countries, M/F ratio seems to have
stabilised at just over the expected value of 0.515, while in
more northern countries, M/F ratio seems to have stabilised at
0.513. Should these secular trends increase in magnitude,
with an increasing male deficit, this will naturally have social,
occupational, epidemiological, and community health related
effects. The results of this study yield highly significant p
values because of the large numbers of births involved despite
the comparatively small shifts in secular trends. The p values
are usually used to enable inferences to be drawn about popu-
lations from samples. In this context, the p value is only use-
ful as an indicator that shows how likely it is that these results
should occur by chance alone, assuming that the dataset is
correct. One final point that must be borne in mind is that the
differences that we demonstrated are very small, and could
theoretically have been produced by even very small differen-
tial reporting in male and female births, although such errors
are unlikely to produce the observed, rather smooth secular
trends.

In conclusion, while we have expanded our findings on the
sex ratio at birth by studying secular trends, we still cannot
put forward any reasonable explanation for the observed
trends, which may well be attributable to several factors and
not just one.

. . . . . . . . . . . . . . . . . . . . .
Authors’ affiliations
V Grech, Paediatric Department, St Luke’s Hospital, Malta
P Vassallo-Agius, Medical School, University of Malta
C Savona-Ventura, Department of Obstetrics and Gynaecology, St
Luke’s Hospital, Malta

REFERENCES
1 Dournon C, Houillon C, Pieau C. Temperature sex-reversal in

amphibians and reptiles. Int J Dev Biol 1990;34:81–92.

614 Grech, Vassallo-Agius, Savona-Ventura

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ttp

://je
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.b
m

j.co
m

/
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p
id

e
m

io
l C

o
m

m
u

n
ity H

e
a

lth
: first p

u
b

lish
e

d
 a

s 1
0

.1
1

3
6

/je
ch

.5
7
.8

.6
1
5
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0
0
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. D

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a
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2 Parkes AS. The mammalian sex ratio. Hum Reprod 1926;2:1–51.
3 James WH. The human sex ratio. Part 1: a review of the literature. Hum

Biol 1987;59:721–52.
4 Wyrobek AJ, Robbins WA, Mehraein Y, et al. Detection of sex

chromosomal aneuploidies X-X, Y-Y, and X-Y in human sperm using
two-chromosome fluorescence in situ hybridization. Am J Med Genet
1994;53:1–7.

5 James WH. Parental hormone levels and mammalian sex ratios at birth.
J Theor Biol 1989;139:59–67.

6 Ober C. The maternal-fetal relationship in human pregnancy: an
immunogenetic perspective. Exp Clin Immunogenet 1992;9:1–14.

7 Grech V, Vassallo-Agius P, Savona-Ventura C. Declining male births with
increasing geographical latitude in Europe. J Epidemiol Community
Health 2000;54:244–6.

8 Grech V, Vassallo-Agius P, Savona-Ventura C. BMJ 2002;324:1010–
11.

9 Moller H. Change in male:female ratio among newborn infants in
Denmark. Lancet 1996;348:828–9.

10 Dickinson HO, Parker L. Why is the sex ratio falling in England and
Wales? J Epidemiol Community Health 1996;50:227–8.

11 van der Pal-de Bruin KM, Verloove-Vanhorick SP, Roeleveld N.
Change in male:female ratio among newborn babies in Netherlands.
Lancet 1997;349:62.

12 Bromen K, Jockel KH. Change in male proportion among newborn
infants. Lancet 1997;349:804–5.

13 Allan BB, Brant R, Seidel JE, et al. Declining sex ratios in Canada. Can
Med Assoc J 1997;156:37–41.

14 Marcus M, Kiely J, Xu F, et al. Changing sex ratio in the United States,
1969–1995. Fertil Steril 1998;70:270–3.

15 Vartiainen T, Kartovaara L, Tuomisto J. Environmental chemicals and
changes in sex ratio: analysis over 250 years in Finland. Environ Health
Perspect 1999;107:813–15.

16 Moynihan JB, Breathnach CS. Changes in male:female ratio among
newborn infants in Ireland. APMIS 1999;107:365–8.

17 Lancaster PA, Day PL. Declines in population sex ratios at birth. JAMA
1998;280:1139–40.

18 Minakami H, Sato I. Change in male proportion of newborn infants in
Japan. Am J Obstet Gynecol 1998;178:624.

19 James WH. Declines in population sex ratios at birth. JAMA
1998;280:1139.

20 Fleiss JL. Statistical methods for rates and proportions. 2nd edn. New
York: Wiley, 1981:14–15.

21 Kraemer S. The fragile male. BMJ 2000;321:1609–12.
22 National Center for Health Statistics. Vital statistics of the United

States, 1992: mortality part A. Vol 2. Washington, DC: Public Health
Service, 1996.

23 Davis DL, Gottlieb MB, Stampnitzky JR. Reduced ratio of male to female
births in several industrial countries: a sentinel health indicator? JAMA
1998;279:1018–23.

24 Astolfi P, Zonta LA. Reduced male births in major Italian cities. Hum
Reprod 1999;14:3116–19.

25 Nicolich MJ, Huebner WW, Schnatter AR. Influence of parental and
biological factors on the male birth fraction in the United States: an
analysis of birth certificate data from 1964 through 1988. Fertil Steril
2000;73:487–92.

26 Jacobs A. Different sex ratios at birth in Europe and North America.
[Letter]. BMJ 2002;325:334.

27 Voracek M, Fisher ML. Different sex ratios at birth in Europe and North
America. [Letter]. BMJ 2002;325:334.

28 Shields MD, O’Hare B, Nelson J, et al. Different sex ratios at birth in
Europe and North America. [Letter]. BMJ 2002;325:334.

29 James WH. Sex ratio at birth and latitude. [Letter]. J Epidemiol
Community Health 2001;55:216.

30 Parazzini F, La Vecchia C, Levi F, et al. Trends in male:female ratio
among newborn infants in 29 countries from five continents. Hum Reprod
1998;13:1394–6.

31 Parazzini F, La Vecchia C, Chatenoud L, et al. Change in male
proportion among newborn infants. Lancet 1997;349:805–6.

32 van den Broek JM. Change in male proportion among newborn infants.
Lancet 1997;349:805.

33 James WH. What stabilizes the sex ratio? Ann Hum Genet
1995;59:243–9.

34 Sachar RK, Soni RK. The role of secondary sex ratios in the declining sex
ratio of India. Indian J Public Health 1995;39:12–15.

35 Coale AJ, Banister J. Five decades of missing females in China.
Demography 1994;31:459–79.

36 Rinehart W. Sex preselection–not yet practical. Popul Rep I
1975;2:21–32.

37 Williams RJ, Gloster SP. Human sex ratio as it relates to caloric
availability. Soc Biol 1992;39:285–91.

APHORISM OF THE MONTH .........................................................................
Starting a rumour, following Christopher Columbus, and spending other
people’s money

R
eaders will recall that Columbus was adept at following his strategy for exploration by

spending other people’s money.1 Bridging the gap between starting a rumour and mobi-

lising resources for change is at the heart of public health—how often have you heard

somebody say that they can’t do anything to improve health without knowing where the

money is coming from first, yet public health is essentially about shaping and influencing the

actions of others and other sectors. An effective public health practitioner should be adept at

spending other people’s money, but for this to happen on any meaningful scale it is essential

for other players involved to feel a sense of ownership.2 I have long contended that in health

and health care we need not Directors of Finance, but Directors of Resources. The mindset

that starts with financial resources in health finishes up with doctors, nurses, bricks and

mortar, and bits of kit. In reality, the resources for health run much wider, including many

human and environmental resources that lie outside the bailiwick of something called health.

JRA
References

1 Ashton JR. Columbus on the need for strategy. J Epidemiol Community Health 2003;57:235.
2 Ashton JR. Communities and sustaining change. J Epidemiol Community Health 2002;56:561.

Trends in sex ratios at birth 615

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