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1For most human populations the sex ratio at birth, defined as the number of male births per 100 female births, is around 105 (Cavalli-Sforza and Bodmer, 1971; Pressat, 1983; Henry and Blum, 1988; Sieff, 1990; Caselli and Vallin, 2001). Several biological mechanisms have been suggested to explain the remarkable constancy of this ratio. Nonetheless, notable variations from this mean value have been observed in certain populations (Teitelbaum, 1972; James, 1987; Chahnazarian, 1988; Sieff, 1990). Besides biological factors, other demographic or socio-economic characteristics also have been studied to better understand these differences.

2Analysis of over 20,000 births from the Saguenay region of Quebec (Canada) during the period 1850 to 1880 has revealed an unusually high sex ratio at birth of more than 108 (Tremblay, 1997). Subsequent analyses have shown that this value persists beyond the end of the nineteenth century and into a good part of the twentieth. The aim of the present study is a detailed examination of these high ratios and an identification of their possible demographic determinants such as parental age, birth order, the sex of prior births, the length of birth intervals, and seasonality. Certain family components have also been examined using information on family names (patronyms).

I – Factors influencing the sex ratio at birth

3The sex ratio at birth (SRB) depends on the sex ratio at conception and sex-specific intrauterine mortality differentials. Although these factors have already been the subject of numerous studies, they remain difficult to measure (Cavalli-Sforza, 1971; Teitelbaum, 1972; James, 1987; Chahnazarian, 1988). Nonetheless, it appears that the timing of conception has an effect on the sex of the child, with males more frequently conceived at the beginning and at the end of the menstrual cycle (James, 1987, 1997a; Sieff, 1990). Hormonal variations at the moment of conception could also explain observed differences among certain ethnic groups (Chahnazarian, 1988; James 1996).

4As a practical matter, the difficulties of measuring these biological factors have persuaded most researchers to rely on less direct, and more easily observable on a large scale, factors underlying the sex ratio [1]. This is the reason why variables such as the ages of the parents and the birth order have been widely studied. Although results often differ from one study to another in both direction of effects and their significance, a general picture emerges that birth order is negatively correlated with the sex ratio, i.e., the proportion of male births decreases as the number of prior births increases (Teitelbaum, 1972; Erickson, 1976; James 1987; Chahnazarian, 1988; Sieff, 1990). Father’s age appears to have a greater influence than mother’s age, older fathers having a tendency to produce fewer sons than younger fathers (Chahnazarian, 1988). Since birth order is closely tied to parental ages, some researchers have attempted to separate their relative contributions. It appears that the influence of parental age is diminished when birth order is taken into account (Teitelbaum, 1972; Erickson 1976; Chahnazarian, 1988). Shorter birth intervals (taking as the first birth interval the length of the interval from marriage to first birth) also seem to be associated with a higher proportion of males (James, 1996, 1997a; Nonaka et al, 1998). In addition, several researchers have examined the effect of spousal age difference. Manning et al. (1997) provide evidence of a positive relationship between parental age gap and the sex ratio for first births. Astolfi and Zonta (1999) reported a slight increase in the sex ratio among children born to parents where the age difference was greater than 15 years. However, other studies have not found a significant effect for spousal age difference (Arnold and Rutstein, 1997; Bocklage, 1997; James, 1997b).

5Sex of the preceding child seems to have some effect on the sex ratio; thus, families comprised only of boys would be more likely to have another boy than other types of families, and this probability increases with the size of the family (James, 1987; Biggar et al., 1999). Once again, results are inconsistent and other researchers have not found any significant relationship between the sex of successive children (Curtsinger et al., 1983; Maconochie and Roman, 1997; Jacobsen et al., 1999; Nonaka et al., 1999).

6Seasonality and period effects are among the other factors that have attracted attention. Significant variations in the sex ratio by month of birth have been observed in certain populations (James 1987; Lerchl, 1998; Nonaka et al., 1998, 1999), but these variations are generally very small. A decline in the sex ratio has been observed in several countries over the course of the twentieth century (Ulizzi, 1983; Feitosa and Krieger, 1992; Allan et al., 1997; Dodds and Armson, 1997; Davis et al., 1998; Marcus et al., 1998; Vartiainen et al., 1999). According to some researchers, this decline could be explained by environmental changes. Finally, some research has suggested significant influence from particular period effects, such as wars or natural disasters (James, 1987, 1997a; Ulizzi and Zonta, 1995; Fukuda et al., 1998; Graffelman and Hoekstra, 2000).

II – Data and methods

1 – The Saguenay population and the BALSAC Population Register

7The Saguenay region is situated about 200 km north of Quebec City. The settlement of this region began in the second quarter of the nineteenth century by emigrants from the neighbouring region of Charlevoix (Pouyez and Lavoie, 1983; Bouchard, 1996). Between the middle of the nineteenth century and the middle of the twentieth century, the population grew rapidly, mostly due to natural increase. During this time, the population increased from about 5,000 to 196,000 inhabitants, with an annual growth rate of 3.7% (Pouyez and Lavoie, 1983). Today the region’s population is close to 285,000 (Institut de la statistique du Québec, 2002).

8At the start of the 1970s, Bouchard and colleagues initiated what would later become known as the BALSAC Population Register (Bouchard, 2002). Initially containing about 660,000 records of baptisms, marriages, and burials recorded in the Saguenay region between 1842 and 1971, these data were linked together to reconstitute the entire Saguenay population (Bouchard et al., 1991). For the current study we have exploited the 419,467 live births that occurred in the Saguenay region between 1 January 1850 and 31 December 1971 and for which we know the sex and date of the birth.

2 – Variables studied

9Besides the date of birth and the sex of the newborn, the BALSAC register allows us to link together other pertinent data. Family reconstitution provides data on the ages of the parents at the time of birth of each child, the birth order of that child, the sex of the preceding child, and the length of the birth interval (or marriage-to-first-birth interval). We have also used the patronym (the father’s family name) to test for certain family-specific variations in the sex ratio. Although these data may not be available (or usable) for every birth observed during the entire course of the study period, those that are available have proved sufficient for the purposes of the current study.

3 – Statistical analyses

10The sex ratio is characterized by a markedly asymmetric distribution which makes statistical tests and confidence interval estimation difficult. In contrast, the male proportion of all births (m) has a distribution that is almost Gaussian. Accordingly, all statistical analyses are done using the latter variable and the results are thereafter re-transformed to SRB units to facilitate interpretation. Thus, if m is the male proportion of all births and N is the number of total births, the 95% confidence interval for m can be obtained from the formula:

12Annual variations in the SRB have been modelled with a nonparametric regression of m, with the result being transformed for presentation purposes. This nonparametric regression is of the form:

14where f is a combination of spline functions (Chambers and Hastie, 1992). The method produces a smooth curve f for observed values of x; hypothesis tests allow the determination of statistical significance and confidence intervals. Plotting the smooth curve helps in interpreting the results, as will be seen below in the section on “Annual variations.”

15The effect of birth intervals on the SRB has been modelled with a generalized additive model (Hastie and Tibshirani, 1990). Additive models are a nonparametric multivariate technique that models a variable as a sum of smooth functions of other variables:

17where the functions fi, estimated by spline functions, represent the marginal contributions of each explanatory variable. This is a straightforward multivariate extension of the technique shown above and, similarly, hypothesis tests allow the determination of statistical significance of each explanatory variable xi and graphical representations show the marginal relationship of each one to the dependent variable Y. This approach is particularly well-suited to this study since it permits the detection of nonlinear contributions by the explanatory variables.

III – Results

1 – Annual variations

18Figure 1 shows annual values for the sex ratio at birth for the Saguenay population from 1850 to 1971. Superimposed on the values is a smooth curve that shows the pattern of changes in the ratio over time, with confidence intervals indicated. Over the first years of the period there are notable annual fluctuations in the sex ratio, partially due to small numbers of observations. Several observed values are quite far from the expected values; before 1900, the ratio ranges from a low of 93 to a high of 120. After 1900, the fluctuations are more moderate.

Figure 1

Sex ratio at birth by year, Saguenay, 1850-1971 (n = 419,467)

Figure 1

Sex ratio at birth by year, Saguenay, 1850-1971 (n = 419,467)

Source: BALSAC Population Register.

19As a general rule, the sex ratio is much more often above rather than below the “theoretical” value of 105. The smooth curve also exhibits several inflection points: a plateau around 1885, followed by a marked decrease until about 1920 then, starting around the 1930s a slight increase though not to the pre-1900 level. Accordingly, we distinguish three separate sub-periods for the following analysis: 1850-1899, 19001929, and 1930-1971. Table 1 shows the distribution of births by sex for each of these three periods.

Table 1

Births by sex and period, Saguanay, 1850-1971

Table 1
1850-1899 1900-1929 1930-1971 1850-1971 Males 24,924 49,923 141,775 216,622 Females 22,884 47,240 132,721 202,845 Total 47,808 97,163 274,496 419,467 (%) (11.4%) (23.2%) (65.4%) (100.0%) Sex ratio at birth 108.9 105.7 106.8 106.8 (95% confidence interval) (107.0–110.9) (104.4–107.0) (106.0–107.6) (106.1–107.4) Source: BALSAC Population Register.

Births by sex and period, Saguanay, 1850-1971

20Over the period 1850-1899, the sex ratio approaches 109, which is quite remarkable considering the number of the births involved (47,808 births). During the following period (1900-1929), it approaches a more “normal” value. Afterwards (1930-1971) the ratio rises towards the overall mean for the entire 122-year period of 107, which is statistically significantly different from 105.

2 – Seasonal variations

21Monthly values for the sex ratio for the entire period 1850-1971 are shown in Figure 2, with 95% confidence intervals around each monthly value. The sex ratio is significantly greater than 105 for the months of March (108.9), June (108.1), July (107.6) and January (107.5). Only the April value is less than 105, and that just barely so. Differences among the months are not statistically significant, but nonetheless note the difference between March and April. During the sub-period 1930-1971, which contains nearly two-thirds of all births, the month of March has a particularly high ratio of 110.2. For the 1850-1899 period, only the March ratio differs significantly from 105. It is during the period from 1900-1929 that the monthly variations are strongest, with about a 7 point difference between February’s sex ratio and those of May and September.

Figure 2

Sex ratio at birth by birth month, Saguenay, 1850-1971 (n = 419,467)

Figure 2

Sex ratio at birth by birth month, Saguenay, 1850-1971 (n = 419,467)

Source: BALSAC Population Register.

3 – Parental age and birth order

22Except at the ends of the age distribution, the sex ratio varies little with the ages of either parent at the birth of their children (Figure 3). At the ages with the highest fertility, the ratio oscillates around 107 in each case. Because of small numbers of cases, variability in the sex ratio is larger at the youngest ages (less than 20 for mothers and 23 for fathers), and at the oldest (mothers older than 41 years, or fathers older than 45). However, the observed differences are not statistically significant and there is no obvious trend; the same is true for parental age difference (data not shown). When the age gap (father’s age minus mother’s age) is greater than + 12 years or less than –5 years, the ratio fluctuates broadly, but in both directions and the differences are not significant. With respect to birth order (from 1 to 18), we observed values significantly different from 105 for birth orders 1, 2, 5, and 9, though all differences were of small magnitude.

Figure 3

Sex ratio at birth by mother’s age and by father’s age, Saguenay, 1850-1971 (Mothers: n = 311,508) (Fathers: n = 288,349)

Figure 3

Sex ratio at birth by mother’s age and by father’s age, Saguenay, 1850-1971 (Mothers: n = 311,508) (Fathers: n = 288,349)

Source: BALSAC Population Register.

23The additive model based on parental ages and birth orders provides some interesting results (Figure 4). Between about ages 35 and 45, the age of the father appears to have a positive and significant effect on the male proportion. This is opposite to the effect seen for mothers between the ages of about 30 and 37. One can also observe a tendency for the sex ratio to rise with birth order, but these results reflect the period 1850-1899 during which high birth orders were most common in the Saguenay population.

Figure 4

Marginal contributions (m.c.) of parental ages and birth order to the male proportion of births (MPB), Saguenay, 1850-1971 (n = 181,543)

Figure 4

Marginal contributions (m.c.) of parental ages and birth order to the male proportion of births (MPB), Saguenay, 1850-1971 (n = 181,543)

Source: BALSAC Population Register.

4 – Birth intervals

24The relationship between the sex ratio at birth and birth intervals is shown in Figure 5. In total, it appears that shorter birth intervals (less than about 10 months) have a positive effect on the ratio (this also applies to the marriage-to-first-birth interval). For longer intervals, there does not appear to be an obvious trend. A slight rise in the sex ratio was detected at intervals of 30 to 40 months in the 1850-1899 and 1900-1929 periods.

Figure 5

Sex ratio at birth by birth interval, Saguenay, 1850-1971 (second births and higher, n = 284,258)

Figure 5

Sex ratio at birth by birth interval, Saguenay, 1850-1971 (second births and higher, n = 284,258)

Source: BALSAC Population Register.

5 – Sex of preceding child

25Table 2 presents values of the sex ratio for birth order 2 and above by the sex of the preceding child for the entire period 1850-1971. The sex ratio is significantly higher when the preceding child is a boy (p = 0.006). This effect is statistically significant for the period 1930-1971 (p = 0.017) but not for the earlier periods (p = 0.256 for 1850-1899 and p = 0.539 for 1900-1929).

Table 2

Sex ratio at birth by sex of preceding child, Saguenay, 1850-1971

Table 2
Births Sex of preceding child Male Female Males 76,097 70,599 Females 70,651 66,911 Sex ratio 107.7 105.5 p = 0.006 Source: BALSAC Population Register.

Sex ratio at birth by sex of preceding child, Saguenay, 1850-1971

6 – Patronyms

26We have calculated the sex ratio for the 269 most common patronyms (i.e., those with more than 200 birth events during the entire period 1850-1971). In the vast majority of cases (249 of the 269), the ratio is within the 95% confidence interval around 105. Table 3 presents the values for the 20 patronyms beyond the 95% confidence interval. For certain family names, the sex ratio is surprisingly different from expected. In particular, six family names are associated with ratios in excess of 135. The patronym Cyr has the highest value at 147.2. However, these patronyms have little overall influence on the total population since their frequencies are all less than 1,000. Among the more common patronyms, we observed a sex ratio of 110.4 for Côté, comprising more than 8,000 births; it reached 112.5 during 1930-1971 (p = 0.0122, n = 5,258). The ratio for the most common family name in the Saguenay region, Tremblay, was 110.6 (p = .06, n = 5,242) during 1850-1899. In contrast, very few patronyms have a sex ratio significantly below 105. The lowest, at 77.4, is observed for Labonté; however, this family name is not common (204 total births during the entire period 1850-1971). Among other family names with low values for the sex ratio are Brassard (98.6) with 4,434 births and Morin (97.6) with 2,974 births.

Table 3

Sex ratio at birth for selected patronyms, Saguenay, 1850-1971

Table 3
Patronym SRB Number of births p Bernier 122.2 811 0.0315 Bérubé 125.0 657 0.0262 Bilodeau 116.7 1,942 0.0198 Bonneau 135.5 796 0.0004 Boucher 122.7 1,071 0.0113 Bouliane 115.5 1,761 0.0451 Brassard 98.6 4,434 0.0353 Caouette 139.8 271 0.0197 Cormier 83.9 353 0.0350 Côté 110.4 8,114 0.0234 Cyr 147.2 262 0.0070 Dionne 136.0 479 0.0051 Doucet 120.8 914 0.0350 Gilbert 114.9 2,018 0.0432 Labonté 77.4 204 0.0301 Lefebvre 136.0 439 0.0072 Morin 97.6 2,974 0.0465 Munger 122.2 1,209 0.0085 Saulnier 144.5 313 0.0053 Turcotte 117.3 1,358 0.0421 Source: BALSAC Population Register.

Sex ratio at birth for selected patronyms, Saguenay, 1850-1971

IV – Discussion

27In light of the above findings it would be premature to try to identify only one or two factors that could explain the unusually high sex ratio values for the Saguenay population. In fact, there are several candidate variables that could play roles, either independently or jointly.

28The sex ratio of 109 observed for the period 1850-1899 (approximately 48,000 births) is particularly interesting. Henry and Blum (1988) suggest that such a value could be an indication of under-registration of female births. However, Bouchard et al. (1991) find no evidence to support this for the Saguenay. Since the sex of newborns is known in 99.1% of all cases, missing sex attribution is not large enough by itself to account for a difference of this magnitude. Moreover, Henry and Blum used a 70% confidence interval around a mean sex ratio of 105 (Henry and Blum, 1988, p. 47); using a 95% confidence interval, the overall value observed for the 1850-1899 period is still significantly above 105.

29The decrease in the sex ratio observed during the beginning of the twentieth century is consistent with other similar decreases in the ratio in other populations during this same period. Ulizzi (1983) has noted a decrease in the sex ratio over the course of the last century in Italy. Allan et al. (1997) have also observed a decrease in Canada, but only after 1970. Davis et al. (1988) have found significant decreases since 1950 in several industrialized countries. In the case of Canada, Dodds and Armson (1997) believe that the sex ratio has stabilized over the last few decades, which is consistent with the Saguenay data. In fact, it is interesting to note that the decrease observed in the Saguenay population coincides with a period of rapid industrialization in the region (roughly, the end of the nineteenth and the beginning of the twentieth century). However, Vartiainen et al. (1999) emphasize that, for Finland, the decrease in the sex ratio did not appear to be linked to industrialization. Moreover, other than the catastrophic fire of 1870 (Saguenayensia, 1959), the Saguenay population did not suffer any disasters during the entire period from 1850 to 1971, unlike several European populations for which the ratio apparently rose during the two World Wars (James, 1987; Ulizzi and Zonta, 1995; Graffelman and Hoekstra, 2000).

30Monthly fluctuations in the sex ratio reveal particularly high values for January, March, June, and July. These results differ markedly from those observed by Nonaka et al. (1999) in their study of the sex ratio in the French-Canadian population while under French rule (1621-1765). Their study showed slightly lower values for the February-April and May-July quarters, and slightly higher values for the August-October and November-January quarters. Note, however, that comparisons are not perfect because of the quarterly groupings that were used. In a study of postwar Germany, Lerchl (1998) observed significantly higher values for the months of May and December, and lower values for March and October. Other patterns in other populations have been shown by James (1987). In the United States, it appears that the ratio may be slightly higher at the beginning of summer, and lower at the end of autumn (James, 1987). In every case, however, these fluctuations are relatively minor.

31As elsewhere, in the Saguenay region the relationship between the sex ratio and parental ages is unclear. High values for younger fathers observed in some populations (Chahnazarian, 1988) are not seen in the Saguenay, nor does the ratio appear to increase with increasing age differences between parents (Astolfi and Zonta, 1999). However, we note a slight positive influence of father’s age (between 35 and 45) and a negative influence of mother’s age (between 30 and 37) when birth order and parental age difference are taken into account. Slightly higher values of the sex ratio for first and second births are not inconsistent with reports from other populations for which the ratio decreases with increasing birth order (Chahnazarian, 1988; Sieff, 1990). However, we do not observe any negative effect for births of very high orders (10 or higher) even for the nineteenth century when such births were not rare: during the period 1850-1899, when the sex ratio attained its highest values, almost 11% of births were of order greater than or equal to 10.

32The relationship between the sex ratio and short marriage-to-first-birth intervals observed in the Saguenay population has also been observed in the Canadian population of the seventeenth and eighteenth centuries (Nonaka et al., 1998). For first birth intervals of 8 to 10 months, the sex ratio was 110 in that population, compared to 108 for Saguenay (113 for intervals of 8 to 9 months). It appears that biological factors linked to coital frequency after marriage could partially explain this phenomenon (James, 1997a). High sex ratio values for births which occur soon after marriage cannot, however, explain by themselves the difference between the all-birth ratio and the mean value of 105, nor can they explain the trend in the ratio over the period, since for high birth orders the sex ratio is itself also high.

33The sex of the preceding birth could have some influence on the sex ratio in this population. The probability of having a boy is higher when the preceding birth is also a boy. Similar results have been obtained by Biggar et al. (1999) for Denmark. James (1987) gives similar evidence that births of a given sex are more often followed by births of that same sex than of the other sex.

34Analysis of patterns in the sex ratio by family name has revealed some interesting results. Certain patronyms exhibit very high male proportions of births, while others show the opposite. Is there a hereditary component in the propensity to procreate more boys or more girls? Insofar as biological factors probably account for most of the variation in the sex ratio within or between populations, some of these factors could have a genetic or hereditary component that contributes to determining the sex at birth. These factors cannot fully explain the values observed in the Saguenay population, but it could be interesting to further explore such family-specific effects.

Acknowledgements

The authors thank Mario Bourque (Projet BALSAC) and Diane Brassard for their technical support. This study received financial support from the Social Sciences and Humanities Research Council of Canada and from the Fonds pour la formation des chercheurs et l’aide à la recherche du Québec.

Notes

  • [*]
    Groupe de recherche interdisciplinaire en démographie et épidémiologie génétique, université du Québec à Chicoutimi.
    Translated by Robert Chung.
  • [1]
    Unless stated otherwise, the term “sex ratio” will refer to the sex ratio at birth.

Outline

  1. I - Factors influencing the sex ratio at birth
  2. II - Data and methods
    1. 1 - The Saguenay population and the BALSAC Population Register
    2. 2 - Variables studied
    3. 3 - Statistical analyses
  4. III - Results
    1. 1 - Annual variations
    2. 2 - Seasonal variations
    3. 3 - Parental age and birth order
    4. 4 - Birth intervals
    5. 5 - Sex of preceding child
    6. 6 - Patronyms
  6. IV - Discussion

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  • Ulizzi L., L.A. Zonta, 1995, “Factors affecting the sex ratio in humans: multivariate analysis of the Italian population”, Human Biology, 67(1), pp. 59-67.
  • OnlineVartiainen T., L. Kartovaara, J. Tuomisto, 1999, “Environmental chemicals and changes in sex ratio: analysis over 250 years in Finland”, Environmental Health Perspectives, 107(10), pp. 813-815.

Authors

Marc Tremblay
Marc Tremblay, Groupe de recherche interdisciplinaire en démographie et épidémiologie génétique, université du Québec à Chicoutimi, 555, bd de l’Université, Chicoutimi G7H 2B1, Tel: (418) 545-5011, ext. 6554, Fax: (418) 545-5518
e-mail: marc_tremblay@uqac.ca
Hélène Vézina
Louis Houde [*]
  • [*]
    Groupe de recherche interdisciplinaire en démographie et épidémiologie génétique, université du Québec à Chicoutimi.
    Translated by Robert Chung.
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