Temporal Trends in Migration in the Åland Islands: Effects of Population Size and Geographic Distance

This is the published version. Copyright 1994 Wayne State University Press.Using a model developed by Relethford (1992), we assess temporal trends (1750-1949) in marital migration in the Aland Islands, Finland, in relation to both geographic distance and population size. The 200-year time period was divided into four 50-year periods. For all time periods both geographic distance and population size are important determinants of migration among 15 Lutheran parishes. The geographic distance parameter of the model decreases significantly over time, and the population size parameter fluctuates slightly but shows no significant change over time. For all time periods migration is negative density dependent, indicating that there is greater relative flow from larger to smaller subdivisions. Even though both the geographic distance and population size parameters are statistically significant, the analysis suggests that geographic distance has a greater relative effect on migration than population size. There is a clear indication of isolate breakdown during the last two time periods (1850-1899 and 1900-1949). Residual analysis indicated that the smallest parish (Sottunga) was a major outlier that showed greater exogamy (less endemicity) than expected from the model

Infant Mortality Patterns in Åland, Finland

This is the published version. Copyright 1983 Wayne State University Press.Infant mortality in Åland, Finland, from 1751 to 1935 is examined. The 18th and early 19th century rates in Åland were more characteristic of Eastern European populations than Western Europe or England. A steady decline in the mortality rates and a reduction in the year-to-year variation began about 1810. This decline is linked to medical innovations, decreases in household and family size, and socioeconomic changes. Stillbirth ratios appear to be relatively high and stable from 1751 to 1935 (ranging from 17 to 37/1000). Regional variation in infant mortality rates in the Åland archipelago does not appear to be shaped by either geographic location of the parishes or population density. Data are provided for causes of death during the neonatal and postneonatal periods. A time series analysis demonstrated that high birth rates tend to co-occur with periods of high infant mortality. Factors that may have influenced the infant mortality rates include household size and complexity, twinning rates, and breast feeding patterns

Weinberg’s Differential Rule Reconsidered

A central problem in research on twins is the estimation of the rates of monozygotic and dizygotic twin maternities. The estimation usually follows Weinberg’s differential rule. According to this rule, the rate of dizygotic twinning is twice the rate of twin maternities in which the twins are of opposite sex. The monozygotic twinning rate is the difference between the rates of same-sex and opposite-sex twin sets. Weinberg’s differential rule is implicitly based on the assumptions that the probability of a male equals the probability of a female and that the sexes in a dizygotic twin set are independent. Although Weinberg’s differential rule has been the target of continuing discussions, the reliability of these assumptions has never been conclusively verified or rejected. In this study we present new variance formulas for the monozygotic and dizygotic twinning rates obtained using Weinberg’s differential rule and stress that these new formulas have to be used.We analyze the accuracy of Weinberg’s differential rule by considering alternative attempts. Especially, we build a general likelihood function and show that the maximum- likelihood estimates differ only slightly from the rates obtained using Weinberg’s differential rule. In addition, our methods are applied to twinning data from Finland and Sweden. We compare our results with findings in the literature. In conclusion, our findings indicate that Weinberg’s differential rule is rather robust and that despite its simplicity, it gives reliable results when official birth registers are analyzed

Biometric Analysis of the Multiple Maternities in Finland, 1881—1990, and in Sweden since 1751

Hellin’s law states that if the twinning rate is w, then the triplet rate is w^2, the quadruplet rate is w^3, and so forth. The opinion of today is that Hellin’s law holds only approximately. In this study the inaccuracy of Hellin’s law is studied and the discrepancies are explained mathematically. In our earlier studies we built linear models for the twinning rate. Because most of the mothers are younger than 40 years of age and because in this age interval the twinning rate depends linearly on age, linear regression methods have been applied. Hellin’s law suggests using the square-root transformation of the triplet rate r. Statistical arguments speak in favor of using the arcsin √r transformation. We discuss both transformations. Despite the fact that Hellin’s law is only approximate, the arcsin transformation proves valuable. The transformed triplet rate can be modeled in a way similar to the twinning rate. We consider secular data from Finland for 1881-1990 and from Sweden since 1751. Using Hellin’s law, we compare the triplet rates and the twinning rates and study the time trends of the observed twinning and triplet rates. The data are standardized. Our theoretical results are applied to multiple maternity data for Finland. Using maternal age as the regressor, we build a linear model for the twinning rate and for the arcsin-transformed triplet rate. This analysis shows a decreasing linear time trend in the triplet series for the period 1881—1950 but not in the twinning series. The triplet rate has an increasing trend after 1960, which seems to be mainly caused by artificial induction of ovulation

Demographic Analysis of the Variation in the Rates of Multiple Maternities in Sweden Since 1751

To elucidate the causes and mechanisms of twinning and higher multifetal maternities, we have taken advantage of the statistical sources of Sweden, where continuous statistics for the whole population are the oldest available. We found strong secular and regional fluctuations. The rates of multiple maternities were the highest during the last three decades of the 18th century, when the twinning rate was more than 17 per 1,000, the triplet rate was more than 3 per 10,000, and the quadruplet rate was almost 7 per 1 million maternities. During 1849–1873 the twinning rate in Sweden was 14.2 per 1,000, but this rate showed great regional differences, being 18.0 per 1,000 on the island of Gotland and 12.6 per 1,000 in the county ofA¨ lvsborg. During this period the twinning rate in the countryside in the county of Stockholm was 20.4, but in the city of Stockholm it was only 14.1 per 1,000. In Sweden after the 1930s there was a marked decrease in the twinning rate, which by the 1960s had fallen to only about half of what it had been two centuries earlier. The corresponding reductions for triplet and quadruplet rates were about 75%. The aim of this paper was to study the temporal and regional variations in multiple maternities in Sweden from 1751 to 1960 based on demographic and some socioeconomic data for the counties. We confirmed our earlier studies that maternal age and parity cannot satisfactorily explain the secular and regional differences in the twinning rates. In contrast to studies in France (1901–1968), we found no unequivocal association between the twinning rates and the crude birth rates. The correlation coefficients between the twinning rate and the crude birth rate showed statistically significant regional and temporal variations. After eliminating the temporal trends, regional differences in the correlation coefficients remained. The twinning rates for the counties seem to converge toward a common low level, 10–12 per 1,000. The observed convergence toward relatively similar levels may be caused by the increased matrimonial migration distances and decreased endogamy of the citizens as a consequence of better communications. The increased urbanization and industrialization that started in the last decades of the 19th century broke up the old static agrarian isolates and caused Sweden, within 2–3 generations, to develop from a poor nation to one of the most prosperous in the world. A more urban and affluent lifestyle, a better diet, and increased stress and sedentary occupations may have reduced the physical capacity of mothers to carry gestations with multiple embryos or fetuses to completion

Correlations Between Live and Stillbirth Outcomes in Twin Pairs

A relationship has been proposed to exist between individual outcomes (live or stillbirth) of twins in the same set. Here, we analyze this association between live births and stillbirths among individuals in different twin pairs. When national birth registers are analyzed, individuals in opposite-sex twin sets can be identified and the correlation between individual outcomes estimated. However, full information about the individuals in same-sex twin sets is not, as a rule, available, and consequently, correlation coefficients cannot be estimated, but upper and lower limits of the correlation coefficients can be obtained. The methods introduced here were applied to data from Sweden (1869–1967), the Åland Islands (Finland) (1750–1949), the Kingdom of Saxony (1881–1900), and England andWales (1940–2003). Comparisons between the correlation coefficients among opposite-sex twins and the lower bound (minimum) of correlation coefficients among same-sex twins indicate that in all populations studied a stronger association exists between twins in same-sex rather than opposite-sex twin sets or pairs. For opposite-sex twin sets no general association between the correlation coefficient and the stillbirth rate was identified