The Convergence of the Regional Twinning Rates in Sweden, 1751–1960 Johan Fellman and Aldur W. Eriksson Folkhälsan Institute of Genetics, Population Genetics Unit, Helsinki, Finland weden has, as a whole nation, the oldest contin- shown that maternal age and parity (birth order) Suous population register of births, including twin cannot satisfactorily explain the temporal and and higher multiple maternities, starting in the 17th regional differences in the TWR (Eriksson et al., century. Until the 1920s, the rates of multiple mater- 1996; Fellman & Eriksson, 1993). Recently, the nities in Sweden were among the highest known regional and temporal variations in the TWRs were among Europeans. Strong secular and regional fluc- analyzed in some counties in Sweden and, in the tuations were noted. Some of the eastern counties absence of parity data, the crude birth rate (CBR; showed especially marked decreasing trends. The usually defined as the number of live births per year twinning rate had no clear associations with the per 1000 inhabitants) was considered as a proxy vari- anthropometric and serological data. In this paper we study the temporal and regional variations of the able for parity (Fellman & Eriksson, 2003). The main twinning rate in the 25 counties of Sweden from result stressed previously was confirmed, namely that 1751 to 1960. Different statistical methods were differences in the TWRs cannot be satisfactorily applied in order to test the hypothesis that, irrespec- explained by demographic data on a macro level tive of the initial levels, the twinning rates for the (Fellman & Eriksson, 1987). counties converge towards a common low level. We In this study the temporal and regional variations present and interpret a geometrical model for the in the TWR are considered in the 25 counties of trends of the regional twinning rates. We also Sweden between 1751 and the start of the era of arti- analyze the regional heterogeneity using the ranges ficial reproduction techniques in the 1960s. The data and the coefficients of variation of the regional twin- are pooled for 10-year periods. Several different sta- ning rates. All the methods gave consistent results, tistical methods are applied in order to test the supporting our convergence hypothesis that the hypothesis that, irrespective of the initial levels, the regional differences in the twinning rate are gradu- TWRs for the counties converge towards a common ally disappearing. In addition, this study supports our low level. A geometrical model for the trends in the earlier findings that the regional heterogeneity regional TWRs is presented and interpreted. The cannot be explained by differences in the distribu- range of the regional TWRs during the period 1751 tion of maternal age and parity. We suggest that the to 1960 are also considered. The regional heterogene- convergence may be caused by increased urbaniza- ity of the TWRs is also measured with the coefficient tion and industrialization and by the increased interregional migration of citizens as a consequence of variation (CV). These methods all give consistent of better communications, which lead to the break- results supporting the convergence hypothesis that the ing up of isolates and decreased endogamy. regional variation is gradually disappearing. In addi- tion, this study supports earlier findings that the regional heterogeneity cannot be explained by differ- Sweden has the oldest continuous population records ences in the distribution of maternal age. It is for a whole nation, beginning in the 17th century with suggested that the convergence may be caused by information about twinning or higher multiple mater- increased urbanization and industrialization and by nities. The rates of multiple maternities have been the increased interregional migration of citizens as a among the highest known in Europeans. Strong tem- consequence of better communications, leading to the poral and regional fluctuations have been noted. After breaking up of isolates. A detailed discussion of the the 1920s, an accentuated decrease in the rates of multiple maternities took place in Sweden as a whole; in the 1970s, the twinning rate (TWR) was hardly Received 21 October, 2004; accepted 10 January, 2005. 50% of what it had been 200 years earlier (Eriksson, Address for correspondence: Professor Johan Fellman, Folkhälsan 1962, 1964, 1973; Eriksson & Fellman, 1973; Institute of Genetics, Population Genetics Unit POB 211, FIN-00251, Fellman & Eriksson, 2003). Earlier studies have Helsinki, Finland. E-mail: [email protected] Twin Research and Human Genetics Volume 8 Number 2 pp. 163–172 163 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 26 Sep 2021 at 08:00:42, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1375/twin.8.2.163 Johan Fellman and Aldur W. Eriksson effects of different factors on the TWR as well as a ered for four regions: the city of Stockholm and the map of Sweden with its counties are presented in counties of Stockholm, Gotland and Älvsborg. For the Eriksson and Fellman (2004). county of Älvsborg, the TWR was low (between 10.6 and 14.6 per 1000 for the whole period 1751 to 1960 Materials with exception of 1811 to 1820, when it was 15.9). Demographic data was analyzed for the years 1751 to Careful reading of the published data gives the impres- 1960 in the 25 counties of Sweden. The data were sion that the number of twin maternities for 1811 may pooled for 10-year periods. The data set analyzed contain an error resulting in too high a TWR. A possi- included all the periods for which regional data was ble explanation for this is that for this year, the number available. For some counties, annual twinning data of twins and not the number of twin sets (maternities) were occasionally missing, the whole year for this is registered. The controls, based on other registers, county was therefore ignored. In spite of this, the have not resolved this problem and as a result, no cor- remaining available data was assumed to represent the rections have been made in the database. whole decade. A gap exists in most of the counties for The continuously low TWR indicates that Älvsborg the time series 1774 to 1794 as the parish data were shows no statistically significant trend. In the counties pooled during this period, according to the partition of Gotland, Stockholm and in Stockholm city, the of Sweden into dioceses. The diocese and the county decreasing trends of the TWR were statistically signifi- partition coincide only for the counties Stockholm city cant. The decreasing TWRs for these counties converge and Gotland. Consequently, data for the rest of the towards the low rate which has been observed for Älvs- counties for the period 1781 to 1790 are completely borg since the 1750s. In Figure 1, the trends for these missing. A detailed description of the data is given in four counties are presented. Gotland starts from a Fellman and Eriksson (2003). very high level with an extremely steep slope, Stockholm city and county start from medium high Methods and Results levels with medium steep slopes, and Älvsborg starts Disappearing Regional Differences in the TWRs from a low level with a slightly increasing slope. It is In an earlier study (Fellman & Eriksson, 2003), the also of special interest that Stockholm city and county marked regional differences in the TWRs were consid- are neighbouring regions, urban and rural respectively. Temporal trends for TWR in some counties in Sweden, 1751-1960 27 Gotland Stockholm county 24 Stockholm city Älvsborg 21 18 TWR perTWR 1000 15 12 9 1751-60 1781-90 1811-20 1841-50 1871-80 1901-10 1931-1940 Period Figure 1 The twinning rates for Stockholm city and the counties of Stockholm, Gotland and Älvsborg. Linear trends are also included in the figure. The counties discussed in Fellman and Eriksson (2003) represent counties with different temporal patterns. Counties with high (Gotland), medium (Stockholm city and county) and low (Älvsborg) starting levels are noted. Gotland shows the steepest decreasing trend, Stockholm city and county show moderate, but still statistically significant, decreasing trends. Älvsborg shows a slightly increasing, but not signifi- cant trend. Note also that Stockholm city and county are neighbouring regions, the former urban and the latter rural. 164 Twin Research and Human Genetics April 2005 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 26 Sep 2021 at 08:00:42, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1375/twin.8.2.163 Convergence of Twinning Rates Linear cluster 0.02 0.00 P -0.02 A -0.04 B b (temporal trend) b (temporal I -0.06 -0.08 10 12 14 16 18 20 22 24 a (initial twinning rate) Figure 2 The estimates of the pairs of parameters in the linear trends in the regional TWRs. The approximate linearity indicates a tendency for the disappearance of regional heterogeneity. The correlation between a and b is –.964. The points corresponding to Stockholm city (A) and the counties of Stockholm (B), Gotland (I) and Älvsborg (P) in Figure 1 are located well within the cluster. In this study, all 25 counties of Sweden are consid- parameter pairs should form a cluster of points gath- ered and, using three different methods, analyzed to ered around a line with a negative slope. see whether the TWRs in the counties all converge The points (ai,bi) are distributed in the graph towards a common low level resulting in a gradually (Figure 2) according to the following rules: If the disappearing regional heterogeneity of the TWR. TWR for a specific county starts from a high level, The Linear Cluster then the corresponding point is located on the right- hand side and, accordingly, if the starting level is low, If exact linear trends are assumed in the regional twin- the point is located on the left-hand side of the figure.