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In an attempt to improve our understanding of the factors that affect human twinning, we further developed the models given by Hellin (1895) and Peller (1946). The connection between these models and our own model (“Fellman's law”) were studied. These attempts have resulted in a more general model, which was then applied to data from Åland Islands (1750-1939), Nîmes (1790-1875), Stuttgart (about 1790-1900) and Utah (1850-1900). The product of the mean sibship size and the total twinning rate can be considered as a crude estimate of the expected number of sets of twins in a sibship. The same can be said about the twinning parameter in our model. These estimates are in good agreement. If we consider twinning data only, we obtain the geometric distribution, and log (Nk), where Nk is the number of mothers with k twin maternities, is a linear function of the number of recurrences. Graphically, this property can easily be checked. For sibships containing three or more sets of twins, all four populations show higher values than expected, particularly the populations from Stuttgart and Utah, which data also show poor agreement according to a χ2-test. A more exact model would demand more detailed demographic information, such as distribution of sibship sizes, age-specific twinning rates and temporal variations in twinning.
The osberved number of mothers in Åland with several recurrences of multiple maternities shows a considerable excess over the expected number as predicted by Peller's rule. The parameters in our model can be estimated by the maximum likelihood method and the obtained model fits the data better then Peller's model.
Twinning rates were studied in Swedes, Åland Islanders, Finns, Germans, and Dutch during years of starvation when death rates were two to three times higher than average. In contrast to the situation among some animals, this study suggests that nutrition above a certain threshold is unimportant for human reproduction, including twinning. The twinning rates for these different populations display marked temporal differences, but low values in the twinning rate are not consistently associated with periods of epidemics, famine, or similar nutritional stress. After years of privation and/or separation of spouses, a rapid “catch-up effect” can often be seen in the twinning rates, as well as marriage and birth rates. Psychoendocrine factors and interparental immunological conditions that may be involved in this phenomenon are discussed.
Linear regression models are used to explain the variations in the twinning rates. Data sets from different countries are analysed and maternal age, parity and marital status are the main regressors. The model building technique is also used in order to study the secular decline in the twinning rate. Linear regression technique makes it possible to compare the effect of different factors but the method requires sufficiently disaggregated data.
The rates of human multiple maternities in the Nordic countries were studied from continuous series of data. In the Åland and Åboland archipelagos the parish records for births and baptisms since the 1650's were used. Various sources, some unpublished, in the archives of statistics were used for Sweden (since 1749) and Finland (since 1859) as a whole. Until recently, the rates of multizygotic multiple maternities in isolated island populations in the Åland and Åboland archipelagos have been some of the highest known among Whites (15-20‰). Highly significant temporal fluctuations in the twinning rates were noted. In Sweden, the twinning rate during the last part of 18th century was about twice as high as it was in 1966-70. The triplet and quadruplet rates were about three to four times as high as they are nowadays. There has been a secular decline in DZ twinning. This downward trend set in first in the isolated populations. In Sweden, it started in the 1930's, but in Finland, not until the 1960's. The steep downward trend in the twinning rates is shown to set in about one generation after the break-up of isolation. This can be interpreted as evidence that the changes in matrimonial migration patterns have affected the rates of DZ twinning.
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