Spatial variation in the twinning rate in Sweden, 1751-1850.

Strong geographical variations in the twinning rate have been presented in the literature. In general, the rate is high among people of African origin, intermediate among Europeans and low among most Asiatic populations. In Europe, a progressive increase has occurred in the twinning rate from south to north, with a minimum around the Basque provinces. The highest twinning rates in Europe have been found among Nordic populations with the exception of the Saamis (Lapps). Within larger populations, some small isolated subpopulations have been identified to have extreme, mainly high, twinning rates. In this study, we investigated the regional variation of the twinning rate in Sweden. We analysed twinning rates for different counties for the period 1751-1850. From the middle of the 19th century, the environmental and genetic differences have decreased and the regional twinning rates have converged towards a common decreased level. The models applied have the geographical coordinates as regressors. The optimal model for the twinning rate has the longitude, the latitude and their product as regressors, indicating both horizontal and vertical trends. According to this model, the maximum twinning rate in Sweden is located in the eastern part of central Sweden, mainly on the island of Gotland and in the counties around Stockholm. Relatively low twinning rates are seen in the western and northern parts of Sweden.