Within the fields of historical demography and family history, a dichotomy has been drawn between western and eastern European marriage patterns and family systems. Expansive discourse on this subject goes back to Malthus, and was further developed by Le Play, Laslett and Hajnal, among others. The last of these drew a conceptual line from present-day St Petersburg in Russia to Trieste in Italy and, based on extant statistical evidence, surmised that people who lived west of this line married late, with a considerable share of the population remaining single for life. East of the line, and elsewhere globally, early marriage was universal.

Hajnal believed that the (western) European marriage pattern, thus defined, had existed for at least the two centuries preceding the Second World War. According to him, the difference in the western and eastern European marriage patterns resulted from different family formation systems. In western Europe, the children of a married couple were supposed to leave the parental home and to form their own household. This form of neo-locality in a nuclear family system required economic independence, which the majority of the population acquired only at an advanced stage in the life course, while for others it was unattainable. East of the Hajnal line, economic independence was no requirement for marriage, as married individuals became part of an extended, patriarchal household. Consequently, in eastern Europe, marriage was early and universal, and multi-generational households were the rule. Although important adjustments and refinements have been proposed regarding the classification of European family systems, the dichotomy between East and West has remained dominant in mainstream scholarship.

Against this background, Mikolaj Szołtysek sets out to study marriage patterns and family systems among the rural population of the eighteenth-century Polish-Lithuanian Commonwealth, which was one of the largest and most populous countries of Europe at the time. Located between the German states in the West and the Russian empire in the East, it occupied territories on both sides of the imaginary Hajnal line, and it had characteristics associated with western European civilisation, such as Western Christianity, but also traits that were typical of eastern European civilisation at the time, such as serfdom. Because of its geographical position, and also because of the preservation of high-quality data from the region, the Polish-Lithuanian Commonwealth offers an interesting case study, for which it is possible to ‘reconstruct’ marriage patterns and family systems, and to test empirically the paradigmatic East-West model.

Szołtysek studies patterns of leaving home, domestic service, marriage behaviour, household size and structure, and living arrangements on the basis of data from preserved nominative censuses and household listings of 155,818 individuals, who lived in 26,654 households and 234 parishes and estates. These data were collected, transcribed, cleaned and stored in the Central European Family Forms Database, which is the empirical foundation of this extensive book. In order to gain a better view of the diversity within this large state, Szołtysek groups the available parishes and estates into 12 regions and four regional clusters, based on the administrative status, geographic proximity and the cultural and regional identity of inhabitants, among other factors.

What truly makes this work a landmark in the field is the application of a life-course perspective in combination with the use of microsimulations. Contrary to former works on marriage patterns and family systems in East-Central Europe, Szołtysek's units of analysis are individuals rather than households or families. He systematically compares observed patterns of home-leaving, co-residence, household size and structure and so on, with the range of possibilities that were feasible in a given demographic regime according to CAMSIM microsimulations, taking such crucial parameters as mortality, fertility and age at marriage into account. The comparison of actual rates and demographically feasible rates obtained from the CAMSIM simulations results in propensity scores, which tell us more about the actual preferences of individuals regarding family formation than the rates themselves, as these do not account for variations in mortality, nuptiality and fertility rates in different regions. This is especially important with respect to patterns of co-residence, as household composition is to a certain degree determined by demographic constraints. Put differently, multi-generational households can only be formed when there are offspring over at least two generations and when grandparents, parents and children do not die prematurely. Controlling for demographic constraints is thus key to understanding behaviour, for low rates of multi-generational households might simply result from high mortality and low fertility, rather than reflecting individual preferences or societal norms and values.

Szołtysek's painstaking analyses show that there was a much larger diversity in family forms and marriage patterns within the Polish-Lithuanian Commonwealth than one would expect on the basis of Hajnal's simple East-West model. Moreover, Szołtysek shows that the differences in marriage patterns within the Commonwealth were not merely caused by variations in economic constraints. The environment and manorial practices also had an effect on family forms, and the same is true of cultural and religious factors. Nevertheless, Hajnal's prediction that there were significant differences in terms of marriage patterns and family forms between East and West is basically confirmed, in the sense that people in the western part of the Commonwealth left the parental home earlier and more often, married at a later age, and tended to form nuclear households, while ages at marriage in the east were significantly lower and multi-generational families were more common. However, the idea of a rigid East-West dividing line is clearly an inadequate description of the historical reality. The Polish-Lithuanian Commonwealth should rather be seen as a land of transition with a high degree of internal diversity in marriage patterns and living arrangements.

This magnum opus will serve in many ways as an example for current and future generations of historical demographers and family historians, and the importance of this work goes well beyond the region of study. It sets an example of how marriage and co-residence patterns can be examined from a life-course perspective, on the basis of preserved census data from pre-modern and modern societies used in combination with microsimulations.