When young adult women born between 1954 and 1973 left the parental home in Italy and Spain they did so to get married, while in Sweden and Germany the majority started to live single. In France, just over half of them started a union, either married or not. Looking at those who made a first transition into a union, almost all in Sweden started to cohabit, and so did a majority in France and about half in Germany, while in Italy and Spain marriage was the obvious choice. Although women born more recently may behave differently, we follow Nazio’s extensive analysis of the Family and Fertility Surveys (FFS) collected in the 1990s (coordinated by the UN’s Economic Commission for Europe) to see how this geographical variation can be explained. Nazio starts from the key idea that the emergence of cohabitation is the outcome of an individual-level decision-making process, whereby individuals are influenced by their knowledge about others’ previous experiences with that behaviour. Diffusion theory is thus connected with an empirical application to help understand social stability and changing union behaviour: what drives the diffusion process in selected countries with different family traditions and institutional contexts, and what forces might lead to divergence or convergence over time?

Also related to women’s educational and labour force expansion, as well as secularization and urbanization, family living has undergone profound transformations over the past decades: marriage postponement, rise in cohabitation, lower and later fertility and increasing divorce rates, which have impacted the prevalence and meaning of marriage. Currently, cohabitation, a co-residential union of two partners in an intimate relationship without being legally married, has become much more prominent. All these social and normative changes may have long-term consequences for the organization of family living, for inter-household and intergenerational relationships and for fertility rates, welfare provision and poverty.

Nazio’s fundamental assumption underlying social diffusion is that the adoption of innovative practices is primarily a learning process. She believes that comparative static analyses cannot easily reveal causal processes, and therefore developed a micro-sociological dynamic framework to bridge the macro–micro divide. Cohabitation is seen as an innovative behavioural option for entering a union when it emerges within a national context, i.e. via its adoption by previous cohorts and by peer groups. More experience leads to less deviant and socially more accepted innovations. Mass media play an important role in its dissemination, and so in its awareness.

The core finding of the study is that cohabitation is indeed ‘contagious’, diffusing through the enacted behaviour of others. The shape of this influence varies across countries, reflecting their institutional contexts and their stage in the diffusion process. In the long run, social approval may prevail over individually perceived advances in adopting cohabitation. It may even become the norm when it also appeals to those who normally would not have had a strong preference for cohabitation: the higher the rate of cohabiting unions, the faster the change in social norms.

Hence, the start of the diffusion process of cohabitation seems to be driven mainly by peers adopting this behaviour, but the influence of peers diminishes as the stage of diffusion advances. Later, the adoption of cohabitation seems rather influenced by an acquired abstract knowledge. The faster the process in turning cohabitation into an accepted alternative to marriage and the more widely institutions follow up on this change, the lower the effect of direct social modelling.

Living single, thus having already overcome housing obstacles and having reached some degree of economic autonomy, has everywhere—and even more so in Spain and Italy—a significant positive effect on entry into cohabitation.

Why has cohabitation risen much more slowly in southern Europe? It turns out not to be an attractive option under welfare regimes such as those of Italy or Spain. That may be related to the fact that young adults have great difficulties in finding suitable accommodation there. The cultural and institutional setting does not provide fertile ground for the diffusion of living arrangements with a lower degree of commitment than marriage. Only a small and highly selective group of early adopters have good reasons to break with traditional gender roles and family models.

Nazio presents a wealth of information in a concise and straightforward manner. Although the book may focus on diffusion theory, she also provides very detailed (demographic) information on the geographical variation of cohabitation. The FFS has enabled us to make innovative advances in theoretical and practical knowledge, by comparing various countries. Nazio also observes that cohabitational unions are less stable: maybe due to (1) a process of self-selection into cohabitation of those individuals whose partnerships originally have a high risk of disruption; (2) higher risks of mismatch between partners; and (3) lower legal barriers and financial costs to exit the union. Subsequently, she argues for a progressive reduction in the legal distinction between marriage and cohabitation, in order to avoid the risk that cohabitation (compared with marriage) makes the already weaker spouse’s position even weaker while only partially compensating for the investments made in reproduction of the family at the price of economic and career sacrifices.

It would have been helpful if Nazio had repeated her summary table of the expected effects and magnitudes of her 12 hypotheses (Table 4.2 on p. 97) with a similar one at the end of the book giving the observed test results. But even without it, this book is an eye-opener not only for those who would like to better understand the demographics of cohabitation, but also for those who are mainly interested in the nature of behavioural diffusion processes and their various stages: in the first stage cohabitation emerges as a rare and deviant practice of a selected group of forerunners; in the second stage cohabitation becomes a more widely practiced form of living arrangement (a kind of premarital living arrangement); in the third stage cohabitation is well established and constitutes a non-deviant institutional form of union and in the fourth stage cohabitation is the largely prevalent norm, also with respect to the birth of children.