Local marriages LM versus the fraction of potential available couples Γ for France (panel a), Germany (panel b), Italy (panel c and d), Spain (panel e), and Switzerland (panel f).

The data points (circles) presented here are obtained from the raw data LMi,y and Γi,y representing, respectively, the number of local marriages in the province i at time y divided by the total number of marriages. Details on the manipulation of raw data are provided in the Theoretical Protocol Section. The error circles should be understood with respect to the legend reported in each graph. In order to establish a direct comparison among the states, all the plots were realized dividing the interval of data in 30 bins. For each data set (circles), the best fit(s) (solid line) according to Eq 2 is also provided. As it is clear from the plots, and numerically confirmed by the tables presented in Fig 2, Italy is best fitted by two square root curves that, remarkably, naturally split the country exactly into Northern and Southern regions as reported by Eurostat. The two-color map shown for Italy depicts the definition of Northern and Southern Italy as given by the European Union (Eurostat data) and that perfectly matches results from our data analysis. The black circles, whose sizes mirror the related amount of available data, represent examples of cities whose marriages along the years 2000–2010 have been studied.