Mixed marriages MM (that is involving a native citizen, either male or female, with an immigrant) versus the fraction of potential couples Ω.

Here the raw data used to build the points are MMi,y, which is the number of marriages between native citizens and immigrants in the province i at time y divided by the total number of marriages (defined as before) in the province i at time y, and Γi,y, that is the number of mixed couples (here the number of native citizens times the number of immigrants) in the province i at time y divided by the square of the sum of the two group of people in the province i at time y. The error circles should be understood with respect to the legend reported in each graph. To optimize the visualization of the trends, the plots were realized dividing the interval of data of each state in bins of the following numbers (we write the name of the country followed by the amount of bins used): Germany 25, France 21, Italy 35, Switzerland 21, Spain 27. The red lines in the plots are the best fit functions; we refer to Fig 9 for the related R2. With respect to Fig 1, here the different countries do not show the same behavior: France and Germany follow a linear behavior, which is derived from a non-interacting particles approach, while Spain and Switzerland maintain a square-root trend. Italy displays a mixed behavior which highlights even more the difference between the north and south of the country: in fact, the north is best fitted by a square root function while the south by a line.