Figure 1 shows the trend in inequalities in average income between the 95 departments of metropolitan France since 1966 (black curve). I break down the Theil index to distinguish the inequalities between the eight Ile-de-France departments (light blue curve), the inequalities between the 87 departments in the rest of France (dark blue curve), and the inequalities between the Ile-de-France region as a whole and the rest of France (red curve).
The black curve confirms the results of Bonnet et al. (2021). Inter-departmental inequalities in France have decreased considerably in the last 50 years: the Theil index has declined by nearly 60% between 1966 and 2019. France is situated in the descending part of the Williamson’s curve. Interestingly, by extending the analysis to the period 2016–2019, I show that the Theil index reached a minimum in 2009 and has since increased by 30%. This corresponds to an increase in spatial inequalities for extremely high levels of development, consistent with the work Amos Jr (1988), List & Gallet (1999) or Lessmann (2014).
Based on analysis of the various components of the Theil index, Fig. 1 highlights a specific trend in inequalities between Ile-de-France departments, which have tripled since 1966. These inequalities have followed a U curve, falling from 1966 to 1978 before rising continuously from 1978 to 2019, increasing ninefold over four decades. Their contribution to overall inequalities was non-significant in the 1970s (3%) but rose to 27% in 2019. This trend contrasts with that of (1) inequalities between the Ile-de-France region and the rest of France, and (2) inequalities between departments outside the Ile-de-France, which declined sharply over the period.
To better understand the increase in inequalities between Ile-de-France departments, in Fig. 2I show the trend in relative average income for the Ile-de-France region (dotted black curve) and for its eight constituent departments. Relative average income is the ratio between average income in the department and national average income, a value of over 100 meaning that average income in the department is higher than national average income.
Figure 2 demonstrates the advantage still enjoyed by the Ile-de-France region: the income of an adult living in Ile-de-France is 29% higher than the average income of a French adult. But this advantage has eroded over time, having fallen from 45% in 1966. This downward trend runs counter to the results of Rosés & Sanchis (2018), who attribute the recent rise in regional inequalities in France above all to the divergence of Ile-de-France with the rest of France. This contradiction can be explained by the difference in the variable used: the added value created in Ile-de-France is increasingly redistributed to the rest of France, which increases the difference between the region’s share of added value and its share of income (Davezies, 2008).
Looking beyond the regional trend, Fig. 2 shows strongly divergent situations and trends between departments. For now, I will set aside the case of the Seine-et-Marne department (dotted green curve), which has followed a distinct trend (to which I will return later) and focus on the seven other departments.
Figure 2 reveals the extent of the differences prevailing today. In 2019, Paris had a substantial advantage: its average income was 80% higher than national average income and the highest of any department in France. This advantage stood at 60% in Hauts-de-Seine, 40% in Yvelines, and nearly 10% in Val-de-Marne, Essonne, and Val d’Oise. In contrast, average income in Seine-Saint-Denis in 2019 was 20% lower than the national average, making it the poorest department in France.
Figure 2 also shows the trend in these differences. Echoing the changes revealed in Fig. 1, it demonstrates that the inequalities between the seven departments rose sharply between 1966 and 2019: in particular, the gap between Seine-Saint-Denis and Paris was 100 points in 2019, against just 40 points in 1966. The difference dipped to a low of 30 points around 1980. I refer to this substantial increase in inequalities between the seven Ile-de-France departments as the “Ile-de-France divide” and consider that the phenomenon emerged in 1978, based on the values of the Theil index.
Lastly, Fig. 2 demonstrates that the 2019 departmental income rankings were already in place in 1968 and did not change over the period. Based on this observation, I group the seven Ile-de-France departments into three distinct groups according to their relative positions and their trend between 1966 and 2019. The first group is made up of Paris, Hauts-de-Seine, and Yvelines (in yellow), where relative average income has held steady or increased. The second group comprises Val-de-Marne, Essonne, and Val d’Oise (blue), where relative average income has declined but remains over 100. The last group, made up solely of Seine-Saint-Denis (red), shows a singular trend, with relative average income having dropped strongly since 1970 and currently standing below 100. Seine-Saint-Denis is considered today as the poorest department in metropolitan France, but interestingly this was not the case in 1966. Though the department already had the lowest average income of the seven departments, it was still 25% higher than the national average. Average income has declined steadily since 1970 and dipped below the national average on the cusp of the third millennium.
To better grasp the contrasting trajectories of Ile-de-France departments, I used departmental distributions of income by tax unit to build two series of indicators. In the first, I present relative average income by quantile for each department, echoing Fig. 2. The relative average income of a quantile is the ratio between the average income of the tax units belonging to this quantile in the department and the average income of tax units belonging to this quantile at national level. This series provides information on differences in income, between those considered as the poorest or the wealthiest in each department. In the second series of indicators, I define the national income thresholds to be attained for a tax unit to belong to a quantile, and I then calculate for each department the share of tax units belonging to each of these quantiles. Defining the lower classes as tax units belonging to the 50% of the lowest incomes, a value of over 50% for this quantile means that the lower classes are over-represented in the department. The results for the first series of indicators are shown in Fig. 3; those of the second series are presented in Figure A2 in the Supplementary Information. Both series are available for the 1986–2019 period. This period, right of the dotted black vertical line in Fig. 2, provides information on the sharp rise in inequalities having occurred since 1978.
Figure 3 shows that for each of the six quantiles studied, the average income of Ile-de-France households belonging to these quantiles (black curve) was higher than the national average in 2019. However, the trends differed substantially over the period. While in 1986 the advantage was close to 30% for each of the quantiles, the advantage today differs considerably from one quantile to the next. For the nine deciles with the lowest incomes, the Ile-de-France advantage has declined over time, the decrease being particularly significant for the lower classes (deciles 1 to 5). From 30% above the national average in 1994, their advantage has fallen sharply to just 7% in 2019. In contrast, the income advantage has increased over time for the decile with the highest incomes, particularly for those in the top percentile. The advantage of the upper wealthy classes in Ile-de-France (100th percentile) was 60% in 2019. These results are similar to those observed at the state level in the United States (Gaubert, et al., 2021).
An analysis of the departmental curves in Fig. 3 also clearly reveals that the Ile-de-France divide highlighted in Fig. 2 is based above all on the trend in high incomes. For the lower classes, the differences in trends between departments are small, each department showing zero or slight growth in relative average income until 1995 and then a decrease until 2019. Similarly, for the intermediate classes (deciles 6 to 8), the trends are not as emphatic as in Fig. 2, relative average income declining in Seine-Saint-Denis (in red), Val-de-Marne, Essonne, Val d’Oise (all blue) and in Yvelines, while rising slightly in Paris and Hauts-de-Seine. The Ile-de-France divide becomes much clearer starting with the lower middle classes, with the curves rising sharply for Yvelines, Paris, and Hauts-de-Seine (yellow) and falling for the other four departments. In Seine-Saint-Denis, the average income of the middle classes was close to the national average in 1986 but is 20% lower today. The differences are considerable today regarding the upper wealthy classes of each department, their income being 5 higher in Paris than in the neighbouring department of Seine-Saint-Denis.
Figure A2 sheds further light on the Ile-de-France divide through information on the share of tax units belonging to each social class. This figure is all the more informative regarding the upper wealthy classes. In Ile-de-France, the share of tax units living in the region and belonging to this social class increased by 0.25 points between 1986 and 2019, to 2.25%. In Paris, this share rose by 1.3 points to 4.6%. The increase was even higher in Hauts-de-Seine, climbing 1.8 points to 4.2% in 2019. Overall, Paris and Hauts-de-Seine accounted for just 0.44% of the surface area and 6% of the tax units of France in 2019 but were home to 24% of the tax units belonging to the upper wealthy classes. Meanwhile, in Seine-Saint-Denis, the share of tax units belonging to the upper wealthy classes was already low in 1986 and has since declined further, standing at just 0.37% today.
The Ile-de-France divide in terms of income is even more concerning because it comes hand in hand with increasingly distinct territorial inequalities regarding mortality. To illustrate this growing divide, in Fig. 4I show the trend in life expectancy at birth for the seven Ile-de-France departments and for the Ile-de-France region. Life expectancy is calculated for both sexes combined and is available from 1968 to 2019. The thin lines represent annual values, while the thick lines represent smoothed values.
Figure 4 demonstrates that life expectancy at birth in Ile-de-France was higher than the national average for the entire period, at 84.1 versus 82.8. Seine-Saint-Denis is the sole Ile-de-France department where life expectancy was lower than the national average for the entire period.
Figure 4 also reveals emphatic differences in life expectancy in 2019 between the three previously identified groups of departments. Life expectancy at birth in the departments of Paris, Hauts-de-Seine, and Yvelines (in yellow) was 84.9, compared with 84.1 in Val-de-Marne and Essonne (blue), 83 in Val d’Oise (dotted blue) and 82.7 in Seine-Saint-Denis (red). These departmental rankings correspond to those observed for average income. Interestingly, these clear-cut rankings can be confirmed only since the start of the 2000s as regards life expectancy at birth. Before that, the values for the departments excluding Seine-Saint-Denis were relatively similar, and even slightly lower in Paris, owing notably to the AIDS epidemic, which had a particularly strong impact on the capital city in the 1990s (Jougla, et al., 1990). The departments with the lowest incomes are today subject to a further penalty in the field of health, which, excluding Seine-Saint-Denis, did not exist in the second half of the 20th century. These results had already been demonstrated for all departments in metropolitan France as well as at the state level in the United States (Couillard, et al., 2021).
In another interesting aspect, the Ile-de-France divide does not extend to neighbouring departments, where the trend resembles that of Seine-et-Marne, as shown in Fig. 2. In Fig. 5I present the evolution of relative average income between 1950 and 2019 in Seine-et-Marne and the seven departments surrounding Ile-de-France: Aube, Eure, Eure-et-Loir, Loiret, Marne, Oise, and Yonne.
Figure 5 shows that the trend in relative average income is similar for all these departments. In an initial phase, from 1950 to the start of the 1980s, relative average income increased. Apart from Yonne and Aube, situated to the south-east of the Ile-de-France region, an adult living in these departments had an income superior to that of an average French adult in 1980, while in 1950 this had been true solely for the inhabitants of Seine-et-Marne. In a second phase, starting in the 1980s and continuing today, the average income of all the inhabitants of the departments included in Fig. 5 increased less rapidly than average income in France. As a result, relative average income declined below 100 for all the departments but Seine-et-Marne, with income now once again lower than the national average.