Germany is the latest example: its pension commission presented its recommendations in June, tying retirement age to average life expectancy automatically.

As usual, the devil is in the details. In this case, the key word is “average”.

Here, it gets a bit technical: in Europe's pension systems, once you reach retirement age, your contributions are divided into a monthly amount paid out to you until you die. And here comes the snag: to calculate this amount, pension systems use the average life expectancy. However, how long you live is influenced by different factors such as gender, income, or occupation. Depending on those, you could live above or below the average.

The system is blind to these factors, and here hides a crucial inequality: Whether you live to 70 or to 95, each euro you paid into the pension system during your working life bought you the same monthly pension, and those who die early leave their contributions behind to finance the people who outlive them. In the end, those who die younger pay for the pensions of those who live longer.

To get rid of these inequalities completely, we would need to know everyone's exact life expectancy. Since that’s virtually impossible, some unfairness is built in.

Two examples of how this inequality plays out: Since richer people live longer than poorer people do, contributions paid by the poor end up financing the pensions of the rich. And since women live longer than men on average, a higher share of men’s contributions pay for women's pensions than the other way around.

In France, the richest 5% of men can expect to live 13 years longer, on average, than the poorest 5%. Among women, the gap is smaller at eight years.

Among German male pensioners, the richest tenth outlives the poorest by seven years.

The following graph shows how much a person in France gets in pension for each €1 they contribute throughout their lifetime, sorted by their income.

From left to right, all people are ordered by lifetime income, from poorest to richest. The black line shows what each group actually gets back for what they contributed over their lifetime. The red line shows what they would have received if they all had lived the same number of years. The shaded areas highlight what each group wins or loses purely due to a longer or shorter life than average.

As one would expect, women, who live longer, get more from the system: on average, €1.44 for every euro they paid in.

More striking, though, is what happens among men. If everyone lived as long as the average man, a low earner would get back €1.14 for every euro he paid in. However, since a low earner tends to have a shorter life than the average, he collects his pension for fewer years. In the end, he gets €0.96 for each euro he paid into the system.

Pension systems have ways of correcting these inequalities, but they aren't enough to make male low earners break even.

In the current German proposal for pension reform, if the average German lives one year longer, everyone retires eight months later.

The rule treats the average life expectancy gain as if it were everyone's gain. But it isn't.

In Germany the poorest 20% of men gained 1.7 years in life expectancy between 1997 and 2016, as opposed to 3.6 years for the richest. This is not unique to Germany. Denmark, Sweden, and France show similar trends.

If life expectancy keeps rising faster for the rich than for the poor, tying the retirement age to the average life expectancy will only mean more inequalities.