I spent the tail end of last week at the Pontignano conference, which brings together Britons and Italians of different stripes (politicians, journalists, business-people, even some academics) to discuss matters of common interest.

The theme of the conference was “the shock of the new”, and one of the more recent shocks discussed was Jeremy Corbyn. The Italians were, as a rule, fascinated. The Britons (again as a rule) had difficulty concealing their schadenfreude (if they were on the right) or their sense of resignation (if they were on the left).

One interpretation of Corbyn's success (put forward by one of the British participants) was that social democracy in Europe more generally was failing to renew itself, and that Corbyn therefore represented an attempt to reach back into the past.

I'm skeptical of continent-sized generalizations. In this case, however, it does seem as though social democratic parties in Europe are struggling, and have been struggling over the past ten years.

The graph below shows an index based on the simple and population-weighed average vote share for social democratic parties in the old EU15 from 1946 until the end of 2014. The base year for the index is 1970.

Where the two series diverge, it’s because social democracy was under-performing in larger countries (the case for the early post-war period), or under-performing in smaller countries, relative to the situation in those countries in 1970.

The lines along the bottom show whether the social democratic share of the vote increased or decreased in each election.

The graph isn't pretty reading for social democrats. Social democratic performance at the polls has fallen consistently from 2005 onwards.

That’s not just a result of new parties which are harder to classify. If you repeat this exercise for conservative parties, you would see that they are on average above their 1970 baseline.

I'm not sure what the answer is for social democrats. But it seems clear that the renewal which occurred at the end of the nineties has exhausted itself.