Every so often, data emerging from social surveys stop me in my tracks. I look at a certain number, or proportion, and just know something is awry. It defies logic — it cannot be. There must be a sampling or computational error. So I review it and check it, and check it again. And then, suddenly, the penny drops. There is no error. That bizarre anomaly —that inconceivable reality — is, in fact, entirely accurate.

I had this experience some time ago when I first started studying Hungarian Jews. And I was reminded of it recently as I was working on a survey for the European Union looking at Jewish people’s perceptions and experiences of antisemitism. Over the past few months, my team has been gathering data in 13 EU member states, including Hungary, and carefully monitoring response levels every 12 hours to continually assess the representativeness of the sample in each country.

One of the ways we have done this is by looking at age distributions — the proportions of respondents in each age band — because we know, from existing research, what these should look like in most cases.

But the age distributions of the Hungarian Jewish population are known to be odd. There’s one particular age-band that has long appeared to be abnormally small. And sure enough, in this study, we saw it again. About 20 per cent of all respondents were aged in their 40s and another 20 per cent were in their 60s, but only ten per cent were in their 50s. It’s not unusual to see some slight differences in the proportions of different age-bands in a population – indeed, it’s completely normal. But not like that. For one ten-year age band to be half the size of the two ten-year age bands either side of it, there are only three likely possibilities: something has gone wrong with the fieldwork, or with the maths, or something rather unusual has happened to that cohort.

I know now that it’s not a fieldwork or mathematical mistake. So what is it? Fifty-somethings in 2018 would have been born sometime between 1959 and 1968. Did anything happen in Hungary during that decade that could explain why that cohort was so much smaller than the two cohorts either side of it? Quite a few Hungarian Jews fled the country after the Hungarian uprising in 1956; certainly, the population declined over the course of the following decade. But that migration didn’t involve one very particular age band — it affected Jews of different ages. That said, younger people were more likely to migrate than older people — that’s often the case — so it’s reasonable to assume that birth rates might have dropped somewhat in the decade after that. But by half? And then to see birth rates recover in the following decade to the levels they were before that? No. That cannot explain it completely.

So what else might be involved? Assuming parents are aged about 25 years-old when they have children— a reasonable assumption in this context — the parents of children born between 1959 and 1968 would have been born between about 1934 and 1943. When the Nazis invaded Hungary in March 1944, this cohort was aged 10 or under.

Between May and July 1944, 424,000 Hungarian Jews were deported to Auschwitz. Ninety per cent of those arriving there at that time were murdered immediately. But young children fared even worse. Few, if any, survived. And that’s the key explanatory factor. The main reason why there are so few 50-something Hungarian Jews today is because the people who would have been their parents never reached adulthood. The Nazis murdered almost all of them.

The cohort above them — teenagers during the war— were a little more likely to survive and go on to have children. And the cohort below them, born immediately post-war, were the children of 20- and 30-somethings during the war, who also had an above-average survival rate.

History often leaves its traces in unexpected ways. The horrors of the Shoah can still be seen clearly in the age distributions of contemporary Hungarian Jews — even those born years after the war ended. And the effects remain devastating. Fifty-somethings are typically the age group that takes on many of the most senior leadership positions in the community. But in Hungary right now, there are desperately few of them. There’s a vacuum. And that is a direct result of what the Nazis did, three-quarters of a century ago.

Jonathan Boyd is Executive Director of the Institute for Jewish Policy Research (JPR)