1. Haredim and Corona, a thread.

About 50% of the coronavirus cases in Israel are among Haredim, even though they make up only about 10% of the Israeli population. I've seen a lot written about what's going on, but none that get it right

2. To understand what's going on, you have to realize that you can't understand demographic phenomena without understanding population structure. Another way of expressing this is @nntaleb's famous dictum "Never cross a river that is on average four feet deep"

3. Before I get into it, I'll briefly address other factors. They really do play a role, but not the critical one. Haredi Jews have a lot more communal activities than the secular population, e.g. communal prayers three times a day and studying in pairs, arguing over the Talmud

4. Enormous weddings and funerals of public figures are commonplace. Moreover, the Haredi public is largely cut off from the general public. Internet usage is very low, they have their own newspapers, their own popular culture

5. Early on, one of the most important rabbis was asked whether the yeshivas (places of Jewish learning) should be closed down because of the coronavirus. He answered "of course not!" It turned out later, that he had never heard of the coronavirus. He later reversed his ruling

6. In addition, the Haredi sector is the poorest sector of Israeli society. They live in the most crowded conditions in Israel. Haredi Bnei Brak, not secular Tel Aviv, is the most densely populated city in the country. It's very hard to self-isolate with 10 people in 4 rooms

7. All of these things are factors, but not *the* factor. The biggest factor is that the Haredi population is very structured, meaning that there are many sub-populations, and a structured population behaves very differently from a non-structured population

8. To illustrate the importance of structure, let's look at fertility. If you had a population with a TFR (total fertility rate) of 2.0 you would guess that it would slowly decline, because replacement rate is 2.1, right? Wrong!

9. This would only be true if there is no population structure, which is almost impossible in the real world. For example, if your population was made up of two sub-populations, one with a TFR of 1.0, the other 3.0, the TFR would be 2.0 on *average*.

10. But the population would increase. Let's look at what's going on. Start with 200 people total, 100 in each sub-population:

Population 1: 100, 50, 25

Population 2: 100, 150, 225

Total: 200, 200, 250

After two generations the population has grown by 25%!

11. Even taking into account that 5% die before being counted (that's where the 0.1% comes in) that's still some hefty population growth, considering that the average TFR is 2.0! Why am I talking about population growth?

12. Because population growth is geometric in the same way that the spread of the coronavirus is geometric. You have to take demographic structure into account. Averages between sub-groups are meaningless. You have to divide your population into *non*-structured groups.

13. You can get meaningless results just by putting your boundaries in meaningless places. For *this* purpose, putting all Haredim into one group is meaningless

14. Yikes! I pushed "tweet all" by accident!

15. It so happens that one of the earliest big successes of fighting the coronavirus in Israel was in a Haredi community. Kiryat Yearim is a Haredi community that was hit hard early on, and succeeded in bringing the new case level down to zero. https://en.globes.co.il/en/article-kiryat-yearim-successfully-kicks-coronavirus-out-1001324115

16. Kiryat Yearim was *more* strict than the Israeli guidelines. But at the same time, other Haredi communities were ignoring the guidelines. What is going on?

17. What is going on is that the Haredi community is skeptical of the Israeli government, in general, and they follow their own leaders rather than the secular leaders. But there are many, many Haredi leaders. The Haredi community is to a large degree many little communities

18. Each community has its own leaders. Some of those leaders were stricter than the general population. But *some* of those leaders ignored or resisted the lockdown. And that made all the difference