Every person after taking just one 101 statistics course tells everyone "correlation doesn't equal causation". But here, in the abstract is clear:
> National-survey evidence on time use and sexual behavior is consistent with the iPhone reducing in-person interactions, increasing pornography use, and reducing sexual frequency.
Not every rich person got an iPhone. The rich people without an iPhone did not had equal amount of less kids.
There are two groups, one has an iPhone, the other has not. The assumption is that two groups are big enough to have equal amount of people from any other group that can explain the decline in fertility, i.e. equal amount of rich/poor, educated, etc.
They can control for this because they know which people had access to the iphone based on the AT&T network coverage.
At the end of the abstract they state the likely explanation of this seemingly spurious correlation:
> National-survey evidence on time use and sexual behavior is consistent with the iPhone reducing in-person interactions, increasing pornography use, and reducing sexual frequency.
"A4 paper sheet-sized ceramic film could hold more than 2 TB of data" Just 2 TB? Makes you appreciate just how much 2 TB of data is.
Also, what's the point of "storing our future" if it is so difficult to read and decode? How would one even know where to look for the data if it's get lost. So besides doing it for fun I don't see the point of this and how it can be ever scaled to be in everyday use.
Off-topic, but anyone knows where to buy such [1] old but not rare (so they are cheap enough) math books? In UK and globally? Is e-bay and perhaps amazon the best place? How to avoid fakes?