TIL that millennials in the ages of 18-34 are more likely to live with their parents instead of a spouse/partner - the first time this has occurred since record-keeping began in the 1880s

Various governments, universities, non-profits, and other entities have been recording data like this for centuries... It just wasn't widespread for a long time. So it while there may be data from say, the 1850s, it would be disingenuous to compare the modern data to that data because that 1850s data may far less complete/all encompassing, for example, relative to more modern data which would would be far more exhaustive.

The reason you see this kind of date for the data being so common (particularly with regard to the US) is because the turn of the century, the late 19th early 20th century, saw the emergence of tons of federal agencies that started officially tracking this data nationally... Things like the Census Bureau, the Bureau of Labor Statistics, Department of Labor, Department of Commerce, etc... They all started ~1880-1920, but they didn't just appear out of thin air, there were already state agencies and nascent unofficial federal agencies doing that before they became official.

The same is true for climate data and the like, around the same time governments and universities started officially collecting data globally, where most climate data the proceeding centuries was done only in some regions and usually unofficially by groups such as the British East India Company as it related to their businesses/record keeping/risk management, etc.

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