When we look at the [facts], it becomes crystal clear that something happened … in the early nineteenth century. … [Up] until approximately 1820, per capita world economic growth [the single best way of measuring human material progress] registered near zero. … Then, not long after 1820, prosperity began flowing in an ever-increasing torrent; with each successive generation, the life of the [child] became observably more comfortable, informed, and predictable than that of the father.
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