Spanish Flu killed millions...............
OK, it was higher in the “Dark Ages” than it was in 1915.
Also... there were only about 200 days during which you were ALLOWED to work, you couldn’t possibly labor beyond the length of the day, teeth indicate a better diet than in 1920.
For good or for ill, he population was held in check by a long infancy before weening, a huge celibate caste, a lack of personal privacy and a large number of chaste fasts.