Most of its population live/lives in cities or close to them.
One third of its population is/are farm workers.
NOT A TEACHER
(1) I cannot answer your question with confidence, but I can share some of my research:
(a) Mesdames Celce-Murcia and Larsen-Freeman's admirable The Grammar Book (1983 edition, pages 38- 39) tells us that:
Fractions and percentages take ... either the singular or the plural ... when they modify a collective noun.
The two scholars give these examples:
One tenth of the population is Christian/ are Christians.
Ten percent of the population is Christian/ are Christians.
(2) Can we conclude, therefore, that the "correct" sentence would be:
One third of its population are farm workers.
(3) Regarding your first sentence, I dare not venture a guess, but I shall offer these items:
(a) I did some googling, and I noticed examples of "Most of the
population work/works ...."
(b) Back in 2006, I copied this sentence from the Los Angeles Times (supposedly one
of our better newspapers): "50% of the population lives in poverty."
(c) I once heard a well-known (and well-educated?) TV journalist say this:
"80% of the population disagrees."
(d) I once copied this sentence from a Los Angeles Times columnist: "The overwhelming majority of Third World inhabitants still lives outside urban centers." (I guess that we can say that "majority" = "most.")
(3) I shall sit back and wait for the teachers to join the discussion. Thank you for your great question.
