user7792: Human population is increasing whether we like it or not and might even hit 10bn in the next few decades. But why is this happening according to a Buddhist's perspective? Some that I know that don't believe in kamma use this as a counter example saying that if people are becoming more sinful they ask how come the population is increasing. While I have my own understanding (devas being born back among humans, kamma alone doesn't determine the next birth "cuti sitha" does) I would like your expert opinion this.

user7049: here's an interesting perspective on this from the Pali Canon, which implies that it's rather when human morals are high that the population is growing, not the other way round
superstitious quite a bit

Depopulation
Then a certain affluent brahmin approached the Blessed One … and said
  to him:
“Master Gotama, I have heard older brahmins who are aged, burdened
  with years, teachers of teachers, saying: ‘In the past this world was
  so thickly populated one would think there was no space between
  people. The villages, towns, and capital cities were so close that
  cocks could fly between them.’ Why is it, Master Gotama, that at
  present the number of people has declined, depopulation is seen, and
  villages, towns, cities, and districts have vanished?”
(1) “At present, brahmin, people are excited by illicit lust, overcome
  by unrighteous greed, afflicted by wrong Dhamma. As a result, they
  take up weapons and slay one another. Hence many people die. This is a
  reason why at present the number of people has declined, depopulation
  is seen, and villages, towns, cities, and districts have vanished.
(2) “Again, at present people are excited by illicit lust, overcome by
  unrighteous greed, afflicted by wrong Dhamma. When this happens,
  sufficient rain does not fall. As a result, there is a famine, a
  scarcity of grain; the crops become blighted and turn to straw. Hence
  many people die. This is another reason why at present the number of
  people has declined, depopulation is seen, and villages, towns,
  cities, and districts have vanished.
(3) “Again, at present people are excited by illicit lust, overcome by
  unrighteous greed, afflicted by wrong Dhamma. When this happens, the
  yakkhas release wild spirits. Hence many people die. This is yet
  another reason why at present the number of people has declined,
  depopulation is seen, and villages, towns, cities, and districts have
  vanished.”
“Excellent, Master Gotama! … Let Master Gotama consider me a lay
  follower who from today has gone for refuge for life.”

Paloka sutta (AN 3.56)

user254: Here is not an expert opinion: which is that I don't know why you expect that the total number of sentient beings is finite, countable, and constant.
The perspective in the OP seems to be that the total number is constant, and is asking, "If the number of humans is increasing, then where (which realms) are they (pre-existing beings) coming from and why?"
It has a view of, "this used to be (countably) one old sentient being, and that (same) being is now being reborn as (countably) one new sentient being" ... it has a view that, the type of being and/or realm might change (hell, animal, human, deva), but the total number of beings must be constant.
I don't see why you'd expect that view to be true, though: it seems to me an atman (self/soul) view, not an anatman view.
So far as I know a more anatman view might be look "there's no person now" (there are are just skandhas, an assemblage of parts, c.f. the parable of the Milinda's chariot), "the number of sentient beings isn't countable" (except perhaps to the Buddha), and fabricated things are inconstant.

7792: <p>Human population is increasing whether we like it or not and might even hit 10bn in the next few decades. But why is this happening according to a Buddhist's perspective? Some that I know that don't believe in kamma use this as a counter example saying that if people are becoming more sinful they ask how come the population is increasing. While I have my own understanding (devas being born back among humans, kamma alone doesn't determine the next birth "cuti sitha" does) I would like your expert opinion this.</p>
 7049: <p>here's an interesting perspective on this from the Pali Canon, which implies that it's rather when human morals are high that the population is growing, not the other way round</p>

<p>superstitious quite a bit</p>

<blockquote>
  <p><strong>Depopulation</strong></p>
  
  <p>Then a certain affluent brahmin approached the Blessed One … and said
  to him:</p>
  
  <p>“Master Gotama, I have heard older brahmins who are aged, burdened
  with years, teachers of teachers, saying: ‘In the past this world was
  so thickly populated one would think there was no space between
  people. The villages, towns, and capital cities were so close that
  cocks could fly between them.’ Why is it, Master Gotama, that at
  present the number of people has declined, depopulation is seen, and
  villages, towns, cities, and districts have vanished?”</p>
  
  <p>(1) “At present, brahmin, people are excited by illicit lust, overcome
  by unrighteous greed, afflicted by wrong Dhamma. As a result, they
  take up weapons and slay one another. Hence many people die. This is a
  reason why at present the number of people has declined, depopulation
  is seen, and villages, towns, cities, and districts have vanished.</p>
  
  <p>(2) “Again, at present people are excited by illicit lust, overcome by
  unrighteous greed, afflicted by wrong Dhamma. When this happens,
  sufficient rain does not fall. As a result, there is a famine, a
  scarcity of grain; the crops become blighted and turn to straw. Hence
  many people die. This is another reason why at present the number of
  people has declined, depopulation is seen, and villages, towns,
  cities, and districts have vanished.</p>
  
  <p>(3) “Again, at present people are excited by illicit lust, overcome by
  unrighteous greed, afflicted by wrong Dhamma. When this happens, the
  yakkhas release wild spirits. Hence many people die. This is yet
  another reason why at present the number of people has declined,
  depopulation is seen, and villages, towns, cities, and districts have
  vanished.”</p>
  
  <p>“Excellent, Master Gotama! … Let Master Gotama consider me a lay
  follower who from today has gone for refuge for life.”</p>
</blockquote>

<p><a href="https://suttacentral.net/en/an3.56" rel="nofollow">Paloka sutta (AN 3.56)</a></p>
 254: <p>Here is <em>not</em> an expert opinion: which is that I don't know why you expect that the total number of sentient beings is finite, countable, and constant.</p>

<p>The perspective in the OP seems to be that the total number is constant, and is asking, "If the number of humans is increasing, then where (which realms) are they (pre-existing beings) coming from and why?"</p>

<p>It has a view of, "this used to be (countably) one old sentient being, and that (same) being is now being reborn as (countably) one new sentient being" ... it has a view that, the type of being and/or realm might change (hell, animal, human, deva), but the total number of beings must be constant.</p>

<p>I don't see why you'd expect that view to be true, though: it seems to me an atman (self/soul) view, not an anatman view.</p>

<p>So far as I know a more anatman view might be look "there's no person now" (there are are just skandhas, an assemblage of parts, c.f. the parable of the Milinda's chariot), "the number of sentient beings isn't countable" (except perhaps to the Buddha), and fabricated things are inconstant.</p>
