減少、増加、上がる、下がる

Today's interesting lesson has to do with increasing and decreasing.

My wife and I were trying to figure out the different between 減少、増加、上がる、and 下がる last night. In English:

減少: decrease
増加: increase
上がる: increase (go up)
下がる: decrease (go down)

Note: There are other words for increase and decrease in Japanese - but we're focusing on these four today.

Neither of us could really figure out a good explanation for why some things use 上がる and some use 増加 and vice versa. We though about it for a tick, and then I came to this conclusion:

When the subject is singular and the value increases or decreases: 上がる・下がる
When the subject is a singular word composed of many items and the value increases or decreases: 増加・減少

Here's the break down:

Temperature is a singular thing whose value can increase or decrease. You could say:

もうすぐ春に成るから、温度を上がる。

Because it will be Spring soon, the temperature will rise.

Population is a singular idea composed by the number of citizens in a given location. It's one from many. So you would say:

最近人口が減少している。

Recently, the population is decreasing.

My wife agreed this is the likely distinction. It's possible that some more highly educated scholar has already come to this conclusion and written a dissertation on it somewhere. If so - laud his brilliance. For the lay man, I wrote a simple blog entry about it so students who don't spend their time in the dissertation section of their university's library can pick up on this important distinction.