grammar - What is the usage of 〜げ and how does it differ from 〜そう or 〜っぽい?

How do these two differ, for example:

寂しそう vs 寂しげ

楽しそう vs 楽しげ

言いたそう vs 言いたげ

大人げ vs 大人っぽい（...？ Not sure if this one works.)

Answer

They are the same ("seems like") but 〜げ has more of a connotation of 「それらしい」 or 「っぽい」with the げ coming from the character 気 as in 気分. I remember it as "that sort of feeling".

Arguably this makes 〜げ more subjective whereas 〜そう is more objective but only so far as the observation is shared with others in the same/similar view point as the speaker.

A word about usage:- You commonly use 〜げ when expressing your feelings. It comes from the kanji 気, so this serves as a reminder. It matters "what" you are talking about, not just that an adjective comes before it.

Examples revised:-

So observing the rule about expressing your feelings.

1A. 寂しそうな顔をしている = OK (with a lonely face)
1B. 寂しげな顔をしている = OK

2A. 楽しそうな雰囲気 = OK (seemingly enjoyable atmosphere)
2B. 楽しげな雰囲気 = OK

3A. 言いたそう = OK (with a desire to say ...)
3B. 言いたげ = OK (NB: this is common, but not widely used with other verbs)

4A. 大人っぽい = OK
4B. 大人げ = (corrected: this is ok as in 大人がある or 大人げない or

Alternatively these can be followed by に, as in 言いたげに見ている (looking with an expression of wanting to say something).

Origins:- Apparently this word has been around for sometime in certain parts of Japan (Ref: http://otasuke.goo-net.com/qa5447963.html?order=DESC&by=datetime > Answer 2), but some consider it to be 若者言葉 that is making the transition to accepted vocabulary (Ref: http://ja.wikipedia.org/wiki/%E8%8B%A5%E8%80%85%E8%A8%80%E8%91%890 + own experience)