Are there any studies on what motivates people to learn an international auxiliary language? (The focus of the answers should be on the motivation to learn them, not on the motivation to construct them.)
Even in the case of Esperanto it is hard to find good sources, so I doubt there are any such studies about other IALs. In any case, Tazio Carlevaro (1989:179-180) lists three main types of motivations for learning Esperanto, that can be divided into a few subcategories:
Motives related to character: "an attitude inclined favorably towards education"
Motives related to language: "The Esperantists are people interested in language and culture [...]. They are often polyglots for whom Esperanto is the ultimate linguistic esperience."
Motives related to the movement
3.1. Internal motives
3.1.1 Pragmatic motive: "I'm interested in a language like Esperanto"
3.1.2 Sympathetic motive: "I'm interested in the ideas which the Esperanto movement carries, and for this reason also in the Esperanto movement itself"
3.1.3 Motives of factual and symbolic gain (prestige): "Through Esperanto movement I am looking for a prestigious position, either within the movement itself, or within the society I live in"
3.2. External motives: ("Esperanto is very useful, because it is important socially and economically attractive"). Carlevaro asserts that this motivation is probably the weakest in the case of Esperanto.
Carlevaro, Tazio (1989). Planned auxilary language and communicative competence. In: Schubert & Maxwell (eds.), Interlinguistics: Aspects of the Science of Planned Languages
Start with this: European Identity, Zlatko Tisljar Assoc. for European Consciousness Maribor 3/8'11 http://www.debatingeurope.eu/2011/08/03/should-esperanto-be-the-language-of-europe/#.V53JpLh95hF
Language is a package (communication part) of cultural (+) IDentity.
For cross-cultural communication we need a Neutral language that is not forced by commercial gains and is fair on all sides.
Soviets & Yugoslavia were unaware of role of language in identity, & they tried to develop Soviet/Yugoslav language of largest population; it did work as language of communication but could not be accepted as language of communal identity, because it wasn’t neutral. For this reason, they failed to develop (cross)communal identity. In time of crisis this was decisive & fundamentally influenced collapse of these multi-ethnic states. Without solution to this problem European Union will fall apart whenever first large scale economic or political crisis occurs...