The Australian study [1] that is cited in the Wikipedia article compared pupils that learned Esperanto, Indonesian, German or Japanese, respectively, in primary school and then went on to learn a possibly different language in secondary school.
Let me note a few problems with that study up front:
- There was apparently an additional class per week by a professional Esperanto teacher (the other Esperanto teachers were learning on the go).
- The Japanese teachers in the comparison rated themselves 0.25-0.5 on a number of 5-point scales (Esperanto: 1.6-2.4, German 4.5-5, Indonesian: 2-3). This is essentially a plus for Esperanto (easy to learn for the teachers), but taints the comparison since the Japanese teachers were basically incompetent.
- Due to attrition of study subjects, their evaluation after one year of secondary school has only 21 students of Esperanto. (306 students in total)
- They do not differentiate by (or even talk about) the languages learned in secondary school.
Now to the findings (on a 5-point scale [1,2,3,4,5], rated by the secondary-school teachers, given as percentages in each category):
- Motivation to learn the language (presumably the new one): Esperanto [0, 5, 10, 67, 19], others [ 13, 9, 22, 24, 31]. This one is difficult to analyze, since their motivation might change due to liking the new language better (or worse), or due to a general effect on motivation to learn any language (the authors claim the latter). I'll just point out that both the lowest and the highest scores are rarer among Esperanto students.
- "attainment level" (no idea what that specifically means): Esperanto [0, 4, 26, 61, 9], others [10, 9, 23, 26, 32]. Again, both the lowest and the highest scores are rarer among Esperanto students. The striking resemblance to the motivation results is interesting.
- speaking ability: Esperanto [0, 0, 48, 43, 10], others [12, 8, 46, 27, 6]. This is the only measure where Esperanto seems to be a uniform improvement.
In my opinion, this study mostly shows that this specific Esperanto course did not completely kill off the students ability to learn a foreign language, unlike apparently some other courses. The improvement in speaking ability could be caused by the focus on an immersive classroom experience, but it did apparently transfer to other languages, which is a plus.
The study definitely doesn't show anything like "those who learned Esperanto and another target language (including Japanese) became better at the target than those who learned only the target." (Since they didn't test any such thing.) In fact, my best guess at explaining the higher rate of perfect motivation/attainment scores for students of other languages is that they continued studying the same language they had already had in primary school. If this were the case, it would pretty clearly show that learning Esperanto before learning Japanese is not going to help someone learn Japanese faster overall.
[1] http://web.archive.org/web/20040215074307/http://www.educati...