Short-Term and Long-Term Effects of Incidental Vocabulary Acquisition and Instructed Vocabulary Teaching

The present empirical study was conducted to compare instructed vocabulary teaching and incidental vocabulary acquisition that are two common approaches to teaching second language (L2) vocabulary in the literature.  For this purpose, 53 Iran learners of English as a Foreign Language were selected from a larger sample and were then randomly assigned into a control group and two experimental groups as the participants of the study.  The participants in the groups received placebo instruction while those in the experimental groups were either explicitly instructed or incidentally exposed to a number of targeted words (TWs) selected for the purposes of the study.  The results of an immediate posttest of the TWs demonstrated that the participants in both experimental groups benefited from instruction on/exposure to the TWs compared to the participants in the control group who were neither instructed on nor exposed to the TWs.  The results of a delayed posttest indicated that, though there was a difference between the two experimental groups in the immediate posttest with respect to the acquisition of the TWs, the difference faded away in fiveweek interval as the experimental groups performed rather similarly on the delayed posttest.  At the end, the implication of these findings for L2 vocabulary research and pedagogy would be discussed, along with some suggestions for researchers who wish to follow this trend of research.