One of the reasons as I think, is fearing of others' mocking.

Two closely related areas offering research and theoretical insights here, I suggest, are those concerning the topics of self-efficacy and of cognitive mindset. Prominent among contributors to the former is Barry J. Zimmerman, and to the latter, Carol Dweck. Mindset theory differentiates learners whose behaviour is governed by a "performance" mjndset (tending towards obtaining social approval for their actions) or, contrastingly, by a "growth" mindset (motivated to seek ways of improving their current knowledge and proficiency level, perhaps beyond accepted educational principles and regardless of social responses). Self-efficacy - strength of one's conviction that one's actions will be, or eventually prove, successful - is seen by Zimmerman to be dependent upon capacity for cognitive self-regulation in learning-situations, that is, one's responsiveness to, and productive use-making of, task-relevant feedback for steering performance and learning. Both perspectives predict that the kind of feedback to which individuals respond in learning-situations - task-relevant versus social - will have a critical impact upon their readiness to participate in them.

Another insightful explanatory perspective is that of ideomotor theory, positing the necessity of being able to couple an appropriate motor-act to a conceptualized intention to achieve a certain, perceived corresponding effect in one's environment. Here, obviously, appropriate motor-acts are those of speech. Ability to participate in a discussion is dependent not merely upon comprehending the subject and immediate interpreting of the content of other participants' utterances (often ones contributed much earlier), both requiring near-instantaneous memory-retrieval, but equally upon alacrity at accessing suitable words for contributing one's own thoughts as well as at converting the accessed words into motor speech sequences. Contributing will obviously be undermined if one has difficulty imagining suitable words or generally in articulating them sequentially (e.g. as per stammerers). In addition, ideomotor theory has spawned extensive research investigating effects of individuals' focus of attention upon their motor performance and rate of improvement. Individuals who tend to focus on imagining the external goal they are intending to attain are found to perform and improve better than those who tend to focus on trying to produce an effective action for attaining it. This general finding, too, may apply as regards proficiency in articulating thoughts in demanding, formal contexts of speaking, e.g. in an academic discussion, in teaching or responding to audience questions during a lecture.

Other potent determinants of self-efficacy include the individual's general background regarding learning-contexts and the opportunities made possible thereby for furthering their proficiency as communicators. The extensive study headed by Bloom (1985) revealed that expert performers had typically received exceptional levels of parental support throughout childhood and adolescence, providing, among other factors, access to highly expert teachers in their domain, frequent experiences of performing to, and with, others, and opportunities for interacting with like peers. All such factors may be reasonably assumed to promote individual's self-efficacy as communicators of their intentions, and would have constituted for them normal phenomena in their ongoing life. By contrast, the less-privileged majority may grow up in a background of indifference towards their personal accomplishments and potential for growth, exposure to teaching-styles that foster dependency on being told what to think and do, and to ridicule from peers and teachers when they fail to deliver what is expected of them. Acquiring proficiency in articulating one's thoughts requires long-term practice and abundant opportunity for learning by trial-and-error that are often prevented in such an upbringing. Given that, readiness to contribute to a tutorial discussion may be influenced more by individuals' personality traits and personal cognitive style as determinants of self-efficacy than by anything else.

I do monitor the PBL classes and based on my experience I could say there are number of reasons why students do not contribute in the PBL process.

1. Some students dominant the class talk most of the time. They do not bother about others' participation

2. The less participatry students feel infereior and have less confidence to speak in the group

3. The leader and the tutor (who are responsible to see the delivery process) often focus on the content and ignore the significance of delivery process

4. Some students do not like PBL