Final answer:
The term for the reduction in personal identity and sense of personal responsibility in group settings is deindividuation.
Step-by-step explanation:
The reduction in personal identity and erosion of the sense of personal responsibility when one is part of a group is known as deindividuation. This phenomenon occurs in situations where individuals may feel a sense of anonymity, leading to less accountability and a diminished sense of self when they are amongst others. It is particularly noticeable in behaviors such as those exhibited by mobs or during riots, where people act in ways they typically would not if they were alone.