1. Summarize: It should concisely synthesize existing research relevant to the current study's topic. This isn't just a list of papers; it should identify key findings, trends, and disagreements in the literature.
2. Provide rationale: It should clearly explain *why* the current study is needed. This often involves identifying gaps, inconsistencies, or limitations in previous research. The rationale shows how the study addresses an unanswered question, corrects a flaw, or explores a new aspect of the topic.
3. Show how it extends previous research: This explains how the current study builds upon existing knowledge. It might involve testing a different population, using a novel methodology, examining a different outcome, or exploring a mediating or moderating variable that wasn't considered before. The researcher should explicitly state how their work will contribute to the field.
If a literature review fails to do one or more of these, it's weak and doesn't effectively justify the current study.