A meta-analysis is often justified as a way to avoid the problem of, well, just one study. These are topics where someone else had already decided that a meta-analysis was worthwhile doing and had done one. And the second issue is that these meta-analyses pertain to topics probably on average have more studies than the average topic that has been studied in the literature.
John Ioannidis of Stanford University talks with EconTalk host Russ Roberts about his research on the reliability of published research findings. They discuss Ioannidis's recent study on bias in economics research, meta-analysis, the challenge of small sample analysis, and the reliability of statistical significance as a measure of success in empirical research.