Robert Williams gives an overview of the putative main drivers of the effect. He notes that far from being uniform, “the gains have been large, small, variable and even negative”. Some researchers have found that the gains were on g loaded items, whilst more have found no g loadings, suggesting that the gains may be empty. The rate of gain varies by the dates chosen for study. Data from behind the former Iron Curtain countries are particularly informative, since there have been real changes in education and in society. A feature of the literature is the frequent contradictions: for every finding there seems to be an opposite finding. As to causes: education may be a driver of change in emerging nations, but probably no longer in wealthy nations; test sophistication is no longer much of an issue; guessing answers may boost gains somewhat; nutrition may boost gains, particularly at the lower levels; nutrition may be boosting both height and intelligence, but not in the same way at the same times; measurement invariance strongly suggests that the meaning of IQ is not constant over time. Williams says: “It is likely that most of the Flynn Effect gains that have been reported are hollow”. However, he also says that the effect remains enigmatic because there are varying combinations of multiple drivers, and methodological problems are confounded with real world issues.