 that the instrumental variable, `nearc4`, is actually correlated with `IQ`, at least for the subset of men for which an IQ score is reported. However, the correlation between `nearc4“ and `IQ`, once the other explanatory variables are netted out, is arguably zero. At least, it is not statistically different from zero. In other words, `nearc4` fails the exogeneity requirement in a simple regression model but it passes, at least using the crude test described above, if controls are added to the wage equati