A second inverted-U-shaped effect is then superimposed on this monotonic first effect, whereby (starting from zero) a rise in the share of highly-educated individuals initially raises earnings inequality as the earnings of some workers now differ from that associated with a low education level, but eventually inequality declines as more and more individuals have higher education and heterogeneity in education attainment is reduced. The results suggest that, on average across countries, an upper-secondary or post-secondary non-tertiary degree is associated with an earnings premium of 700 USD per month (relative to a lower-secondary degree) and a tertiary degree is associated with a further premium of more than 1100 USD per month. The vertical axis shows the impact of the same change on the log earnings of the 90th quantile.