Very interesting finding! Two notes attached to this: being critical may only work well in the US. In places like China, and in most Asia, kissing bosses’ asses well may brings more benefits than challenging them. Another thing is that critical people tend to be more creative. They have better ideas to share, which explains why they earn more.
Timothy Judge, Beth Livingston, and Charlice Hurst examined this trait in a paper [PDF] in the Journal of Personality and Social Psychology this year. By way of background, conventional personality research defines agreeableness as two related qualities: (1) the extent to which you value getting along with others, and (2) the degree to which you are willing to be critical of others.
Using earnings data, the researchers found that men who rank high in agreeableness make substantially less than men who are less agreeable. Across studies, this difference was as high as $10,000 per year. Conversely, women’s earnings were less affected. There was only a small earnings difference between women high and low in agreeableness, and it was often not statistically reliable.