I recently (2021) had a study published that shows that workers who rate themselves to be less productive than other workers are more likely to show mental vulnerability. For example, in the graph for Major Depression , the group that rated themselves to be much poorer producers (the "Poorest"), had the highest levels of major depression at some time in their lives (over 35%). In comparison, those who deemed themselves to be the equals of their co-workers did much better (about 15% depressed). Not surprising. However, those who thought that they were better than others at work were not better in terms of mental health (about 19% depressed -and about 22% for the "Best"). This surprising "U-shaped graph" was repeated for 10 of the 11 other human conditions shown here (anxiety not being so clear). In many cases, high levels of confidence seemed even more troubling. So, if you if you observe someone appearing to be unusually confident, have another look (even if it is yourself).
     Another point coming from this is that many research studies report only straight line (linear) findings, even when they have data that are suited to a curvilinear analysis (like testing for a U-function). So, if we had only looked at linear extremes here (Poorest and Best), we would get a straight line that would have incorrectly indicated the meaning for most of the findings!
     A viewable copy of the journal article can be found at https://rdcu.be/b4VXZ

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Source: Thompson AH (2021). Measures of mental health and addictions conditions show a U-shaped relationship with self-rated worker performance. Social Psychiatry and Psychiatric Epidemiology 56, 1823-1833.  https://doi.org/10.1007/s00127-020-01894-w. Click here for a copy.