Under certain circumstances, well‐known others (so‐called informants) may possess unique insights into targets' personality traits beyond the targets' self‐views. Specifically, as proposed by the self–other knowledge asymmetry model, an incremental predictive ability of informants is most likely for traits and corresponding behaviours that are clearly visible to others and highly evaluative in nature. In two studies, we provide an empirical test of this proposition and extend prior research to one of the most important domains of interpersonal interaction: prosocial and moral behaviours. Specifically, we investigate the unique predictive power of informant reports in trait Honesty–Humility for fairness in the dictator game and dishonesty in a cheating paradigm. Importantly, while both these classes of behaviour are highly evaluative in nature, only fairness is clearly visible to others. Correspondingly, in line with the self–other knowledge asymmetry model, our results reveal unique predictive accuracy of informant reports for fairness. For dishonesty, by contrast, there was no conclusive evidence for incremental predictive power of informant reports. This implies that informants may indeed provide valuable information beyond targets' self‐reports on trait aspects driving fair behaviour, but that targets themselves are their own best experts when it comes to judging trait aspects driving dishonest behaviour. Copyright © 2017 European Association of Personality Psychology