We attempt to develop an integrative model of the factors that give rise to the overestimation of their abilities among the incompetent and underestimation of the competent by decomposing the specific conditions of the cognitive and motivational components underlying the self-assessment phenomenon from a statistical point of view. Hong Kong (Study 1) and European American participants (Study 2) took an ability test and assessed their performance. By plotting estimated relative ability against actual ability and fitting a regression line, we found that a comparative bias (intercept), reflecting participants’ self-enhancement motivation, and a less-than-perfect estimation accuracy (slope), reflecting participants’ cognitive bias, jointly contributed to the ability estimation line wherein low-performance participants overestimate and high performance participants underestimate their performance. In testing and validating the model, the relationship between participants’ estimated relative ability and actual performance was examined as a function of participants’ self-esteem (motivational factor) and perceived difficulty level (cognitive factor). Regarding the extent of self-enhancement, we found that participants with higher self-esteem rated their ability more favorably independent of their actual performance. Moreover, participants who perceived the task to be more difficult made more unfavorable self-assessments. Regarding the relationship between estimated and actual performance, we found that when the task was seen as more difficult, and, hence, less diagnostic of ability, participants were less likely to base their ability assessment on their actual performance, resulting in less accurate self-assessments.