First, it has been shown that underestimation of one’s performance, even in regard to a single school subject, has been consistently associated to costs and no benefits. Secondly, accurate or realistic self-appraisals are associated to benefits and no costs. Thirdly, overestimation of one’s performance is more likely to be associated with more costs but not necessarily fewer benefits than realistic self-beliefs. These findings clearly point out the maladaptive functioning of negative illusions and the adaptive functioning of realistic self-appraisals. In addition, the beneficial character of positive illusions is brought into question by the support provided to the ‘mixed blessing’ view. From an applied perspective, our findings offer new insights to teachers, parents and care-givers seeking to enhance students’ motivation. Boys and girls should be encouraged to develop high but at the same time accurate efficacy beliefs while acquiring the knowledge and skills required for academic success.