372 tendency but only for stimuli presenting their own children as opposed to other children. On the basis of these con- tradictory results, Snarr et al. argued that the biases in maternal ratings of their own children may not be universal but depend on the severity of the behaviour problems and the particular parent–child history. A more general explanation for the observed variability in child ratings by close acquaintances or strangers can be provided by researchers and theorists in the domains of social and personality psychology. Ample evidence from these domains shows increased acquaintance to clearly affect the accuracy of judgments about personality char- acteristics of a person. For example, Blackman and Funder [5] showed both interjudge consensus and accuracy, deﬁned as self-other agreement, to be much higher for observers who had known the person being judged for an average of 14 months than for those who were unac- quainted with the person being judged and only observed the person on video for 30 min. In terms of Funder’s [7, 8] Realistic Accuracy Model, increased information about the target—greater acquaintance between target and judge— should generally lead to enhanced detection and interpre- tation of behavioural cues and thus to increased accuracy in the judgment of a person’s personality characteristics. Blackman and Funder [5] explain the effect of acquaintance on accuracy in personality judgements as follows. Acquaintances have the opportunity to observe a person on various occasions and thus observe and detect more behaviours that are relevant to certain personality characteristics. In other words, acquaintanceship not only leads to more accurate personality judgment but also—as a prerequisite—to the detection of more related behaviours. As a consequence, an observer who knows a person better not only can give a more accurate rating of his personality, but should also be able to give a more accurate rating of his behaviour. Applied to the ratings of behaviour problems of children referred to child psychiatric services or other kinds of child services, the acquaintanceship hypothesis predicts the report of greater amounts of behaviour problems by acquaintances as opposed to strangers. Moreover, this does imply that the child behavioural ratings of acquaintances (such as parents) are not necessarily biased when compared to the ratings of independent observers, but, on the contrary might be accurate. The acquaintanceship hypothesis provides not only an alternative explanation for the observed variation in maternal perceptions of children’s behaviour but can also help us re-frame one of the most frequently discussed methodological issues in research on observer biases due to informant personality traits—namely, the use of indepen- dent observers to provide criterion ratings [17]. Up to the turn of the century most of the research on this topic has been concerned with the demonstration of bias due to 123 Eur Child Adolesc Psychiatry (2010) 19:371–377 maternal depression, depressed mothers were found to report greater child behaviour problems than both non- depressed mothers and other informants serving as criterion raters (e.g., teachers, group-care workers). As Richters [17] has pointed out, however, none of these studies provides convincing evidence of depression-related distortion because the mothers and criterion raters invariably rate different child behaviours in different contexts. According to Richters, carefully validated and independent ratings of the same child behaviours under more or less identical circumstances are needed to provide conclusive evidence of bias in the perception of child behaviour. More recently, a few laboratory studies have indeed been undertaken with video ratings by independent observers not previously acquainted with the child serving as the criterion for evaluation of depression-related distortion of maternal perceptions of child behaviour [14, 20, 21]. These studies have produced mixed results and thus mirror the contra- dictory results obtained in ﬁeld studies. In using judges not previously acquainted with the children being observed as criterion raters the laboratory studies have unintentionally raised some doubts about the use of independent observers to evaluate child behaviour. In all of the laboratory studies, that is, considerable dif- ferences were detected between the ratings provided by knowledgeable versus independent observers. In the study by Kroes et al. [14], in fact, the mothers, teachers and group-care workers reported twice as many problem behaviours as independent observers for the exact same children on the same videotape. In the study by Weis and Lovejoy [20], the mothers also reported more than twice as many—both positive and negative—child behaviours as extensively trained independent observers, which led the authors to conclude that ‘‘mothers and observers used the scales differently, with mothers reporting higher levels of all behaviours’’ (p. 223). Youngstrom et al. [21] also found maternal reports of both positive and negative child behaviours to consistently differ from the reports of inde- pendent observers for the same children. The question, however, is whether the differences between the maternal and independent observer ratings are due to differential use of the rating scales or acquain- tanceship. This implies that bias in maternal reports of their own children is not convincingly demonstrated and that further experimental research is necessary. In the present study, we propose to approach this issue from a different theoretical angle. The present study In light of an hypothesized acquaintanceship effect, the nature of the differences between the ratings provided by