376 higher levels of problem behaviour than independent observers for the same sample of known children. The group-care workers in our study reported the most behaviour problems for all of the children regardless of their acquaintanceship with the children. This suggests that factors other than acquaintanceship or in addition to acquaintanceship may have inﬂuenced their perceptions of child behaviour. It is certainly possible that explicit study and professional experience give group-care workers spe- ciﬁc knowledge of child behaviour problems and thereby make them more qualiﬁed to detect speciﬁc behaviour problems than other informants. This capacity is referred to as expertise by Funder [8]. As part of their training and professional practice, moreover, group-care workers may be more skilled at the observation and analysis of video- taped behaviour than mothers or independent observers. Such professionals may be better prepared to detect and interpret minor behavioural cues in addition to obvious cues and report ambiguous or less conspicuous problem behaviours than other observers. Professional expertise with regard to child behaviour problems may also explain why the group-care workers did not report different levels of behaviour problems for known versus control children. Given that all of the children— including the control children—were being treated in the same clinic, it is reasonable to assume that the level of problem behaviours for the unknown control children did not differ from the average level of problem behaviours for the known children. That is, the similarities in the profes- sional ratings of the known and unknown children by the group-care workers are likely to reﬂect actual similarities in the levels of problem behaviour for the two groups of children. The professionals presumably drew upon their knowledge of child behaviour problems to evaluate the videotaped behaviour samples for both the known and unknown children. Mothers, in contrast, may only beneﬁt from knowledge of their own children. It is nevertheless possible that the similarities in the observations of the group-care workers for the known versus unknown children may still reﬂect some bias as this has been reported in the research literature. Studies of clinical judgments, for example, show a tendency on the part of professionals to adjust their ratings of client behaviour in accordance with information on the clinical status of the client or a phenomenon referred to as anchoring and adjustment [9]. Bias rather than expertise may thus explain the lack of difference in the professional reports of problem behaviours for the known versus unknown children and only additional research will help us unravel the contributions of the different factors to per- ceptions of child behaviour. The present ﬁndings have some important implications for clinical practice and research on the accuracy of child 123 Eur Child Adolesc Psychiatry (2010) 19:371–377 behaviour reports. To start with, the present ﬁndings show mothers to estimate the amount of problem behaviour on the part of their own children more accurately than inde- pendent observers. In keeping with the acquaintanceship hypothesis, mothers have greater access to information regarding their children than other informants and are therefore better equipped to detect and interpret critical behavioural cues with regard to their children. This implies that mothers are accurate informants whose information should be used in diagnosis and treatment in clinical practice. In addition, expertise and not acquaintanceship appears to explain the generally greater amounts of prob- lem behaviours reported by the group-care workers. This raises doubts about the use of independent observers who are often used as criterion informants, but who are unfa- miliar with the problem behaviours of children in their evaluation. That is, the requirement that independent jud- ges be experienced observers of child problem behaviours should be added to Richters’ [17] recommendation that carefully validated and independent ratings of the same child behaviours in identical settings be undertaken to attain accurate perceptions of child behaviour. In the meantime Richter’s [17] conclusion still holds true as he stated that ‘‘…maternal depression–perception associations should be viewed as an occasion for questions, not con- clusions, about the accuracy of depressed mothers’ reports’’ (p. 497). Yet another important ﬁnding is that the professionals in the present study observed more problem behaviours than mothers under controlled circumstances (i.e., when rating exactly the same videotaped behaviour samples). This ﬁnding is in contrast with the more common ﬁnding that mothers tend to report more behaviour problems than professionals using the same behavioural rating scales but observing the children in different settings [12, 22]. While such ﬁeld differences are often ascribed to reporting biases on the part of mothers, our ﬁndings suggest that the differences between the mothers and the professionals may be due to actual differences in the incidence of problem behaviours depending on the particular situation (i.e., in the home vs. in the institution or school setting). This only strengthens our belief in the accuracy of maternal reports, but more research is needed to test both the acquaintanceship and expertise hypotheses under varying circumstances and in different settings. We sug- gest a research design in which the status of the client is manipulated via the use of videotaped behaviour samples from both referred and normal children, for example. The professional ratings of these children can then be com- pared to the ratings provided by their mothers and inde- pendent observers in order to gain greater insight into the accuracy of child behaviour assessment and the factors mediating this.