BackgroundDevelopmental researchers often use a multi-informant approach to measure adolescent behaviour and adjustment, but informant discrepancies are common. In general population samples, it is often found that parents report more positive and less negative outcomes than adolescents themselves. This study aimed to investigate factors associated with informant discrepancy, including adolescent sex, and parental level of psychological distress and education.MethodsInformant discrepancy on the Strengths and Difficulties Questionnaire was investigated using a Latent Difference Score (LDS) approach, which estimates the true difference between parent and adolescent reports in a structural equation model. The sample were parent-adolescent dyads from the seventh wave of the UK Millennium Cohort Study (N= 6947, 49.3% female, aged 17 years).ResultsParents reported lower levels of difficulties (emotion symptoms, peer problems, conduct problems), and higher levels of pro-social behaviour than adolescents themselves. Conditional effects were found, as discrepancy was greater amongst parent-daughter dyads for emotion and peer problems, and greater amongst parent-son dyads for conduct problems and pro-social behaviour. Parent-adolescent discrepancy was also greater generally if parents had a lower level of psychological distress or a higher level of education.ConclusionsIn a large general population sample from the UK, it was found that adolescents tended to report more negative and less positive outcomes than parents reported about them. Conditional effects were found at the parent and adolescent level suggesting that specific informant biases are likely to impact the measurement of adolescent behaviour and adjustment across reporters.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13034-023-00605-y.