One‐hundred and twenty‐seven Norwegian upper‐secondary school students completing college preparatory courses (41% female) were asked to select the texts they wanted to use in order to write a letter to the editor about a socio‐scientific topic, with 60 students assigned the topic of climate change and 67 students assigned the topic of nuclear power. Afterwards, they were asked to justify their text selections, read the selected texts and write their letters to the editor. Across both topics, hierarchical multiple regression analyses showed that students' behavioural engagement in the selection, justification, reading and writing tasks contributed to their performance over and above their reading comprehension, topic knowledge and topic interest. Specifically, behavioural engagement in the selection and justification tasks improved the prediction of their content‐based and source‐feature based justifications for text selection, and behavioural engagement in the reading and writing tasks improved the prediction of the content coverage and content integration observed in their written products, with large effect sizes obtained for behavioural engagement across the two topics. The theoretical and educational significance of the study is discussed and directions for future research are suggested.