The study attempts to investigate whether there is a relationship between individual differences in cognitive abilities, learners’ perceptions about typology relations, and negative transfer in written production. To this aim, data were analyzed from Spanish/Catalan bilingual learners of L3 English (n= 47) aged 10-15, with German as L2. The cognitive abilities measured were working memory, attention control, and attention switching. Furthermore, proficiency in the L3 was also controlled, based on the resultsof the participants’ performance in the Oxford Placement Test. It was found that learners with lower attention alabilities had a hardertime in identifying areas of structural contrast between the L2 and the L3, which, in turn, resulted in a higher rate of transfer from this language.