At the beginning of the 20th Century, researchers became increasingly interested in the effects of bilingualism on cognitive development. With the emergence of the first standardised intelligence tests, it quickly became clear that bilinguals of all ages performed worse on them than their monolingual peers. Bilingualism was subsequently considered to be detrimental to non-verbal cognitive development. In these early studies, poorly matched samples of bilinguals and monolinguals repeatedly produced results suggesting adverse effects of bilingualism on non-linguistic cognitive functions due to unrelated background factors and/or unsuitable tests. This interest in how bilingualism affects non-linguistic cognitive abilities has continued through time, reflecting trends in psychological research. Researcher addressed its effect on meta-cognition and meta-linguistics in the 1970s and 1980s, before the focus shifted towards executive functioning in the 1990s. However, recent research suggests we may be repeating the same mistakes: poorly matched groups, no accounting for background factors, and inappropriate tasks.