Bilingual settings are perceived as exemplary cases of linguistic diversity, and they are assumed to trigger cross-linguistic interaction. The rationale underlying this assumption is the belief that when more than one language is processed in a brain, this will inevitably affect the way in which linguistic knowledge is acquired, stored and used. However, this idea stands in conflict with results obtained by research on children acquiring two (or more) languages simultaneously. They have been demonstrated to be able to differentiate languages from early on and to develop competences qualitatively identical to those of monolinguals. These studies thus provide little evidence supporting the idea that bilingualism must lead to divergent grammatical development. The question then is what triggers alterations of bilinguals’ grammars, especially of the syntactic core, possibly resulting in non-native competences. This has been claimed to occur in the acquisition of second languages, weaker languages of simultaneous bilinguals, or heritage languages. These acquisition types differ from first language development in that onset of acquisition of one language is delayed or that the amount of exposure to one language is reduced. I will argue that age at onset and severely reduced amount of exposure are potential causal factors triggering divergent developments, whereas bilingualism on its own is not a sufficient cause of divergence.