The role of the mother tongue has been a major topic of second language acquisition research over the last few decades, but despite the overwhelming empirical evidence of cross-linguistic influence in learner language a number Of questions still remain to be answered: what and how much is transferred when, how and why? This study explores the extent to which a theory of linguistic naturalness — the conceptual opposite of markedness, as formulated in Natural Phonology and Natural Morphology — might provide some insights into the issue of cross-linguistic influence. As an alternative to Eckman's (1977) 'Markedness Differential Hypothesis', a more detailed 'Naturalness Differential Hypothesis' is formulated in terms of phonological processes and morphological preferences. Unlike typological markedness, which may be regarded as a mere research tool, the notion of naturalness offers an explicit functional explanation of the observed learning difficulty, mainly in terms of ease of production and perception. Empirical support for these Claims will be drawn from research on the spontaneous acquisition of Italian äs a second language.