When a child is first learning a language in a bilingual environment, is it easier or harder to properly acquire the two distinct languages if they are more similar? For example, is it easier for a child to learn Italian and Spanish growing up in an Italian-Spanish household or English and Japanese growing up in English-Japanese household?

Four cases are possible:

  1. [Easier] It might be that similar languages require similar vowels, grammar, and/or vocabulary (or many other factors, the specifics don't matter) and thus are easier to learn.

  2. [Harder] if the languages are close, it might be harder for the child to learn to distinguish them and thus they might develop a mixed-language instead of learning the two separate languages.

  3. [Same] It could be that children's early language adoption is so robust that there is no statistical difference between the two categories.

  4. [Mid] There might be a sweet-spot where the language are distinct enough to not be confusing to the child, but similar enough to allow recycling of learning. If you pick more similar or less similar languages than this sweep-spot then the acquisition deteriorates.

If the case of children is different from adults (i.e. if similar languages are not easier for them) then why is this the case?

  • 3
    You've overlooked several cases: is it easier or harder to learn two languages when the vocabulary is almost the same or very different.
    – kaleissin
    Sep 22, 2011 at 10:55
  • Your just considering first language / native language / mother language right? Not kids learning at say five or ten years old as a second language? Sep 22, 2011 at 10:59
  • 1
    I am considering two native languages learnt at the same time. However, if the results for a second language before the critical period are non-trivial and insightful then I would like to know them, too. Sep 22, 2011 at 11:07
  • Re number 2, this wouldn't be a creole as there's no precursor pidgin, it would be a mixed language. There may not be many studies that compare these things, it would be difficult due to confounding factors, eg differing amount of input, main language/s of peers, domain separation, etc. It's hard to measure similarity/difference between languages, eg Bahasa Indonesia and English are unrelated but could be thought of as more similar in some ways than, say, Russian/English. But assuming optimal and equal input, I suspect it would be hard to measure a difference. Oct 21, 2011 at 4:01

2 Answers 2


I'd recommend taking a look at "Crosslinguistic influence in bilingual language acquisition: Italian and French as recipient languages" by Mueller and Hulk and "Internal and external interfaces in bilingual language development: Beyond structural overlap." by Serratrice and Sorace for starters.

The overall conclusions seem to be that structural overlap between the languages does play a role in competency, but that this is only one of many factors and may not even be the most important one.


An example of similar languages being simultaneously learnt as L1 and L2 is modern Catalonia (currently in Spain), where Spanish and Catalan are co-oficial.

Many households are monolingual, and the other language is learnt early at school, or from TV (despite some slums where Catalan is barely taught despite being mandatory).

In my case, living in a Catalan speaking household, the only Spanish I was exposed to before age 6 was that coming from TV programs, but by that time I had an almost native-like fluency when talking to strangers. And I rarely mixed up both languages. Only a few Catalan words slipped from time to time.

I guess I'd developed some sort of sound change and morpheme correspondence list between both languages.

  • 1
    Welcome to Linguistics SE! While your answer gives an example of someone learning two similar languages at the same time, it doesn't really address the question of "does the similarity between languages in dual-native speakers affect the ease in which they learn their mother tongues?" Could you edit your response to talk about this? For example, do you know of any research on differences in learning Catalan between mono-lingual Catalan children and bilingual Catalan/Castilian children?
    – acattle
    May 12, 2013 at 7:26

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