How can language contact cause neighboring languages to become more similar?

Language contact causes neighboring languages to become more similar through several mechanisms, primarily driven by the interaction of speakers and the diffusion of linguistic features across geographic areas.1

• Borrowing: Languages frequently adopt loanwords from one another, particularly when a language lacks a term for a new object, institution, or technology. Over time, these borrowed elements can become integrated into the recipient language's vocabulary.23
• Bilingualism and Interference: When communities are bilingual, mutual interference often occurs. Speakers may carry pronunciation features, grammatical structures, or word meanings from one language into another. This process can lead to long-term linguistic change as these features become established within the community.4
• Areal Diffusion: Similarities between languages—such as shared grammatical or phonological traits—are often not the result of a common ancestral origin but rather the diffusion of linguistic innovations through space. This transmission can occur readily between neighboring languages, even those that are unrelated.15
• Social and Cultural Factors: The prestige of a dominant language or culture can accelerate these changes, as speakers may adopt features from the more influential group. Conversely, shared environments or contiguous settlement patterns can lead to cultural and linguistic profiles that closely resemble one another.61