When speakers of different languages are in contact, they often borrow features like sounds, words, or syntactic patterns from one language to the other. However, the lack of historical data has hampered estimation of this effect at a global scale. We overcome this hurdle by using genetic admixture and shared geohistorical location as a proxy for population contact. We find that language pairs whose speaker populations underwent genetic admixture or that are located in the same geohistorical area exhibit notable similar increases in shared linguistic patterns across world regions and different demographic relationships, suggesting a consistent trend in borrowing rates. At the same time, the effect varies strongly across specific linguistic features. This variation is only partly explained by cognitive differences in lifelong learnability and by social functions of signaling assimilation through borrowing, leaving much randomness in which specific features are borrowed. Additionally, we find that, for some features, admixture decreases sharing, likely reflecting signals of divergence (schismogenesis) under contact.