Language-dependency of the left-digit effect in number line estimation and the role of number word inversion

Language plays a critical role in shaping cognitive processes. The present study focuses on the impact of language-dependent number representations, specifically examining the effects of number word inversion and the left-digit effect across different linguistic groups. Number word inversion is a phenomenon where in certain languages, such as in German, spoken two-digit numbers start with the unit preceding the decade (e.g., for 91, literally "one-and-ninety" rather than the Arabic numeral order "ninety-one"). Previous studies employing various numerical tasks indicate that the inversion effect-where the unit digit exerts a disproportionate impact on how the total magnitude of a two-digit number is perceived-influences children's performance. This study addresses the inversion effect in adults by comparing German-speakers to English- and Mandarin-speakers in a number line estimation task. Linear regression slope analyses of estimated positions conducted across decades identified the inversion effect, demonstrating that German-speakers provided comparatively higher estimates for numbers with larger unit digits compared to the other language groups. Additionally, the left-digit effect, which conversely is characterised by the smaller impact of the unit digit on perceived number magnitudes, became evident in all three language groups. This left-digit effect is further modulated by membership of a specific language group. The structure of Mandarin number words is discussed as a significant factor contributing to the pronounced amplification of the effect. Our findings suggest that language plays a pivotal role in persistently shaping the cognitive processing of numbers.