Does individual-level socio-economic position modify the association between area-level deprivation and early childhood language development? A multilevel analysis of cross-sectional population data from Germany.

Early childhood is a public health priority for enhancing health equity across the life course. Children are susceptible to individual-level socio-economic position (SEP) and the socio-economic deprivation of their place of residence, both of which contribute to differences in psychomotor development, including language. This study investigates whether individual-level SEP modifies the association between area-level deprivation and language development. This study used population-based, cross-sectional data from a School Entry Survey in Germany (2021; female: n = 9751, male: n = 10 623; age in years: ≤6 n = 11 423; 6-≤7 n = 8 746; >7 n = 205; native language: German n = 18 752; not German n = 943; bilingual n = 679), which was linked to the 'German Index of Socioeconomic Deprivation' (GISD). Binary multilevel models [odds ratio (OR), 95% CI] were used to analyse the interaction of GISD (measured continuously) with individual-level SEP (categorized as high/medium/low) according to delayed language development (DLD in % yes/no), which was assessed using validated instruments of 'social-pediatric screening of developmental status for school entry'. Individual SEP was tested as a moderator by stratifying the analyses. The interaction of GISD with SEP was associated with DLD (ORmediumSEP*GISD = 1.13; 1.04-1.24; ORlowSEP*GISD = 1.27; 1.13-1.43), with the main effect of GISD (OR = 0.85; 0.77-0.93). In SEP stratification, GISD was only partially associated with DLD (ORhighSEP = 0.81; 0.73-0.91; ORmediumSEP = 0.97; 0.89-1.07; ORlowSEP = 1.14; 0.99-1.32). This study demonstrates an association between area-level deprivation and DLD that is modified by individual-level SEP. The findings underscore the importance of stratification by individual-level SEP in analysing area-level effects on health, as the area-level effects can be contradictory according to individual-level SEP.