The dynamics of rapid stunting reduction among children under 5 : a comparative study of age-specific trajectories and wealth disparities across six high-performing countries using JME and DHS data.

BACKGROUND: Childhood stunting remains a premier global public health challenge, with progress recently plateauing. Despite this, certain 'Success Outliers' defined in this study as nations that have achieved an Average Annual Rate of Reduction (AARR) ≥ 4.0% demonstrate that accelerated progress is achievable even in high-burden contexts. OBJECTIVES: The primary objectives of this study are (1) to evaluate the global association between initial stunting burden and the velocity of progress, measured by the Average Annual Rate of Reduction (AARR); (2) to characterize temporal shifts in age-specific growth dynamics among 'Success Outlier' nations defined here as those achieving an AARR ≥ 4.0%; and (3) to measure the evolution of wealth-based stunting inequalities using the Concentration Index. METHODS: This study analyzed six 'Success Outlier' nations - Albania, Armenia, Bolivia, Peru, Tajikistan, and Turkey; selected based on an Average Annual Rate of Reduction (AARR) ≥ 4.0%. The analysis utilized a pooled dataset of 142,028 children from Demographic and Health Surveys (1992-2023). We employed non-parametric Kernel-weighted local-mean regressions (lpoly) to estimate age-specific stunting trajectories and calculated the Concentration Index (CI) to evaluate longitudinal shifts in socioeconomic inequality. RESULTS: The six selected "Success Outliers" achieved an Actual Annual Average Rate of Reduction (AARR) ≥ 4.0%, led by Albania (6.3%) and Turkey (5.54%), which significantly outperformed the global median AARR of 2.24%. High-resolution analysis shows Peru reduced peak stunting from nearly 50% in 1992 to approximately 20% by 2012 by flattening the growth failure curve between 6 and 24 months. Socioeconomic equity improved most notably in Peru, where the Concentration Index (CI) shifted from - 0.312 to - 0.214, and Turkey, where it moved from - 0.285 to - 0.182. CONCLUSIONS: Accelerated stunting reduction is fundamentally characterized by a "vertical upward shift" in the entire age-specific growth distribution. While the "first 1000 days" remains a vital window, sustaining high reduction velocities requires nutritional and environmental support that extends into the third year of life to prevent the persistent "deep dip" seen in stagnant contexts. Furthermore, the stability of the Concentration Index in "Success Outlier" nations suggests that rapid progress is only achievable when child growth is structurally decoupled from socioeconomic status.