  Objectives: Current standards for comparing stunting across human populations
assume a universal model of child growth. Such comparisons ignore population
differences that are independent of deprivation and health outcomes. This paper
partitions variation in height-for-age that is specifically associated with
deprivation and health outcomes to provide a basis for cross-population
comparisons.
  Materials & Methods: Using a multi-level model with a sigmoid relationship of
resources and growth, we partition variation in height-for-age z-scores (HAZ)
from 1,522,564 children across 70 countries into two components: 1) "accrued
HAZ" shaped by environmental inputs (e.g., undernutrition, infectious disease,
inadequate sanitation, poverty), and 2) a country-specific "basal HAZ"
independent of such inputs. We validate these components against
population-level infant mortality rates, and assess how these basal differences
may affect cross-population comparisons of stunting.
  Results: Basal HAZ differs reliably across countries (range of 1.5 SD) and is
independent of measures of infant mortality. By contrast, accrued HAZ captures
stunting as impaired growth due to deprivation and is more closely associated
with infant mortality than observed HAZ. Ranking populations by accrued HAZ
suggest that populations in West Africa and the Caribbean suffer much greater
levels of stunting than suggested by observed HAZ.
  Discussion: Current universal standards may dramatically underestimate
stunting in populations with taller basal HAZ. Relying on observed HAZ rather
than accrued HAZ may also lead to inappropriate cross-population comparisons,
such as concluding that Haitian children enjoy better conditions for growth
than do Indian or Guatemalan children.
