Marital rank, input allocation, and child growth disparities in polygynous family units under malt-barley contract farming in Arsi Zone, Ethiopia

Agrarian intra-household allocation models often assume unitary households, yet Sub-Saharan polygynous hierarchies are associated with distributional frictions. This study examines intra-household child growth disparities in polygynous smallholder family units in Arsi Zone, Ethiopia, evaluating how maternal rank (senior vs. junior wives), son preference, and malt-barley contract farming are associated with input allocation and child nutrition. Using primary microdata, the identification uses a family unit fixed-effects model to compare children and plots managed by co-wives within the same family unit. Metrics include plot-level fertilizer, male adult labor, crop yields, and child height-for-age z -scores (HAZ) and stunting prevalence among children aged 6–59 months. Fixed-effects estimates reveal a monotonic gradient. Children of third-or-higher-order wives have 0.72 standard deviation lower HAZ ( p < 0.001) and 20.8%-point higher stunting probability than children of first wives within the same family unit. This penalty is accompanied by input disparities. Junior wives’ plots receive 29.4–43.8 kg/ha less fertilizer, 9.1–13.4 fewer male labor days/ha, and 466–720 kg/ha lower yields. Son preference is positively associated with fertilizer allocation, but the association is attenuated for junior wives, indicating rank remains the dominant axis. Malt-barely contract farming is associated with a ~ 40% larger first-to-third-wife HAZ gap (β = − 0.29, p < 0.05). These findings challenge the unitary household model and suggest market integration can be associated with internal nutritional inequality. Policy implications include disaggregating polygynous family units in interventions, integrating distributional monitoring into contract-farming schemes, strengthening support for junior wives, and operationalizing parcel-specific land certification for co-wives.