Lower externalising behaviour was associated with female sex (−0.30 (−0.53, –0.08), p=0.009), higher soluble-transferrin-receptors (mg/L) (−0.07 (−0.14, –0.01), p=0.024) and initiation of solids/semisolids≥6 months (−0.16 (−0.31, –0.01), p=0.033), but higher externalising behaviour was associated with underweight (0.35 (0.07, 0.62), p=0.014), more diarrhoeal episodes (0.03 (0.004, 0.06), p=0.022) and higher Maternal Depression Score (0.04 (0.01, 0.07), p=0.003) in the first year. The gene discussed is TFRC; the disease is depressive disorder.