Tropical maize cultivars generally yield less than their temperate counterparts; improvement of the former is urgently needed, and limiting physiological responses must be identified. Experiments were conducted in south eastern Zaire and Kansas, USA, to compare physiological source factors (leaf area index, LAI) and sink factors (kernel number per unit leaf area, KN m-2 LA, and kernel weight, KW) that might limit yield. The tropical study compared three open-pollinated cultivars in two experiments: one consisting of three plant densities and three split applications of N fertilizer and the other consisting of four plant densities and two N rates. The temperate experiment compared three US-Cornbelt hybrids of known characteristics at four density levels and one N level. The results suggested that sink size (KN m-2 LA) was the factor limiting grain yield in tropical cultivars and the two low-yielding temperate hybrids, and that the source (LAI) was the factor limiting grain yield in the highest-yielding temperate hybrid. Traits that were associated with high grain yield and high density tolerance in temperate maize hybrids were small tassel size, silking earlier than anthesis, high available carbohydrates at anthesis, and numerous KN m-2 LA. Those traits indicated weak apical dominance that enabled early and rapid growth of the ear. Tropical maize possessed strong apical dominance, which caused small sink size. Tropical maize breeders should emphasize weak apical dominance in breeding for high grain yield and high density tolerance.