Temperature, geo-residential pattern of subpopulations, prevalence of infectious diseases, and UV radiation have been proposed to explain the declining cognitive ability observed with proximity to the equator in the USA. This study tested the cognitive effects of the four variables. The results reveal that the latitudinal decline of cognitive ability is strongly correlated with the UV Index rather than with the other variables among White children. The decline in measured cognitive ability from north to south is absent among African American and Hispanic children, plausibly because the high levels of skin melanin among these ethnic groups, by absorbing and dissipating light, prevent the occurrence of radiation's cognitive effects among these populations at USA latitudes. The possible physiological mediators (oxidative stress, folate degradation, sexual hormones) suggest diet, family planning, and educational methods as mitigating strategies; however, specific studies measuring the mediating variables are needed to confirm their role and further strengthen UV radiation as an explanatory concept.